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Title: Artificial and Natural Flight
Author: Maxim, Hiram S.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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  Transcriber’s Notes:

  Text printed in italics in the original work has been transcribed
  between _underscores_, bold text has been transcribed between =equals
  signs=. Small Capitals in the original work have been transformed to
  ALL CAPITALS.

  More Transcriber’s Notes may be found at the end of this text.



ARTIFICIAL AND NATURAL FLIGHT.



  =A Pocket-Book of Aeronautics.=--By H. W. L. MOEDEBECK. Translated
  from the German by Dr. W. MANSERGH VARLEY. With 150 Illustrations.
  10s. 6d. net.

  CONTENTS.--Gases--Physics of the Atmosphere--Meteorological
  Observations--Balloon Technics--Kites and Parachutes--On
  Ballooning--Balloon Photography--Photographic Surveying from
  Balloons--Military Ballooning--Animal Flight--Artificial
  Flight--Airships--Flying Machines--Motors--Air
  Screws--Appendix--Index.

  “Will be highly welcome to all aeronauts. It may be said to be the
  only complete work practically dealing with such matters. We have no
  hesitation in thoroughly recommending this as an absolutely
  indispensable book.”--_Knowledge._

  “It is without a doubt the best book that has appeared on the
  subject.”--_Aeronautical Journal._

  “The present volume ought certainly to be possessed by every student
  of Aeronautics, as it contains a vast amount of information of the
  highest value.”--_Glasgow Herald._

  WHITTAKER & CO., LONDON, E.C.



  ARTIFICIAL AND
  NATURAL FLIGHT.

  BY
  SIR HIRAM S. MAXIM.

  WITH 95 ILLUSTRATIONS.

  WHITTAKER & CO.,
  2 WHITE HART STREET, PATERNOSTER SQUARE,
  LONDON, E.C.,
  AND 64-66 FIFTH AVENUE, NEW YORK.
  1908.



PREFACE.


It was in 1856 that I first had my attention called to the subject of
flying machines. My father, who was a profound thinker and a clever
mechanician, seems to have given the subject a great deal of thought,
and to have matured a plan identical with what has been proposed by
hundreds since that time. I was then sixteen years of age, and a fairly
good mechanician, and any new thing in the mechanical line interested me
immensely.

My father’s proposed machine, of which he made a sketch, was of the
Hélicoptère type, having two screws both on the same axis--the lower one
to be right hand and mounted on a tubular shaft, and the top one to be
left hand and mounted on a solid shaft running through the lower tubular
shaft. These screws were to be rotated in reverse directions by means of
a small pinion engaging a bevel gear attached to each of the shafts. His
plan contemplated large screws with very fine pitch, and he proposed to
obtain horizontal motion by inclining the axis forward. He admitted that
there was no motor in existence light enough, but thought one might be
invented, and that an engine might be worked by a series of explosions
in the cylinder, that is, what is known to-day as internal combustion;
but he was not clear how such an engine could be produced. He, however,
said that a flying machine would be so valuable in time of war, that it
mattered little how expensive the explosive might be, even if fulminate
of mercury had to be used. It is interesting to note in this connection
that the great Peter Cooper of New York thought out an identical machine
about the same time, and actually commenced experiments. It seems that
this gentleman regarded fulminate of mercury as altogether too feeble
and inert, because we find that he selected chloride of nitrogen as his
explosive agent. However, his work was soon brought to an end by the
loss of the sight of one eye, after which time he had no further
dealings with this lively explosive.

The many early conversations that I had with my father on the subject
kept the matter constantly before me, and I think it was in 1872, after
having seen Roper’s hot-air engine and Brayton’s petroleum engine, that
I took the matter up, and commenced to make drawings of a machine of the
Hélicoptère type, but instead of having one screw above the other, I saw
at once that it would be much better if the two screws were widely
separated, so that each would engage new air, the inertia of which had
not been disturbed. The designing of the machine itself was a simple
matter, but the engine gave me trouble. No matter from what point I
examined the subject, the engine was always too heavy. It appears that
the Brayton engine was shown at the Centennial Exhibition at
Philadelphia in 1876, and that Otto visited this exhibition. Up to that
time, he had been making a species of rocket engine--that is, an engine
in which an explosive mixture shot the piston upward and then sucked it
back, a rack and pinion transmitting movement to the rotating shaft by
means of a pawl and ratchet. He appears to have been much interested in
the Brayton engine, as it was evidently very much in advance of his own.
It actually developed, even at that time, one horse-power per hour for
every pound of crude petroleum consumed, but it was very heavy indeed,
very difficult to start, and not always reliable. The shaft that worked
the valve gear was parallel to the cylinder, and placed in the exact
position occupied by a similar shaft in the present Otto engine, but
instead of revolving only half as fast as the crank shaft, it made the
same number of revolutions. On Otto’s return to Germany, he evidently
profited by what he had seen, and made a new engine, which in reality
was a cross between his own and the Brayton; the result was a very
important invention, which has been of incalculable value to mankind. It
is this engine which is now propelling our motor cars, and it is the
only engine suitable for employment on a flying machine; but even this
motor was not in a sufficiently high state of development as far as
lightness was concerned, to be of any use to me. The drawings which I
made in 1873, although of little or no value, kept my thoughts on
artificial flight, and while I was away from home attending to business,
especially when in foreign countries, I often amused myself by making
mathematical calculations. Quite true, the formula which I used at the
time--Haswell’s--was not correct; still, it was near enough to the mark
to be of considerable value. Moreover, the error in this formula
affected the Hélicoptère quite as much as the aeroplane system, and as I
was working with the view of ascertaining the relative merits of the two
systems, the error, although considerable, did not have any influence at
all in the decision which I arrived at--namely, that the aeroplane
system was the best. The machine that I thought out at that time
contemplated superposed aeroplanes of very great length from port to
starboard. The size in the other direction was more for the purpose of
preventing a rapid fall than for a lifting effect. I saw that it would
be necessary to have horizontal fore and aft rudders placed a long
distance apart, so as to prevent rapid pitching, and it appeared to me
that the further these rudders were apart, the easier it would be to
manœuvre the machine. As I never had any doubts regarding the efficiency
of screw propellers working in the air, I decided to use two of these of
a large size rotating in opposite directions. Of course, all this
speculation was theory only, but I verified it later on by actual
experiments before I built my machine, and it is very gratifying to me
to know that all the successful flying machines of to-day are built on
the lines which I had thought out at that time, and found to be the
best. All have superposed aeroplanes of great length from port to
starboard, all have fore and aft horizontal rudders, and all are driven
with screw propellers. The change from my model is only a change in the
framework made possible by dispensing with the boiler, water tank, and
steam engine. In this little work, I have dealt at considerable length
with air currents, the flight of birds, and the behaviour of kites,
perhaps at the expense of some repetitions; as the resemblance between
kite flying and the soaring of birds is similar in many respects,
repetitions are necessary. To those who go to sea in ships, it is
necessary to know something of the currents they are liable to
encounter; if it be a sailing ship, certainly a knowledge of the air
currents is of the greatest importance, and so it is with flying
machines. If flights of any considerable distance are to be made, the
machine is liable at any time to encounter very erratic air currents,
and it has been my aim in discussing these three subjects--air currents,
birds, and kites--to bring them before the would-be navigators of the
air, in order that they may anticipate the difficulties they have to
deal with and be ready to combat them. Then, again, there has been
almost an infinite amount of discussion regarding the soaring of birds
and the flying of kites. Many years ago, after reading numerous works on
the subject of flight, I became a close observer myself, and always
sought in my travels to learn as much as possible. I have attempted to
discuss this subject in simple and easily understood language, and to
present sufficient evidence to prevent the necessity of any further
disputes. I do not regard what I have said as a theory, but simply as a
plain statement of absolute and easily demonstrated facts. During the
last few years, a considerable number of text-books and scientific
treatises have been written on the subject of artificial flight, the
most elaborate and by far the most reliable of these being the
“Pocket-Book of Aeronautics,” by Herman W. L. Moedebeck, Major und
Battaillonskommandeur im Badischen Fussartillerie Regiment No. 14; in
collaboration with O. Chanute and others. Translated by W. Mansergh
Varley, B.A., D.Sc., Ph.D., and published by Whittaker & Co. This work
does not, however, confine itself altogether to flying machines, but has
a great deal of information which is of little or no value to the
builder of true flying machines; moreover, it is not simple enough to be
readily understood by the majority of experimenters. In some other works
which I have recently examined, I find a confusing mass of the most
intricate mathematical calculations, abounding in an almost infinite
number of characters, and extending over hundreds of pages, but on a
close examination of some of the deductions arrived at, I find that a
good many of the mathematical equations are based on a mistaken
hypothesis, and the results arrived at are very wide of the truth. I
have shown several diagrams which will explain what I mean. What is
required by experimenters in flying machines--and there will soon be a
great number of them--is a treatise which they can understand, and which
requires no more delicate instruments than a carpenter’s 2-foot rule and
a grocer’s scales. The calculations relating to the lift, drift, and the
skin friction of an aeroplane are extremely simple, and it is quite
possible to so place this matter that it can be understood by anyone who
has the least smattering of mathematical knowledge. Mathematics of the
higher order expressed in elaborate formulæ do very well in
communications between college professors--that is, if they happen to be
agreed. When, however, these calculations are so intricate as to require
a clever mathematician a whole day to study out the meaning of a single
page, and if when the riddle is solved, we find that these calculations
are based on a fallacy, and the results in conflict with facts, it
becomes quite evident to the actual experimenter that they are of little
value. For many years, Newton’s law was implicitly relied upon. Chanute,
after going over my experimental work, wrote that Newton’s law was out
as 20 is to 1--that is, that an aeroplane would lift twenty times as
much in practice as could be shown by the use of Newton’s formula. Some
recent experiments, which I have made myself, at extremely high
velocities and at a very low angle, seem to demonstrate that the error
is nearer 100 to 1 than 20 to 1. It will, therefore, be seen how little
this subject was understood until quite recently, and even now the
mathematicians who write books and use such an immense amount of
formulæ, do not agree by any means, as will be witnessed by the mass of
conflicting controversy which has been appearing in _Engineering_ during
the last four months. When an aeroplane placed at a working angle of,
say, 1 in 10 is driven through the air at a high velocity, it, of
course, pushes the air beneath it downwards at one-tenth part of its
forward velocity--that is, in moving 10 feet, it pushes the air down 1
foot. A good many mathematicians rely altogether upon the acceleration
of the mass of air beneath the aeroplane which is accelerated by its
march through the air, the value of this acceleration being in
proportion to the square of the velocity which is imparted to it.
Suppose now that the aeroplane is thin and well-made, that both top and
bottom sides are equally smooth and perfect; not only does the air
engaged by the under side shoot downwards, but the air also follows the
exact contour of the top side, and is also shot downwards with the same
mean velocity as that passing on the underneath side, so if we are going
to consider the lifting effect of the aeroplane, we must not leave out
of the equation, the air above the aeroplane, which has quite as much
mass and the same acceleration imparted to it, as the air below the
aeroplane. Even calculations made on this basis will not bring the
lifting effect of an aeroplane up to what it actually does lift in
practice; in fact, the few mathematicians who have made experiments
themselves have referred to the actual lifting effect of aeroplanes
placed at a low angle and travelling at a high velocity as being
unaccountable. Only a few mathematicians appear to have a proper grasp
of the subject. However, three could be pointed out who understand the
subject thoroughly, but these are all mathematicians of the very highest
order--Lord Kelvin, Lord Rayleigh, and Professor Langley. In placing
before the public, the results of my experiments and the conclusions
arrived at, it is necessary to show the apparatus which I employed,
otherwise it might be inferred that my conclusions were guesswork, or
mathematical calculations which might or might not be founded on a
mistaken hypothesis; this is my excuse for showing my boiler and engine,
my rotating arm, and my large machine. I do not anticipate that anyone
will ever use a steam engine again, because any form of a boiler is
heavy; moreover, the amount of fuel required is much greater than with
an internal combustion engine, and certainly seven times as much water
has to be dealt with. However, the description which I am giving of my
apparatus will demonstrate that I had the instruments for doing the
experimental work that I have described in this work. In the Appendix
will be found a description of my machine, and some of my apparatus. The
conclusions which I arrived at were written down at the time with a
considerable degree of care, and are of interest because they show that,
at that date, I had produced a machine that lifted considerably more
than its own weight and had all of the essential elements, as far as
superposed aeroplanes, fore and aft horizontal rudders, and screw
propellers were concerned, common to all of the successful machines
which have since been made. The fact that practically no essential
departure has been made from my original lines, indicates to my mind
that I had reasoned out the best type of a machine even before I
commenced a stroke of the work.

I have to thank Mr. Albert T. Thurston for reading the proofs of this
work.

  H. S. M.



CONTENTS.


  CHAPTER I.

                                                                    PAGE

  Introductory,                                                        1


  CHAPTER II.

  Air Currents and the Flight of Birds,                               11


  CHAPTER III.

  Flying of Kites,                                                    25


  CHAPTER IV.

  Principally Relating to Screws,                                     31


  CHAPTER V.

  Experiments with Apparatus Attached to a Rotating Arm--Crystal
  Palace Experiments,                                                 62


  CHAPTER VI.

  Hints as to the Building of Flying Machines--Steering by Means
  of a Gyroscope,                                                     77


  CHAPTER VII.

  The Shape and Efficiency of Aeroplanes--The Action of Aeroplanes
  and the Power Required Expressed in the Simplest Terms--Some
  Recent Machines,                                                    99


  CHAPTER VIII.

  Balloons,                                                          120


  APPENDIX I.,                                                       125


  APPENDIX II.--

  Recapitulation of Early Experiments--Efficiency of Screw
  Propellers, Steering, Stability, &c.--The Comparative Value of
  Different Motors--Engines--Experiments with Small Machines
  Attached to a Rotating Arm,                                        130


  INDEX,                                                             163



INDEX OF ILLUSTRATIONS.


  FIG.      PAGE

   1. Diagram showing the reduction of the projected horizontal
      area,                                                            2
   2. Professor Langley’s experiments,                                 5
   3. Eagles balancing themselves on an ascending current of air,     14
   4. Air currents observed in Mid-Atlantic,                          16
   5. Glassy streaks in the Bay of Antibes,                           17
   6. Air currents observed in the Mediterranean,                     18
   7. The circulation of air produced by a difference in
      temperature,                                                    27
   8. Kite flying,                                                    29
   9. Group of screws and other objects used in my experiments,       32
  10. Some of the principal screws experimented with,                 32
  11. The three best screws,                                          33
  12. Apparatus for testing the thrust of screws,                     34
  13. Apparatus for testing the direction of air currents,            35
  14. The ends of screw blades,                                       36
  15. The manner of building up the large screws,                     39
  16. A fabric-covered screw,                                         40
  17. The hub and one of the blades of the screw on the Farman
      machine,                                                        42
  18. Section of screw blades having radial edges,                    43
  19. Form of the blade of a screw made of sheet metal,               44
  20. New form of hub,                                                45
  21. Small apparatus for testing fabrics for aeroplanes,             50
  22. Apparatus for testing the lifting effect of aeroplanes and
      condensers,                                                     51
  23. Apparatus for testing aeroplanes, condensers, &c.,              52
  24. Cross-sections of bars of wood,                                 53
  25. Sections of bars of wood,                                       54
  26. A flat aeroplane placed at different angles,                    55
  27. Group of aeroplanes used in experimental research,              56
  28. An 8-inch aeroplane which did very well,                        57
  29. Resistance due to placing objects in close proximity to each
      other,                                                          58
  30. Cross-section of condenser tube made in the form of Philipps’
      sustainers,                                                     60
  31. The grouping of condenser tubes made in the form of Philipps’
      sustainers,                                                     61
  32. Machine with a rotating arm,                                    63
  33. A screw and fabric-covered aeroplane in position for testing,   64
  34. The rotating arm of the machine with a screw and aeroplane
      attached,                                                       65
  35. The little steam engine used by me in my rotating arm
      experiments,                                                    66
  36. The machine attached to the end of the rotating shaft,          68
  37. Marking off the dynamometer,                                    69
  37_a_. Right- and left-hand four-blade screws,                      70
  38. Apparatus for indicating the force and velocity of the wind
      direct,                                                         71
  39. Apparatus for testing the lifting effect of aeroplanes,         73
  40. Front elevation of proposed aeroplane machine,                  77
  41. Side elevation of proposed aeroplane machine,                   78
  42. Plan of proposed aeroplane machine,                             79
  43. Plan of a hélicoptère machine,                                  82
  44. Showing the position of the blades of a hélicoptère as they
      pass around a circle,                                           83
  45. System of splicing and building up wooden members,              86
  46. Cross-section of struts,                                        86
  47. Truss suitable for use with flying machines,                    87
  48. The paradox aeroplane,                                          88
  49. The Antoinette motor,                                           89
  50. Section showing the Antoinette motor as used in the Farman and
      De la Grange machines,                                          90
  51. Pneumatic buffer,                                               91
  52. Gyroscope,                                                      94
  53. Adjusting the lifting effect,                                   95
  54. Showing that the machine could be tilted in either direction
      by changing the position of the rudder,                         96
  55. Adjusting the lifting effect,                                   97
  56. Adjustment of the rudders,                                      98
  57. Diagram showing the evolution of a wide aeroplane,             102
  58. In a recently published mathematical treatise on aerodynamics
      an illustration is shown, representing the path that the air
      takes on encountering a rapidly moving curved aeroplane,       104
  59. An illustration from another scientific publication also on
      the dynamics of flight,                                        104
  60. Another illustration from the same work,                       105
  61. The shape and the practical angle of an aeroplane,             105
  62. An aeroplane of great thickness,                               106
  63. Section of a screw blade having a rib on the back,             106
  64. Shows a flat aeroplane placed at an angle of 45°,              107
  65. The aeroplane here shown is a mathematical paradox,            107
  66. This shows fig. 65 with a section removed,                     107
  67. Diagram showing real path of a bird,                           108
  68. The De la Grange machine on the ground,                        111
  69. The De la Grange machine in full flight,                       111
  70. Farman’s machine in flight,                                    112
  71. Bleriot’s machine,                                             113
  72. Santos Dumont’s flying machine,                                113
  72_a_. Angles and degrees compared,                                115
  72_b_. Diagram showing direction of the air with a thick curved
      aeroplane,                                                     118
  72_c_. Aeroplanes experimented with by Mr. Horatio Philipps,       118
  73. The enormous balloon “Ville de Paris,”                         123
  74. Photograph of a model of my machine,                           130
  75. The fabric-covered aeroplane experimented with,                131
  76. The forward rudder of my large machine showing the fabric
      attached to the lower side,                                    131
  77. View of the track used in my experiments,                      134
  78. The machine on the track tied up to the dynamometer,           135
  79. Two dynagraphs,                                                136
  80. The outrigger wheel that gave out and caused an accident with
      the machine,                                                   137
  81. Shows the broken planks and the wreck that they caused,        138
  82. The condition of the machine after the accident,               139
  83. This shows the screws damaged by the broken planks,            140
  84. This shows a form of outrigger wheels which were ultimately
      used,                                                          141
  85. One pair of my compound engines,                               142
  86. Diagram showing the path that the air has to take in passing
      between superposed aeroplanes in close proximity to each
      other,                                                         144
  87. Position of narrow aeroplanes arranged so that the air has
      free passage between them,                                     145
  88. The very narrow aeroplanes or sustainers employed by Mr.
      Philipps,                                                      146
  89. One of the large screws being hoisted into position,           149
  90. Steam boiler employed in my experiments,                       157
  91. The burner employed in my steam experiments,                   157
  92. Count Zeppelin’s aluminium-covered airship coming out of its
      shed on Lake Constance,                                        161
  93. Count Zeppelin’s airship in full flight,                       161
  94. The new British war balloon “Dirigible” No. 2,                 162
  95. The Wright aeroplane in full flight,                           162



ARTIFICIAL AND NATURAL FLIGHT.



CHAPTER I.

INTRODUCTORY.


It has been my aim in preparing this little work for publication to give
a description of my own experimental work, and explain the machinery and
methods that have enabled me to arrive at certain conclusions regarding
the problem of flight. The results of my experiments did not agree with
the accepted mathematical formulæ of that time. I do not wish this
little work to be considered as a mathematical text-book; I leave that
part of the problem to others, confining myself altogether to data
obtained by my own actual experiments and observations. During the last
few years, a considerable number of text-books have been published.
These have for the most part been prepared by professional
mathematicians, who have led themselves to believe that all problems
connected with mundane life are susceptible of solution by the use of
mathematical formulæ, providing, of course, that the number of
characters employed are numerous enough. When the Arabic alphabet used
in the English language is not sufficient, they exhaust the Greek also,
and it even appears that both of these have to be supplemented sometimes
by the use of Chinese characters. As this latter supply is unlimited, it
is evidently a move in the right direction. Quite true, many of the
factors in the problems with which they have to deal are completely
unknown and unknowable; still they do not hesitate to work out a
complete solution without the aid of any experimental data at all. If
the result of their calculations should not agree with facts, “bad luck
to the facts.” Up to twenty years ago, Newton’s erroneous law as relates
to atmospheric resistance was implicitly relied upon, and it was not
the mathematician who detected its error, in fact, we have plenty of
mathematicians to-day who can prove by formulæ that Newton’s law is
absolutely correct and unassailable. It was an experimenter that
detected the fault in Newton’s law. In one of the little mathematical
treatises that I have before me, I find drawings of aeroplanes set at a
high and impracticable angle with dotted lines showing the manner in
which the writer thinks the air is deflected on coming in contact with
them. The dotted lines show that the air which strikes the lower or
front side of the aeroplane, instead of following the surface and being
discharged at the lower or trailing edge, takes a totally different and
opposite path, moving forward and over the top or forward edge,
producing a large eddy of confused currents at the rear and top side of
the aeroplane. It is very evident that the air never takes the erratic
path shown in these drawings; moreover, the angle of the aeroplane is
much greater than one would ever think of employing on an actual flying
machine. Fully two pages of closely written mathematical formulæ follow,
all based on this mistaken hypothesis. It is only too evident that
mathematics of this kind can be of little use to the serious
experimenter. The mathematical equation relating to the lift and drift
of a well-made aeroplane is extremely simple; at any practicable angle
from 1 in 20 to 1 in 5, the lifting effect will be just as much greater
than the drift, as the width of the plane is greater than the elevation
of the front edge above the horizontal--that is, if we set an aeroplane
at an angle of 1 in 10, and employ 1 lb. pressure for pushing this
aeroplane forward, the aeroplane will lift 10 lbs. If we change the
angle to 1 in 16, the lift will be 16 times as great as the drift. It is
quite true that as the front edge of the aeroplane is raised, its
projected horizontal area is reduced--that is, if we consider the width
of the aeroplane as a radius, the elevation of the front edge will
reduce its projected horizontal area just in the proportion that the
versed sine is increased. For instance, suppose the sine of the angle to
be one-sixth of the radius, giving, of course, to the aeroplane an
inclination of 1 in 6, which is the sharpest practical angle, this only
reduces the projected area about 2 per cent., while the lower and more
practical angles are reduced considerably less than 1 per cent. It will,
therefore, be seen that this factor is so small that it may not be
considered at all in practical flight.

[Illustration: Fig. 1.--Diagram showing the reduction of the projected
horizontal area of aeroplanes due to raising the front edge above the
horizontal--_a_, _b_, shows an angle of 1 in 4, which is the highest
angle that will ever be used in a flying machine, and this only reduces
the projected area about 2 per cent. The line _c_ _b_ shows an angle of
1 in 8, and this only reduces the projected area an infinitesimal
amount. As the angle of inclination is increased, the projected area
becomes less as the versed sine _f_ _d_ becomes greater.]

Some of the mathematicians have demonstrated by formulæ, unsupported by
facts, that there is a considerable amount of skin friction to be
considered, but as no two agree on this or any other subject, some not
agreeing to-day with what they wrote a year ago, I think we might put
down all of their results, add them together, and then divide by the
number of mathematicians, and thus find the average coefficient of
error. When we subject this question to experimental test, we find that
nearly all of the mathematicians are radically wrong, Professor Langley,
of course, excepted. I made an aeroplane of hard rolled brass, 20 gauge;
it was 1 foot wide and dead smooth on both sides; I gave it a curvature
of about 1/16 inch and filed the edges, thin and sharp. I mounted this
with a great deal of care in a perfectly horizontal blast of air of 40
miles an hour. When this aeroplane was placed at any angle between 1 in
8 and 1 in 20, the lifting effect was always just in proportion to its
angle. The distance that the front edge was raised above the horizontal,
as compared with the width of the aeroplane, was always identical with
the drift as compared with the lift. On account of the jarring effect
caused by the rotation of the screws that produced the air blast, we
might consider that all of the articulated joints about the weighing
device were absolutely frictionless, as the jar would cause them to
settle into the proper position quite irrespective of friction. I was,
therefore, able to observe very carefully, the lift and the drift. As
an example of how these experiments were conducted, I would say that the
engine employed was provided with a very sensitive and accurate
governor; the power transmission was also quite reliable. Before making
these tests, the apparatus was tested as regards the drift, without any
aeroplane in position, and with weights applied that would just balance
any effect that the wind might have on everything except the aeroplane.
The aeroplane was then put in position and the other system of weights
applied until it exactly balanced, all the levers being rapped in order
to eliminate the friction in their joints. The engine was then started
and weights applied just sufficient to counterbalance the lifting effect
of the aeroplane, and other weights applied to exactly balance the drift
or the tendency to travel with the wind. In this way, I was able to
ascertain, with a great degree of accuracy, the relative difference
between the lift and drift. If there had been any skin friction, even to
the extent of 2 per cent., it would have been detected. This brass
aeroplane was tested at various angles, and always gave the same
results, but of course I could not use thick brass aeroplanes on a
flying machine; it was necessary for me to seek something much lighter.
I therefore conducted experiments with other materials, the results of
which are given. However, with a well-made wooden aeroplane 1 foot wide
and with a thickness in the centre of 7/16 inch, I obtained results
almost identical with those of the very much thinner brass aeroplane,
but it must not be supposed that in practice an aeroplane is completely
without friction. If it is very rough, irregular in shape, and has any
projections whatsoever on either the top or bottom side, there will be a
good deal of friction, although it may not, strictly speaking, be skin
friction; still, it will absorb the power, and the coefficient of this
friction may be anything from ·05 to ·40. These experiments with the
brass aeroplane demonstrated that the lifting effect was in direct
proportion to the angle, and that skin friction, if it exists at all,
was extremely small, but this does not agree with a certain kind of
reasoning which can be made very plausible and is consequently generally
accepted.

[Illustration: Fig. 2.--Professor Langley’s experiments--_a_, end of the
rotating arm; _b_, brass plane weighing 1 lb.; _c_ _c_, spiral springs.
When the arm was driven through the air, in the direction shown, the
plane assumed approximately a horizontal position, and the pull on the
springs _c_ _c_ was reduced from 1 lb. to 1 oz.]

Writers of books, as a rule, have always supposed that the lifting
effect of an aeroplane was not in proportion to its inclination, but in
proportion to the square of the sine of the angle. In order to make
this matter clear, I will explain. Suppose that an aeroplane is 20
inches wide and the front edge is raised 1 inch above the horizontal. In
ordinary parlance this is, of course, called an inclination of 1 in 20,
but mathematicians approach it from a different standpoint. They regard
the width of the aeroplane as unity or the radius, and the 1 inch that
the front edge is raised as a fraction of unity. The geometrical name of
this 1 inch is the sine of the angle--that is, it is the sine of the
angle at which the aeroplane is raised above the horizontal. Suppose,
now, that we have another identical aeroplane and we raise the front
edge 2 inches above the horizontal. It is very evident that, under these
conditions, the sine of the angle will be twice as much, and that the
square of the sine of the angle will be four times as great. All the
early mathematicians, and some of those of the present day, imagine that
the lift must be in proportion to the square of the sine of the angle.
They reason it out as follows:--If an aeroplane is forced through the
air at a given velocity, the aeroplane in which the sine of the angle is
2 inches will push the air down with twice as great a velocity as the
one in which the sine of the angle is only 1 inch, and as the force of
the wind blowing against a normal plane increases as the square of the
velocity, the same law holds good in driving a normal plane through
still air. From this reasoning, one is led to suppose that an aeroplane
set at an angle of 1 in 10 will lift four times as much as one in which
the inclination is only 1 in 20, but experiments have shown that this
theory is very wide of the truth. There are dozens of ways of showing,
by pure mathematics, that Newton’s law is quite correct; but in building
a flying machine no theory is good that does not correspond with facts,
and it is a fact, without any question, that the lifting effect of an
aeroplane, instead of increasing as the square of the sine of the angle,
only increases as the angle. Lord Kelvin, when he visited my place, was,
I think, the first to mention this, and point out that Newton’s law was
at fault. Professor Langley also pointed out the fallacy of Newton’s
law, and other experimenters have found that the lifting effect does not
increase as the square of the sine of the angle. In order to put this
matter at rest, Lord Rayleigh, who, I think we must all admit, would not
be likely to make a mistake, made some very simple experiments, in which
he demonstrated that two aeroplanes, in which we may consider the sine
of the angle to be 1/4 inch, lifted slightly more than a similar
aeroplane in which the sine of the angle was only 1/2 inch. Of course,
Lord Rayleigh did not express it in inches, but in term of the radius.
His aeroplanes were, however, very small. We can rely upon it that the
lifting effect of an aeroplane at any practical angle, everything else
being equal, increases in direct proportion to the angle of the
inclination. In this little work, I have attempted to make things as
simple as possible; it has not been written for mathematicians, and I
have, therefore, thought best to express myself in inches instead of in
degrees. If I write, “an inclination of 1 in 20,” everyone will
understand it, and only a carpenter’s 2-foot rule is required to
ascertain what the angle is. Then, again, simple measurements make
calculations much simpler, and the lifting effect is at once understood
without any computations being necessary. If the angles are expressed in
degrees and minutes, it is necessary to have a protractor or a text-book
in order to find out what the inclination really is. When I made my
experiments, I only had in mind the obtaining of correct data, to enable
me to build a flying machine that would lift itself from the ground. At
that time I was extremely busy, and during the first two years of my
experimental work, I was out of England fourteen months. After having
made my apparatus, I conducted my experiments rather quickly, it is
true, but I intended later on to go over them systematically and
deliberately, make many more experiments, write down results, and
prepare some account of them for publication. However, the property
where I made these experiments was sold by the company owning it, and my
work was never finished, so I am depending on the scraps of data that
were written down at the time. I am also publishing certain observations
that I wrote down shortly after I had succeeded in lifting more than the
weight of my machine. I think that the experiments which I made with an
aeroplane only 8 inches wide will be found the most reliable. All the
machinery was running smoothly, and the experiments were conducted with
a considerable degree of care. In making any formula on the lifting
effect of the aeroplane, it should be based on what was accomplished
with the 8-inch plane. Only a few experiments were made to ascertain the
relative value of planes of different widths. However, I think we must
all admit that a wide plane is not as economical in power as a narrow
one. In order to make this matter plain, suppose that we have one
aeroplane placed at such an angle that it will lift 2 lbs. per square
foot at a velocity of 40 miles an hour; it is very evident that the air
just at the rear of this aeroplane would be moving downward at a
velocity corresponding to the acceleration imparted to it by the plane.
If we wish to obtain lifting effect on this air by another plane of
exactly the same width, we shall have to increase its inclination in
order to obtain the same lifting effect, and, still further, it will be
necessary to use more power in proportion to the load lifted. If a third
aeroplane is used, it must be placed at an angle that will impart
additional acceleration to the air, and so on. Each plane that we add
will have to be placed at a sharper angle, and the power required will
be just in proportion to the average angle of all the planes. As the
action of a wide aeroplane is identical with that of numerous narrow
ones placed in close proximity to each other, it is very evident that a
wide aeroplane cannot be as efficient in proportion to its width as a
narrow one. I have thought the matter over, and I should say that the
lifting effect of a flat aeroplane increases rather faster than the
square root of its width. This will, at least, do for a working
hypothesis. Every flying machine must have what we will call “a length
of entering edge”--that is, the sum of entering edges of all the
aeroplanes must bear a fixed relation to the load carried. If a machine
is to have its lifting effect doubled, it is necessary to have the
length of entering edge twice as long. This additional length may, of
course, be obtained by superposed planes, but as we may assume that a
large aeroplane will travel faster than a small one, increased velocity
will compensate in some degree for the greater width of larger
aeroplanes. By careful study of the experiments which I have made, I
think it is quite safe to state that the lifting effect of well-made
aeroplanes, if we do not take into consideration the resistance due to
the framework holding them in position, increases as the square of their
velocity. Double their speed and they give four times the lifting
effect. The higher the speed, the smaller the angle of the plane, and
the greater the lifting effect in proportion to the power employed. When
we build a steamship, we know that its weight increases as the cube of
any one of its dimensions--that is, if the ship is twice as long, twice
as wide, and twice as deep it will carry eight times as much; but at the
very best, with even higher speed, the load carried by a flying machine
will only increase with the square of any one of its dimensions, or
perhaps still less. No matter whether it is a ship, a locomotive, or a
flying machine that we wish to build, we must first of all consider the
ideal, and then approximate it as closely as possible with the material
at hand. Suppose it were possible to make a perfect screw, working
without friction, and that its weight should only be that of the
surrounding air; if it should be 200 feet in diameter, the power of one
man, properly applied, would lift him into the air. This is because the
area of a circle 200 feet in diameter is so great that the weight of a
man would not cause it to fall through the air at a velocity greater
than the man would be able to climb up a ladder. If the diameter should
be increased to 400 feet, then a man would be able to carry a passenger
as heavy as himself on his flying machine, and if we should increase it
still further, to 2,000 feet, the weight of a horse could be sustained
in still air by the power which one man could put forth. On the other
hand, if we should reduce the diameter of the screw to 20 feet, then it
would certainly require the power of one horse to lift the weight of one
man, and, if we made the screw small enough, it might even require the
power of 100 horses to lift the same weight. It will, therefore, be seen
that everything depends upon the area of the air engaged, and in
designing a machine we should seek to engage as much air as possible, so
long as we can keep down the weight. Suppose that a flying machine
should be equipped with a screw 10 feet in diameter, with a pitch of 6
feet, and that the motor developed 40 horse-power and gave the screw
1,000 turns a minute, producing a screw thrust, we will say, of about
220 lbs. If we should increase the diameter of the screw to 20 feet, and
if it had the same pitch and revolved at the same rate, it would require
four times as much power and would give four times as much screw thrust,
because the area of the disc increases as the square of the diameter.
Suppose, now, that we should reduce the pitch of the screw to 3 feet, we
should in this case engage four times as much air, and double the screw
thrust without using any more power--that is, assuming that the machine
is stationary and that the full power of the engine is being used for
accelerating the air. The advantages of a large screw will, therefore,
be obvious. I have been unable to obtain correct data regarding the
experiments which have taken place with the various machines on the
Continent. I have, however, seen these machines, and I should say when
they are in flight, providing that the engine develops 40 horse-power,
that fully 28 horse-power is lost in screw slip, and the remainder in
forcing the machine through the air. These machines weigh 1,000 lbs.
each, and their engines are said to be 50 horse-power. The lifting
effect, therefore, per horse-power is 20 lbs. If the aeroplanes were
perfect in shape and set at a proper angle, and the resistance of the
framework reduced to a minimum, the same lifting effect ought to be
produced with an expenditure of less than half this amount of power,
providing, of course, that the screw be of proper dimensions. It is said
that Professor Langley and Mr. Horatio Philipps, by eliminating the
factor of friction altogether, or by not considering it in their
calculations, have succeeded in lifting at the rate of 200 lbs. per
horse-power. The apparatus they employed was very small. The best I ever
did with my very much larger apparatus--and I only did it on one
occasion--was to carry 133 lbs. per horse-power. In my large machine
experiments, I was amazed at the tremendous amount of power necessary
to drive the framework and the numerous wires through the air. It
appeared to me, from these experiments, that the air resisted very
strongly being cut up by wires. I expected to raise my machine in the
air by using only 100 horse-power, and my first condenser was made so
that it did actually condense water enough to supply 100 horse-power,
but the framework offered such a tremendous resistance that I was
compelled to strengthen all of the parts, make the machine heavier, and
increase the boiler pressure and piston speed until I actually ran it up
to 362 horse-power. This, however, was not the indicated horse-power. It
was arrived at by multiplying the pitch of the screws, in feet, by the
number of turns that they made in a minute, and by the screw thrust in
pounds, and then dividing the product by the conventional unit 33,000. I
have no doubt that the indicated horse-power would have been fully 400.
On one occasion I ran my machine over the track with all the aeroplanes
removed. I knew what steam pressure was required to run my machine with
the aeroplanes in position at a speed of 40 miles an hour. With the
planes removed, it still required a rather high steam pressure to obtain
this velocity, but I made no note at the time of the exact difference.
It was not, however, by any means so great as one would have supposed.
From the foregoing, it will be seen how necessary it is to consider
atmospheric resistance. Although I do not expect that anyone will ever
again attempt to make a flying machine driven by a steam engine, still,
I have thought best to give a short and concise description of my engine
and boiler, in order that my readers may understand what sort of an
apparatus I employed to obtain the data I am now, for the first time,
placing before the public. A full description of everything relating to
the motor power was written down at the time, and has been carefully
preserved. An abridgement of this will be found in the Appendix.



CHAPTER II.

AIR CURRENTS AND THE FLIGHT OF BIRDS.


In Mr. Darwin’s “Voyage of the Beagle” I find:--

“When the condors are wheeling in a flock round and round any spot their
flight is beautiful. Except when rising from the ground, I do not
remember ever having seen one of these birds flap its wings. Near Lima I
watched several for nearly half an hour, without taking off my eyes;
they moved in large curves, sweeping in circles, descending and
ascending without giving a single flap. As they glided close over my
head I intently watched from an oblique position, the outlines of the
separate and great terminal feathers of each wing, and these separate
feathers, if there had been the least vibratory movement, would have
appeared as if blended together; but they were seen distinct against the
blue sky.”

Man is essentially a land animal, and it is quite possible if Nature had
not placed before him numerous examples of birds and insects that are
able to fly, he would never have thought of attempting it himself. But
birds are very much in evidence, and mankind from the very earliest
times has not only admired the ease and rapidity with which they are
able to move from place to place, but has always aspired to imitate
them. The number of attempts that have been made to solve this problem
has been very great; but it was not until quite recently that science
and mechanics had advanced far enough to put in the hands of
experimenters suitable material to attack the problem. Perhaps nothing
better has ever been written regarding our aspirations to imitate the
flight of birds than what Prof. Langley has said:--

“Nature has made her flying machine in the bird, which is nearly a
thousand times as heavy as the air its bulk displaces, and only those
who have tried to rival it know how inimitable her work is, for ‘the way
of a bird in the air’ remains as wonderful to us as it was to Solomon,
and the sight of the bird has constantly held this wonder before men’s
eyes, and in some men’s minds, and kept the flame of hope from utter
extinction, in spite of long disappointment. I well remember how, as a
child, when lying in a New England pasture, I watched a hawk soaring far
up in the blue, and sailing for a long time without any motion of its
wings, as though it needed no work to sustain it, but was kept up there
by some miracle. But, however sustained, I saw it sweep, in a few
seconds of its leisurely flight, over a distance that to me was
encumbered with every sort of obstacle, which did not exist for it. The
wall over which I had climbed when I left the road, the ravine I had
crossed, the patch of undergrowth through which I had pushed my way--all
these were nothing to the bird--and while the road had only taken me in
one direction, the bird’s level highway led everywhere, and opened the
way into every nook and corner of the landscape. How wonderfully easy,
too, was its flight. There was not a flutter of its pinions as it swept
over the field, in a motion which seemed as effortless as that of its
shadow.”

During the last 50 years a great deal has been said and written in
regard to the flight of birds; no other natural phenomenon has excited
so much interest and been so imperfectly understood. Learned treatises
have been written to prove that a bird is able to develop from ten to
twenty times as much power for its weight as other animals, while other
equally learned works have shown most conclusively that no greater
amount of energy is exerted by a bird in flying than by land animals in
running or jumping.

Prof. Langley, who was certainly a very clever observer and a
mathematician of the first order, in discussing the subject relating to
the power exerted by birds in flight and the old formula relating to the
subject, expresses himself as follows:--

“After many years and in mature life, I was brought to think of these
things again, and to ask myself whether the problem of artificial flight
was as hopeless and as absurd as it was then thought to be. Nature had
solved it, and why not man? Perhaps it was because he had begun at the
wrong end, and attempted to construct machines to fly before knowing the
principles on which flight rested. I turned for these principles to my
books and got no help. Sir Isaac Newton had indicated a rule for finding
the resistance to advance through the air, which seemed, if correct, to
call for enormous mechanical power, and a distinguished French
mathematician had given a formula showing how rapidly the power must
increase with the velocity of flight, and according to which a swallow,
to attain a speed it is known to reach, must be possessed of the
strength of a man.

“Remembering the effortless flight of the soaring bird, it seemed that
the first thing to do was to discard rules which led to such results,
and to commence new experiments, not to build a flying machine at once,
but to find the principles upon which one should be built; to find, for
instance, with certainty by direct trial how much horse-power was needed
to sustain a surface of given weight by means of its motion through the
air.”

There is no question but what a bird has a higher physical development,
as far as the generation of power is concerned, than any other animal we
know of. Nevertheless, I think that everyone who has made a study of the
question will agree that some animals, such as hares and rabbits, exert
quite as much power in running, in proportion to their weight, as a
sea-gull or an eagle does in flying.

The amount of power which a land animal has to exert is always a fixed
and definite quantity. If an animal weighing 100 lbs. has to ascend a
hill 100 feet high, it always means the development of 10,000 foot-lbs.
With a bird, however, there is no such thing as a fixed quantity. If a
bird weighing 100 lbs. should raise itself into the air 100 feet during
a perfect calm, the amount of energy developed would be 10,000 foot lbs.
plus the slip of the wings. But, as a matter of fact, the air in which a
bird flies is never stationary, as I propose to show; it is always
moving either up or down, and soaring birds, by a very delicate sense of
feeling, always take advantage of a rising column. If a bird finds
itself in a column of air which is descending, it is necessary for it to
work its wings very rapidly in order to prevent a descent to the earth.

I have often observed the flight of hawks and eagles. They seem to glide
through the air with hardly any movement of their wings. Sometimes,
however, they stop and hold themselves in a stationary position directly
over a certain spot, carefully watching something on the earth
immediately below. In such cases they often work their wings with great
rapidity, evidently expending an enormous amount of energy. When,
however, they cease to hover and commence to move again through the air,
they appear to keep themselves at the same height with an almost
imperceptible expenditure of power.

[Illustration: Fig. 3.--While in the Pyrenees I often observed eagles
balancing themselves on an ascending current of air produced by the wind
blowing over large masses of rock.]

Many unscientific observers of the flight of birds have imagined that a
wind or a _horizontal_ movement of the air is all that is necessary to
sustain the weight of a bird in the air after the manner of a kite. If,
however, the wind, which is only air in motion, should be blowing
everywhere at exactly the same velocity, and in the same
direction--horizontally--it would offer no more sustaining power to a
bird than a dead calm, because there is nothing to prevent the body of
the bird from being blown along with the air, and whenever it attained
the same velocity as the air, no possible arrangement of the wings could
prevent it from falling to the earth.

It is well known that only a short distance above the earth’s surface,
say 30 or 40 miles, we find an extremely low temperature sometimes
referred to as interstellar temperature or absolute zero. In order to
illustrate the extremely low temperature of space, I would cite the
following instance:--

One evening, in the State of Ohio, a farmer saw a very brilliant meteor;
it struck in one of his fields not more than 100 feet from his house. He
at once rushed to the spot, and, pushing his arm down the hole,
succeeded in touching it; but he very quickly withdrew his hand, as he
found it extremely hot. Some of the neighbours rushed to the spot, and
he told them what had occurred, whereupon one of them put his hand in
the hole, expecting to be burnt, but, much to his surprise, the tips of
his wet fingers were instantly frozen to the meteor. The meteor had been
travelling at such an exceedingly high velocity that the resistance of
the intensely cold and highly attenuated outer atmosphere was sufficient
to bring its temperature up to the melting point of iron; but the heat
did not have time to pass into the interior, it only extended inwards
perhaps 1/8 inch, so that when the meteor came to a state of rest, the
heat of the exterior was soon absorbed by the intensely cold interior,
thus reducing the surface to a temperature much below any natural
temperature that we find at the surface of the earth.

Nothing can be more certain than that the temperature is extremely low a
slight distance above the earth’s surface. As the air near the earth
never falls in temperature to anything like the absolute zero, it
follows that there is a constant change going on, the relatively warm
air near the surface of the earth always ascending, and, in some cases,
doing sufficient work in expanding to render a portion of the water it
contains visible, forming clouds, rain, or snow, while the very cold air
is constantly descending to take the place of the rising column of warm
air. I have noticed a considerable degree of regularity in the movement
of the air, especially at a long distance from land, where the
regularity of the up and down currents is, at times, very marked.

On one occasion while crossing the Atlantic in fine weather I noticed,
some miles directly ahead of the ship, a long line of glassy water.
Small waves indicated that the wind was blowing in the exact direction
in which the ship was moving, and as we approached the glassy line, the
waves became smaller and smaller until they completely disappeared in a
mirror-like surface, which was about 300 or 400 feet wide, and extended
both to the port and starboard in approximately a straight line as far
as the eye could reach. After passing the centre of this zone, I noticed
that small waves began to show themselves, but in the exact opposite
direction to those through which we had already passed, and these waves
became larger and larger for nearly half an hour. Then they began to get
gradually smaller, when I observed another glassy line directly ahead of
the ship. As we approached it, the waves again completely disappeared,
but after passing through it, the wind was blowing in the opposite
direction, and the waves increased in size exactly in the same manner
that they had diminished on the opposite side of the glassy streak (Fig.
4).

[Illustration: Fig. 4.--Air currents observed in mid Atlantic, warm air
ascending at _a_, _a_, _a_, and cold air descending at _b_, _b_, _b_.
_c_, _c_, _c_ represent the lines where the waves were the largest].

This, of course, shows that directly over the centre of the first
glassy streak, the air was meeting from both sides and ascending in
practically a straight line from the surface of the water, and then
spreading out high above the sea, setting up a light wind in both
directions.

I spent the winter of 1890-91 on the Riviera, between Hyères les
Palmiers and Monte Carlo. The weather for the most part was very fine,
and I often had the opportunity of observing the peculiar phenomena
which I had already noticed in the Atlantic, only on a much smaller
scale. Whereas, in the Atlantic, the glassy zones were from 8 to 15
miles apart, I often found them not more than 500 feet apart in the bays
of the Mediterranean. This was most noticeable at Antibes (Fig. 5), very
good photographs of which I obtained. It will be observed that the whole
surface of the water is streaked like a block of marble.

[Illustration: Fig. 5.--Glassy streaks showing the centres of ascending
and descending columns of air in the Bay of Antibes, Alpes Maritimes.]

At Nice and Monte Carlo this phenomena was also very marked. On one
occasion, while making observations from the highest part of the
promontory of Monaco on a perfectly calm day, I noticed that the whole
of the sea presented this peculiar effect as far as the eye could
reach, and that the lines which marked the descending air were never
more than 1,000 feet from those which marked the centre of the ascending
column. At about three o’clock one afternoon, a large black steamer
passed along the coast in a perfectly straight line, and its wake was at
once marked by a glassy line, which indicated the centre of an ascending
column. This line remained almost straight for two hours, when finally
it became crooked and broken. The heat of the steamer had been
sufficient to determine this upward current of air.

[Illustration: Fig. 6.--Air currents observed in the Mediterranean,
ascending currents at _a_, _a_, _a_, and descending currents at _b_,
_b_, _b_.]

In 1893 I spent two weeks in the Mediterranean, going and returning by a
slow steamer from Marseilles to Constantinople, and I had many
opportunities of observing the peculiar phenomena to which I have
referred. The steamer passed over thousands of square miles of calm sea,
the surface being only disturbed by large patches of small ripples (Fig.
6), separated from each other by glassy streaks, which, however, were
not straight as on the Atlantic, and I found that in no case was the
wind blowing in the same direction on both sides of these streaks, every
one of which indicated the centre of an ascending or a descending column
of air. If we should investigate these phenomena in what might be called
a dead calm, we should find that the air was rising very nearly
straight up over the centres of some of these streaks and descending in
a vertical line over the centres of others. But, as a matter of fact,
there is no such thing as a _dead calm_. The movement of the air is the
resultant of more than one force. The air is not only rising in some
places and descending in others, but at the same time, the whole mass is
moving forward with more or less rapidity from one part of the earth to
another, so we must consider that, instead of the air ascending directly
from the relatively hot surface of the earth and descending vertically
in other places, in reality the whole mass of rotating air is moving
horizontally at the same time.

Suppose that the local influence which causes the up and down motion of
the air should be sufficiently great to cause the air to rise at the
rate of 2 miles an hour, and that the wind at the same time should be
blowing at the rate of 10 miles an hour, the motion of the air would
then be the resultant of these two velocities. In other words, it would
be blowing up an incline of 1 in 5. Suppose, now, that a bird should be
able to so adjust its wings that it advanced 5 miles in falling 1 mile
through a perfectly calm atmosphere, it would then be able to sustain
itself in an inclined wind, such as I have described, without any
movement at all of its wings. If it were possible to adjust its wings in
such a manner that it could advance 6 miles by falling through 1 mile of
air, it would then be able to rise as relates to the earth while in
reality falling as relates to the surrounding air.

In conducting a series of experiments with artillery and small guns on a
large and level plain just out of Madrid, I often observed the same
phenomena, as relates to the wind, that I have already spoken of as
having observed at sea, except that the lines marking the centre of an
ascending or a descending column of air were not so stationary as they
were over the water. It was not an uncommon thing, when adjusting the
sights of a gun to fire at a target at a very long range, making due
allowances for the wind, to have the wind change and blow in the
opposite direction before the word of command was given to fire. While
conducting these experiments, I often noticed the flight of eagles. On
one occasion a pair of eagles came into sight on one side of the plain,
passed directly over our heads, and disappeared on the opposite side.
They were apparently always at the same height from the earth, and in
soaring completely across the plain they never once moved their wings.
These phenomena, I think, can only be accounted for on the hypothesis
that these birds were able to feel out with their wings an ascending
column of air, that the centre of this column of air was approximately a
straight line running completely across the plain, that they found
upward movement more than sufficient to sustain their weight in the air,
and that whereas, as relates to the earth, they were not falling at all,
they were in reality falling some 4 or 5 miles an hour in the air which
supported them.

Again, at Cadiz in Spain, when the wind was blowing in strongly from the
sea, I observed that the sea-gulls always took advantage of an ascending
column of air. As the wind rose to pass over the fortifications, the
gulls selected a place where they would glide on the ascending current
of air, keeping themselves always approximately in the same place
without any apparent exertion. When, however, they left this ascending
column, it was necessary for them to work their wings with great vigour
until they again found the proper place to encounter a favourable
current.

I have often noticed that gulls are able to follow a ship without any
apparent exertion; they simply balance themselves on an ascending column
of air, where they seem to be quite as much at ease as they would have
been roosting on a solid support. If, however, they are driven out of
this position, they generally commence at once to work their passage. If
anything is thrown overboard which is too heavy for them to lift, the
ship soon leaves them behind, and in order to catch up with it again
they move their wings very much as other birds do; but when once
established in the ascending column of air, they manage to keep up with
the ship by doing little or no work. In a calm or head wind we find them
directly aft of the ship; if the wind is from the port side they may
always be found on the starboard quarter, and _vice versâ_.

One Sunday morning, while living at Kensington, I noticed some very
curious atmospheric effects. The weather had been intensely cold for
about a week, when suddenly the atmosphere became warm and very humid.
The earth being much colder than the atmosphere, water was condensing on
everything that it touched. I went to the bridge over the Serpentine in
Hyde Park, and was not disappointed in finding a large number of
sea-gulls waiting about the bridge to be fed. On all ordinary occasions
these birds manage to move about with the expenditure of very little
energy, but on this occasion every one of them, without a single
exception, no matter in what position he might be, was working his
passage like any other bird, just as I had expected. It is only on very
rare occasions that the surface of the earth is sufficiently cold as
relates to the atmosphere to prevent all upward currents of air.

Everyone who has passed a winter on the northern shores of the
Mediterranean must have observed the cold wind which is generally called
the mistral. One may be out driving, the sun may be shining brightly,
and the air warm and balmy, when suddenly, without any apparent cause,
one finds himself in a cold descending wind. This is the much-dreaded
_mistral_, and if at sea it would be marked by a glassy line on the
surface of the water. On land, however, there is nothing to render its
presence visible. The ascending column of air is, of course, always very
much warmer than the descending column, and this is taking place in a
greater or lesser degree everywhere and at all times. A decided upward
trend of air is often encountered by those who are experimenting with
kites, the kite often mounting higher than can be accounted for on the
hypothesis that the wind is moving in a horizontal direction. I have
heard this discussed at considerable length. When a kite is flown in an
upward current, it behaves in many respects like a soaring bird.

From the foregoing, I think, we may safely draw the following
conclusions:--

First, that there is a constant interchange of air taking place, the
cold air descending, spreading itself out over the surface of the earth,
becoming warm, and ascending in other places.

Second, that the centres of the two columns are generally separated from
each other by a distance which may be from 500 feet to 20 miles.

Third, that the centres of greatest action are not in spots, but in
lines which may be approximately straight, but sometimes abound in many
sinuosities.

Fourth, that this action is constantly taking place over both the sea
and the land; that the soaring of birds, the phenomenon which has
heretofore been so little understood, may be accounted for on the
hypothesis that the bird seeks out an ascending column of air, and while
sustaining itself at the same height in the air, without any muscular
exertion, it is in reality falling at a considerable velocity through
the air that surrounds it.

It has been supposed by some scientists that birds may take advantage of
some vibratory or rolling action of the air. I find, however, from
careful observation and experiment, that the motion of the wind is
comparatively steady, and that the short vibratory or rolling action is
always very near to the earth and is produced by the air flowing over
hills, high buildings, trees, etc.

Tools and instruments used by mechanicians are very often made of the
material most used in their profession; for instance, a blacksmith’s
tools are generally of iron, a carpenter’s tools largely of wood, and a
glass-blower uses many things made of glass, and so on. Mathematicians
are no exception to this general rule, and seem to imagine that
everything can be accomplished by pure mathematical formulæ.

It appears that Prof. Langley was at times considerably puzzled by the
extraordinary behaviour of birds, and was led to believe that they took
advantage of some vibratory or oscillating movement of the air; he
called it “the internal work of the air.” I have been very much amused
in a recent mathematical work that I have read, in which the writer
seeks to solve all questions by pure mathematics. In this case,
notwithstanding that all of the factors are unknown and unknowable,
still, with the use of about two pages of closely written algebraic
formulæ, he appears to have solved the whole question. Just how he
arrived at it, however, is more than I am able to understand.

If a kite is flown only a few feet above the ground, it will be found
that the current of air is very unsteady. If it is allowed to mount to
500 feet the unsteadiness nearly all disappears, while if it is allowed
to mount further to a height of 1,500 or 2,000 feet, the pull on the
cord is almost constant, and, if the kite is well made, it remains
practically stationary in the air.

I have often noticed in high winds that light and fleecy clouds come
into view, say, about 2,000 feet above the surface of the earth, and
pass rapidly and steadily by preserving their shape completely. This
would certainly indicate that there is no rapid local disturbance in
the air in their immediate vicinity, but that the whole mass of air in
which these clouds are formed is practically travelling in the same
direction and at the same velocity. Numerous aeronauts have also
testified that, no matter how hard the wind may be blowing, the balloon
is always practically in a dead calm, and if a piece of gold-leaf is
thrown overboard, even in a gale, the gold-leaf and the balloon never
part company in a horizontal direction, though they may in a vertical
direction.

Birds may be divided into two classes. First, the soaring birds, which
practically live upon the wing, and, by some very delicate sense of
touch, are able to feel the exact condition of the air. Many fish which
live near the top of the water are greatly distressed by sinking too
deeply, while others which live at great depths are almost instantly
killed by being raised to the surface. The swim-bladder of a fish is in
reality a delicate barometer provided with sensitive nerves which enable
the fish to feel whether it is sinking or rising in the water. With the
surface fish, if the pressure becomes too great, it involuntarily exerts
itself to rise nearer the surface and so diminish the pressure, and I
have no doubt that the air cells, which are known to be very numerous
and to abound throughout the bodies of birds, are so sensitive as to
enable soaring birds to know at once whether they are in an ascending or
a descending column of air.

The other class of birds consists of those which only employ their wings
occasionally for the purpose of taking them rapidly from one place to
another. Such birds do not expend their power so economically as the
soaring birds. They do not pass much of their time in the air, but what
time they are on the wing they put forth an immense amount of power and
fly very rapidly, generally in a straight line, taking no advantage of
air currents. Partridges, pheasants, wild ducks, geese, and some birds
of passage may be taken as types of this kind. This class of birds has
relatively small wings, and carries about two and a half times as much
weight per square foot of surface as soaring birds do.

We shall never be able to imitate the flight of the soaring birds. We
cannot hope to make a sensitive apparatus that will work quick enough to
take advantage of the rising currents of air, and he who seeks to fly
has this problem to deal with. A successful flying machine, moving at a
high velocity, is likely at any time to encounter downward currents of
air, which will greatly interfere with its action. Therefore flying
machines must, in the very nature of things, be provided with sufficient
power to propel them through various currents of air, after the manner
of ducks, partridges, pheasants, etc.

  +------------------+-------+--------+-----------------+
  |                  |       |        | Corresponding   |
  |Common Name.      |Sq. Ft.|Lbs. per|Speed for a Plane|
  |                  |per Lb.|Sq. Ft. | at 3° in Miles  |
  |                  |       |        |    per Hour.    |
  +------------------+-------+--------+-----------------+
  |Bat,              | 7·64  |  0·131 |      15·9       |
  |Swallow,          | 3·62  |  0·276 |      23·1       |
  |Lark,             | 3·06  |  0·327 |      25·1       |
  |Sparrow hawk,     | 3·00  |  0·333 |      25·3       |
  |Sparrow,          | 2·42  |  0·414 |      28·2       |
  |Gull,             | 2·35  |  0·426 |      28·6       |
  |Owl,              | 2·26  |  0·443 |      29·2       |
  |Crane,            | 2·02  |  0·495 |      30·9       |
  |Rook,             | 1·74  |  0·575 |      33·3       |
  |Plover,           | 1·38  |  0·725 |      37·4       |
  |Balbuzzard,       | 1·26  |  0·795 |      39·2       |
  |Egyptian vulture, | 1·18  |  0·848 |      40·4       |
  |Duck,             | 0·864 |  1·158 |      44·2       |
  |Grey pelican,     | 0·732 |  1·365 |      51·3       |
  |Wild goose,       | 0·586 |  1·708 |      57·4       |
  |Turkey,           | 0·523 |  1·910 |      60·6       |
  |Duck (female),    | 0·498 |  2·008 |      62·2       |
  |  „  (male),      | 0·439 |  2·280 |      66·2       |
  +------------------+-------+--------+-----------------+



CHAPTER III.

FLYING OF KITES.


It was said of Benjamin Franklin that when he wished to fly a kite in
order to ascertain if lightning could be drawn down from the clouds, he
managed to have a boy with him in order to avoid ridicule. It was
considered too frivolous in those days for grown-up men to amuse
themselves with kites, and a good many besides Benjamin Franklin have
feared to face the ridicule that was inevitable if they took up or even
discussed the question of artificial flight. Nineteen years ago, when I
commenced my own experiments, I was told that my reputation would be
greatly injured, that mankind looked upon artificial flight as an
_ignis-fatuus_, and that anyone who experimented in that direction was
placed in the same category as those who sought to make perpetual-motion
machines or to find the philosopher’s stone. Although I had little fear
of ridicule, still I kept things as quiet as I could for a considerable
time, and I had been working fully six months before anyone ascertained
what I was doing. When, however, it became known that I was
experimenting with a view of building a flying machine, the public
seemed to think that I was making honest and praiseworthy scientific
investigations; true, I might not succeed, still it was said that I
would accomplish something, and find out some of the laws relating to
the subject. No one ridiculed my work except two individuals, and both
of these were men whom I had greatly benefited. As is often the case,
those whom you find in difficulties and place on their feet seek to do
you some injury as compensation for the benefits they have received.

At the present time it is not necessary for any man to take a small boy
with him as a species of lightning-rod to ward off ridicule when he
flies a kite. I have been one of a committee on kite-flying at which
some of the most learned and serious men in England were my colleagues
in investigating the subject. The behaviour of kites is certainly very
puzzling to those who do not thoroughly understand the subject. A kite
may be made with the greatest degree of perfection, and placed in the
hands of one of considerable experience; nevertheless, it may behave
very badly, diving suddenly to the ground without any apparent cause.
Then, again, this same kite will sometimes steadily mount in the air
until it reaches a height difficult to account for. If the surface of
the earth should be perfectly smooth, and the wind should always blow in
a horizontal direction, kites would not show these eccentric
peculiarities, but, as a matter of fact, the air seldom moves in a
horizontal direction; it is always influenced by the heat of the surface
of the earth. Heated air is continually ascending in some places only to
be cooled and to descend in other places. If one is attempting to fly a
kite where the air is moving downwards, he will find it an extremely
difficult matter, whereas, if he is fortunate enough to strike a current
of air which is rising, the kite will mount much higher in the air than
can be accounted for, except we admit of the existence of these upward
draughts of air. On one occasion many years ago, I was present when a
bonded warehouse in New York containing 10,000 barrels of alcohol was
burnt. It was nine o’clock at night, and I walked completely around the
fire, and found things just as I had expected. The wind was blowing a
perfect hurricane through every street in the direction of the fire,
although it was a dead calm everywhere else; the flames mounted straight
in the air to an enormous height, and took with them a large amount of
burning wood. When I was fully 500 feet from the fire, a piece of partly
burnt 1-inch board, about 8 inches wide and 4 feet long, fell through
the air and landed very near me, sending sparks in every direction. This
board had evidently been taken up to a great height by the tremendous
uprush of air caused by the burning alcohol. It is very evident that a
kite made of boiler iron could have been successfully flown under these
conditions providing that it could have been brought into the right
position.

[Illustration: Fig. 7.--The circulation of air produced by a difference
in temperature.]

The sketch (Fig. 7) shows a device consisting of a spirit lamp and a box
of ice. The lamp heats the metallic plate, expands the air which rises
and is cooled by convection on coming in contact with the top plate, and
descends as shown. However, a fire is not necessary to accomplish this
result; it is taking place all over the earth, all the time. A great
number of plants depend upon a rising current of air to transport their
seeds to distant places. Seeds of the thistle and dandelion variety are
sometimes able to travel hundreds of miles, to the great vexation of
farmers; and there is a certain class of small spider known as “Balloon
Spiders” which also depend upon a rising current of air to carry them
from the place of their birth to some distant part where they, of
course, hope to start a colony. When I was a boy of eight, I noticed
small spiders webbing down from the sky. I was greatly puzzled; it
appeared to me that they had attached their web to some stationary
object high in the air and were spinning a web in order to lower
themselves to the earth. What could that stationary object be? As the
sky was clear, I was quite unable to understand this phenomenon, but
afterwards I learned from scientific books that there was a class of
spiders that managed to rise high in the air by the aid of the wind. It
appears that they climb a high tree until they have reached the
uppermost extremity and then, from a leaf or twig that projects into the
air, they wait for an ascending current of air. Although the spider is
exceedingly small--the size of a pin’s head--it has about 200
spinnerets, its ordinary web being formed of no less than that number of
extremely fine threads. These are spun out singly into the air until an
almost invisible mass of fine webs interlacing each other in all
directions and forming an approximately cylindrical network about half
an inch in diameter and 18 inches long is produced. Whenever an upward
draft of air approximately vertical occurs, it takes this weightless
tangle of fine webs with it, and so soon as the spider finds there is
sufficient pull to lift its weight, it lets go and ascends with the air.
When the Nulli Secundus ascended at Farnborough and landed at the
Crystal Palace, Mr. Cody, who was on board, reported what he supposed to
be a very curious and unaccountable phenomenon. The balloon was covered
with many thousands of minute spiders that it had picked up in the air
on the voyage. Certainly this of itself is very strong evidence of the
existence of these ascending currents of air.

[Illustration: Fig. 8.--_a_ represents a kite in a horizontal wind, _e_,
_e_, _e_; _b_, the same kite in a rising column of air, the wind blowing
in the direction shown at _f_, _f_. If the kite is a good one, it may
pass over to the point _c_.]

When in Boston about fifteen years ago, I went to Blue Hill to witness
the remarkable kite flying which was taking place at that time. The
kites experimented with were of the Hargrave type, and of enormous
dimensions. A steel wire and windlass worked by a steam engine was
employed. I was told that on certain occasions the kites mounted
extremely high, much higher than they were able to account for; but on
this particular occasion, although they let out a great amount of wire,
the kite did not mount very high. I have heard much discussion first and
last regarding the flight of kites, and I think it is generally admitted
that they do sometimes rise upwards and continue moving to the windward
until they pass directly over the spot where they are attached to the
earth. It was not, however, till about three years ago that I witnessed
this phenomenon myself. Mr. Cody, who is the inventor of a very good
kite, had been flying kites at the Crystal Palace for some months, and
on one occasion I saw his kite rise, pass to the windward and directly
over our heads. I took hold of the cord with both hands, and was
somewhat surprised to find what the lifting effect was. The kite was,
however, of large dimensions, but by no means so large as Mr. Cody’s
“man-lifting kites.” In the drawing (Fig. 8) I have shown, at _a_, the
action of a kite in a horizontal wind, lines _e_, _e_, showing the
direction of the wind. A good kite will easily mount 45°, the angle
shown, but on the occasion just mentioned, the sun had been shining
brightly into the valley where the experiments took place, and an
upward current of air had been determined. The cooler air was, of
course, rushing in from each side and mounting in about the centre of
the valley, and Mr. Cody’s kite, instead of flying in a horizontal wind,
soon reached a point where the wind was ascending at an angle, as shown
at _f_, _f_. The kite would therefore mount until at _b_, where it
presented the same angle to the wind as with the horizontal wind at _a_,
and if it should be made to fly at a higher angle, it might pass over to
the position shown at _c_. But it must not be imagined that this
phenomenon can be witnessed every day in the year. It is only on rare
occasions that one is fortunate enough to find a wind which is blowing
at a sufficiently sharp upward trend to cause a kite to pass to the
windward over the point of support. Neither must it be supposed that
this favourable condition of things is of long duration. As the centre
of the upward current is constantly moving, it is certain that very soon
it will move away from the point from which the kite is being flown.
What is true of kites is also true of flying machines. It is very
difficult indeed to make a kite mount providing that it is in a
descending current of air, and one is just as likely to find a
descending current as any other. Flying machines will, therefore, have
to be made with a considerable amount of reserve energy, so as to be
able to put on a spurt when they encounter an adverse current. If a
machine is made that is able to maintain itself in the air for any
considerable length of time, it will not be a very difficult task to
know when a current of air of this kind is encountered, because, if the
engine is working up to speed, and everything is in perfect order, and
still the machine is falling, it is very certain that an unfavourable
current has been encountered, and efforts should be made to get out of
it as soon as possible. Then, again, if the machine has an abnormal
tendency to rise without any increase in the number of rotations made by
the screws, the aeronaut may be certain that he has encountered an
upward and favourable current of air which, unfortunately, will not
last. It should, however, be borne in mind that, while the width of the
upward current is not very great, nevertheless, it may extend in a
practically straight line for many miles.



CHAPTER IV.

PRINCIPALLY RELATING TO SCREWS.


In 1887 I was approached by several wealthy gentlemen who asked me if I
thought it was possible to make a flying machine. I said, “certainly;
the domestic goose is able to fly and why should not man be able to do
as well as a goose?” They then asked me what it would cost and how long
it would take, and, without a moment’s hesitation, I said it would
require my undivided attention for five years and might cost £100,000. A
great deal of experimenting would be necessary; the first three years
would be devoted to developing an internal combustion engine of the
Brayton or Otto type, and the next two years to experimenting with
aeroplanes and screws and building a machine. Even at that time I had a
clear idea of the system that would be the best. However, nothing was
then done, but in 1889 I employed for the purpose two very skilful
American mechanics, and put them to work at Baldwyn’s Park, Kent. At
that time the petroleum motor had not been reduced to its present degree
of efficiency and lightness; it was not suitable for a flying machine,
and I saw that it would require a lot of experimental work in order to
develop it. After taking into consideration all the facts of the case, I
decided to use a steam engine. Had I been able to obtain the light and
efficient motors which have been recently developed, thanks to the
builders of racing cars, I should not have had to experiment at all with
engines and boilers, as I could have obtained the necessary motors at
once. As it was, I was obliged to content myself with the steam engine.

[Illustration: Fig. 9.--Group of screws and other objects used in my
experiments.]

[Illustration: Fig. 10.--Some of the principal screws experimented
with--_h_, a screw with very thick blades, and _g_, a screw made after a
French model.]

I found that there was a great deal of misunderstanding regarding the
action of aeroplanes, and also of screws working in the air. I procured
all the literature available on the subject, both English and French,
and attempted to make a thorough study of the question; but I was not
satisfied, on account of the wide difference in the views of the writers
and the conflicting formulæ that were employed. I therefore decided to
make experiments myself, and to ascertain what could be done without
the use of anybody’s formula. Although this was nearly twenty years ago,
I find that there is still a great deal of discussion regarding the
action of aeroplanes and screws, in which the majority taking part in
the discussion are in the wrong. However, several good works on the
subject have recently been published.

[Illustration: Fig. 11.--The three best screws. The screw on the right
has a uniform pitch throughout, the middle screw has increasing pitch,
and the left screw compound increasing pitch.]

Having designed and put my boiler and engine in hand, I commenced a
series of experiments for the purpose of ascertaining the efficiency of
screw propellers working in the air, and the form and size that would be
best for my proposed machine. The illustration Fig. 9 shows a
photographic group of the screws and other objects with which I
experimented. Fig. 10 shows some of the leading types which, as will be
seen, have blades of different shape, pitch, and size. Fig. 11 shows
three of the best screws employed. It will be observed that one has
uniform pitch, another increasing pitch, and the third compound
increasing pitch. In order to test the efficiency of my screws I made
the apparatus shown in Fig. 12. The power for running the screw was
transmitted by means of a belt to the straight cylindrical pulley _c_,
_c_. Shaft _b_, _b_ was of steel, rather small in diameter, and ran
smoothly, and practically without friction, through the two bearings
_d_, _d_. When the first screw, _a_, _a_, was run at a high velocity,
the axial thrust pushed the shaft _b_, _b_ back and elongated the spiral
spring _e_. The degree of screw thrust was indicated in pounds by the
pointer _g_. The power was transmitted through a very accurate and
sensitive dynamometer, so that the amount consumed could be easily
observed by a pointer similar to the one employed for indicating the
screw thrust. A tachometer was also employed to observe the number of
turns that the screw was making in a minute. The whole apparatus was
carefully and accurately made and worked exceedingly well. I was thus
enabled, with my various forms of screws and other objects, to make very
accurate measurements, some of which are exceedingly interesting.

[Illustration: Fig. 12.--Apparatus for testing the thrust of
screws--_a_, _a_, the screw; _b_, _b_, shaft sliding freely in the
bearing _d_, _d_; _c_, cylindrical pulley; _e_, spiral spring; _f_,
steel rod; _g_, pointer for indicating the thrust in pounds.]

[Illustration: Fig. 13.--Apparatus for testing the direction of air
currents caused by a rapidly rotating screw. Silken threads were
attached to the wire _c_, _c_, which indicated clearly the direction in
which the air was moving.]

In many of the treatises and books of that time it was stated that a
screw propeller, working in the air, was exceedingly wasteful of energy
on account of producing a fan-blower action. Some inventors suggested
that the screw should work in a stationary cylinder, or, better still,
that the whole screw should be encased in a rotating cylinder, to
prevent this outward motion of the air. In order to ascertain what the
actual facts were, I attached a large number of red silk threads to a
brass wire, which I placed completely around my screw (see Fig. 13).
Upon starting up I found that, instead of the air being blown out at the
periphery of the screw, it was in reality sucked in, as will be seen in
the illustration. I was rather surprised to see how sharp a line of
demarkation there was between the air that was moving in the direction
of the screw and the air that was moving in the opposite direction. The
screw employed in these experiments was 18 inches in diameter and had a
pitch of 24 inches. It was evident, however, if the pitch of the screw
was coarse enough that there would be a fan-blower action. I therefore
tried screws of various degrees of pitch, and found when the pitch was a
little more than three times the diameter, giving to the outer end of
the blade an angle of 45°, that a fan-blower action was produced--that
is, part of the time when the screw was running, the air would
alternate; sometimes it would pass inwards at the periphery and
sometimes outwards. The change of direction, however, was always
indicated by a difference in the pitch of the note given out, and also
by the thrust. In Fig. 14 I have shown the extremities of the blades of
some of the different forms of screws experimented with, in which _a_
shows a plain screw, the front side being straight and of equal pitch
from the periphery to the hub; _b_ is a screw of practically the same
pitch, but slightly curved so as to give what is known as an increasing
pitch; _c_ shows the extremity of a screw in which the curve is not the
same throughout--that is, it is what is known as a compound increasing
pitch; _d_ is the shape of the screw that gave the angle of 45° above
referred to.

[Illustration: Fig. 14.--This drawing shows the ends of screw blades in
which _a_ is a plain screw; _b_, screw with increasing pitch; _c_, screw
with compound increasing pitch; _d_, end of screw blade 45°; _e_, screw
with very thick blade; _f_, blade with no pitch at all; _g_, blade which
gave a thrust in the direction of the convex side, no matter in which
direction it was revolved; _h_, screw said to have been used in the
French Government experiments.]

The first screw experimented with was _a_. This screw was run at a high
velocity--about 2,500 revolutions per minute--until a screw thrust of 14
lbs. was obtained, and then the governor of the engine was set so that
all screws of the same diameter could be run at the same speed. Wishing
to ascertain the efficiency of the screw and how much was lost in skin
friction, I multiplied the thrust in pounds by the pitch of the screw in
feet and by the number of turns it was making in a minute. This, of
course, gave the exact number of foot-pounds in energy that was being
imparted to the air. I was somewhat surprised to find that it
corresponded exactly with the readings of the dynamometer. I thought at
first that I must have made some mistake. Again I went very carefully
over all the figures, tested everything, and made another experiment and
found, even if I changed the number of revolutions, that the readings of
the dynamometer were always exactly the same as the energy imparted to
the air. This seemed to indicate that the screw was working very well
and that the skin friction must be very small indeed. In order to test
this, I made what we will call, for the moment, a screw without any
pitch at all--that is, the blades were of wood and of the exact
thickness and width of the blades of the screw _a_, but without any
pitch at all. The extremity of the blade is shown at _f_. I placed this
screw on my machine in place of _a_, and although my dynamometer was so
sensitive that the pointer would move away from the zero pin by simply
touching the tip of the finger to the shaft, it failed to indicate, and
thus the screw appeared to consume no power at all. These experiments
were repeated a considerable number of times. I then obtained a sheet of
tin the same diameter as the screws, 18 inches, and upon running it at
the same speed, I found that it did consume a measurable amount of
power, certainly more than the two blades _f_. This no doubt was due to
the uneven surface of the tin. Had it been a well-made saw blade without
teeth, perfectly smooth and true on both sides, it probably would not
have required power enough to have shown on the dynamometer. However, it
is quite possible that there is a little more skin friction with a
polished metallic surface, than with a piece of smooth evenly lacquered
wood. The screws which I employed were of American white pine such as
used by patternmakers. This wood was free from blemishes of all kind,
extremely light, uniform, and strong. When the screw had been formed, it
was varnished on both sides with a solution of hot glue, which greatly
increased the strength of the wood crosswise of the grain. When this
glue was thoroughly dry, the wood was sand-papered until it was as
smooth as glass; the whole thing was then carefully varnished with
shellac, rubbed down again and revarnished with very thin shellac
something like lacquer. In this way the surface of the screw was made
very smooth. The screws, of course, were made with a great degree of
accuracy and as free as possible from any unevenness. Having tested
screw _a_, I next tested screw _b_. I found with the same number of
revolutions per minute that this screw produced more thrust, but it
required more power to run it, and when the energy imparted to the air
was compared with the readings of the dynamometer, it was found that it
did not do quite so well as _a_; still as the thrust was greater and the
efficiency only slightly less, it appeared to be the better screw. Upon
trying screw _c_, under the same conditions, the thrust was very much
increased, but the power required was also increased to a still greater
degree, showing that this form was not so favourable as either _a_ or
_b_. All the screws experimented with had very thin blades, and it
occurred to me that the difference between _a_ and _b_ might arise from
the fact that, when _a_ was running at a very high velocity, the working
side instead of being flat might have become convex to a slight extent,
whereas with _b_, a slight bending back of the edges of the blade would
still leave the working side concave. I therefore made the screw shown
at _e_, which had the same pitch as the other three, but the working
side was of the same shape as _a_. Of course the additional thickness of
the blades made it impossible to give an easy curve to the back.
Curiously enough I found that _e_ did nearly as well as _a_, and quite
as well as _b_. The additional thickness did not interfere to any
appreciable extent with its efficiency. I then made another propeller,
shown at _g_, which was of the same thickness in the middle as _e_. Upon
running this, I found that it required considerable power, and no matter
which way it was run, the thrust was always in the direction of the
convex side, which was quite the reverse from what one would have
naturally supposed.

[Illustration: Fig. 15.--The manner of building up the large screws.]

About the time that I was making these experiments, my duties called me
to Paris, and while there I called on my old friend Gaston Tissandier.
Through his influence I was permitted to see some models of the screws
that were alleged to have been used by Captain Renard in his experiments
for the French Government, and I was somewhat surprised to find the form
of the blades, the same as shown at _h_, Fig. 14, and completely without
any twist. On my return to England, I made a screw of this description.
It is also shown in the photographic illustration, Fig. 9. Upon testing
this screw, I found that its efficiency was only 40 per cent. of that of
_a_--that is, the energy or acceleration imparted to the air was only 40
per cent. of the readings of the dynamometer. It then occurred to me
that this particular form of screw was probably the one that the French
had for exhibition purposes, but not the one they intended to use.
Having tried all the various forms of screws and other objects shown in
Fig. 9, I made some sheet metal screws; also a screw which consisted of
a steel frame covered with woven fabric, and which was identical with
screws that I had seen described in various works relating to aerial
navigation. It was found quite impossible to keep the fabric taut and
smooth, and the results were very bad indeed, it being only 40 per cent.
as efficient as a well-made wooden screw.

[Illustration: Fig. 16.--A fabric-covered screw with a very low
efficiency.]

Having thus ascertained the best form of a screw, I built up my first
large screws, which were 17 feet 10 inches in diameter, after the
well-known manner of making wooden patterns for casting steamship
propellers. Fig. 15 shows the form of the end of the blade, the middle
of the blade, and the hub. My first pair of large screws had a pitch of
24 feet, but these were too great a drag on the engine. I therefore made
another pair with 16 feet pitch which greatly increased the piston
speed, and permitted the engines to develop much more power; the screw
thrust was also increased just in an inverse ratio to the pitch of the
screws. Another pair of screws was tried with 14 feet pitch and 12 feet
in diameter, but these did not do so well. My large screws were made
with a great degree of accuracy; they were perfectly smooth and even on
both sides, the blades being thin and held in position by a strip of
rigid wood on the back of the blade. In order to prevent the thrust from
collapsing the blades, wires were extended backwards and attached to a
prolongation of the shaft. Like the small screws, they were made of the
very best kind of seasoned American white pine, and when finished were
varnished on both sides with hot glue. When this was thoroughly dry,
they were sand-papered again and made perfectly smooth and even. The
blades were then covered with strong Irish linen fabric of the smoothest
and best make. Glue was used for attaching the fabric, and when dry
another coat of glue was applied, the surface rubbed down again and then
painted with zinc white in the ordinary way and varnished. These screws
worked exceedingly well. I had means of ascertaining, with a great
degree of accuracy, the thrust of the screw, the number of turns per
minute, the speed of the machine, and, in fact, all the events that were
taking place on the machine. It was found that when the screw thrust in
pounds was multiplied by the pitch in feet, and by the number of
revolutions made in a minute of time, it exactly corresponded to the
power that the engines were developing, and that the amount of loss in
skin friction was so small as to be practically negligible.

[Illustration: Fig. 17.--The hub and one of the blades of the screw on
the Farman machine. The blade _c_, is a sheet of metal riveted to the
rod _b_, and forms a projection on the back of the blade which greatly
reduces its efficiency. The peculiar form of hub employed makes it
possible to change the diameter and pitch of this screw at will.]

In connection with this subject I would say that many experimenters
claim to have shown that the skin friction on screws is considerable, in
fact, so great as to be a very important factor in the equation of
flight. I am, however, of the opinion that these experimenters have not
had well-made screws. If the surface of the screw is uneven, irregular,
or rough, a considerable amount of energy is lost, as shown in the
French screw and the fabric covered screw. It is simply a question of
having a screw well-made. In those recently employed in France (see Fig.
17), the blades are of hammered sheet metal, the twist is not uniform or
true, and what is worst of all, the arm _b_ projects on the back of the
blade and offers a good deal of resistance to the air. This form of
screw, however, is very ingenious; as will be seen by the drawing, the
pitch and diameter can be changed at will. It is, however, heavy,
wasteful of power, and altogether too small for the work it has to do.
The skin friction of screws in a steamship has led inventors to suppose
that the same laws relate to screws running in air, but such is by no
means the case. In designing a steamship, we have to make a compromise
in regard to the size of the screw. If the screw is too small, an
increase in diameter is, of course, an advantage, and it may also be an
advantage, not only to increase the diameter, but also to reduce the
pitch; however, a point is soon reached where the skin friction will
more than neutralise the advantages of engaging a larger volume of
water. This is because the water adheres to the surface; in fact, the
skin friction of a ship and its screw consumes fully 80 per cent. of the
total power of the engines, but with an air propeller its surface is not
wetted and the air does not stick to its surface. If made of polished
wood, the friction is so extremely small as to be almost unmeasurable.
The diameter of a well-made screw running in air is therefore not
limited in any degree by skin friction, as is the case with a screw
running in water; in fact, it is rather a question of its weight, and
its efficiency ought to increase in direct ratio with its diameter,
because the area of the disc increases with the square of the diameter.
The screw slip is therefore reduced by one-half by simply doubling the
diameter of the screw. It will be understood that by doubling the
diameter of the screw, four times as much air will be engaged. If we
push this back at half the speed, we shall have the same screw thrust,
because the resistance of the air is in proportion to the square of the
velocity that we impart to it, so that one just balances the other, and
the diminution of wasteful slip is just in proportion to the increase in
diameter. In all cases, the screw should be made as large as possible.

[Illustration: Fig. 18.--Section of screw blades having radial edges.
With screws of this form, the blades, of course, become narrower as the
hub is approached, and if it is a true screw and the edges radial, the
sine of the angle will be the same at all points. It is 2 inches in this
case.]

[Illustration: Fig. 19.--Shows the form of the blade of a screw
propeller made of sheet metal. It is riveted at the edges and also to
the arm of a screw with a stiffening piece at the extreme end. However,
it is not necessary to rivet edges together. They may be welded with a
name of acetylene oxygen gases.]

[Illustration: Fig. 19_a_.--Shows the manner of welding and the finished
edge.]

[Illustration: Fig. 20.--A new form of hub, of great strength and
lightness, for use on flying machines.]

In the drawing (Fig. 18) I have shown screw blades of a proper shape to
give the best results--that is, providing a metallic screw is employed.
Instead of having the arm of the screw on the back of the blade to offer
resistance to the air, the arm should be tubular, flattened, and covered
on both sides with sheet metal. This particular formation not only
prevents the air from striking the arm, but, at the same time, prevents
the pressure of the air from deforming the blade, so, if a metallic
screw is to be used, the form of blade which I have shown will be found
much superior to that employed at the present time on continental flying
machines. We should not lose sight of the fact that weight tells very
seriously against the success of a flying machine, and that no expense
should be spared to reduce the weight, providing that it is possible to
do so without reducing the factor of safety. Suppose, for example, that
we use an ordinary hub secured to a solid shaft by a common key. All the
parts have to be made heavy in order to be sufficiently strong to
withstand the strain. In the drawing (Fig. 20) I have shown a hub which
I think is quite as light and strong as it is possible to make it. The
action of the motor is often spasmodic and puts very great strain upon
the parts, and there is a very strong tendency for the shaft to turn
round in the hub. If a key is used, the hub has to be large and strong,
and the key of considerable size, otherwise the parts would be deformed.
In my own experiments, I have found considerable difficulty in securing
a shaft to wooden screws. However, it will be seen in the drawings that
a series of grooves is cut in the shaft and that the hub has internal
projections, so that the one fits the other. This makes a very strong
connection and is of extreme lightness. Both the hub and the shaft
should be of tempered steel. The spokes should be hard drawn steel tubes
with long fine threads, so as to withstand centrifugal force. To prevent
them from rotating in the hub, the nuts _d_, _d_ are provided, which
compress the arms of the steel hub so as to grip the tube with any
degree of force required. It will be seen that with this system the
pitch of the screw may be adjusted to some extent; however, it is better
to have all parts of the screw, from hub to centre, of the same pitch. A
slight deviation from this is admissible in the experimental stage, so
long as the deviation from a true screw, caused by rotating the arm, is
not greater than one half of the slip while in flight.

Many experimenters have imagined that a screw is just as efficient
placed in front of a machine as at the rear, and it is quite probable
that, in the early days of steamships, a similar state of things
existed. For several years there were steamboats running on the Hudson
River, New York, with screws at their bows instead of at their stern.
Inventors of, and experimenters with, flying machines are not at all
agreed by any means in regard to the best position for the screw. It
would appear that many, having noticed that a horse-propelled carriage
always has the horse attached to the front, and that the carriage is
drawn instead of pushed, have come to the conclusion that, in a flying
machine, the screw ought, in the very nature of things, to be attached
to the front of the machine, so as to draw it through the air. Railway
trains have their propelling power in front, and why should it not be
the same with flying machines? But this is very bad reasoning. There is
but one place for the screw, and that is in the immediate wake, and in
the centre of the greatest atmospheric disturbance. While a machine is
running, although there is a marked difference between water and air as
far as skin friction is concerned, still the conditions are the same as
far as the _position_ of the screw is concerned. With a well-designed
steamship, the efficiency of the screw is so great as to be almost
unbelievable; in fact, if a steamship had never been made, and the
design of one should be placed before the leading mathematicians of
to-day, with the request that they should compute the efficiency of the
screw, none of them would come anywhere near the mark. They would make
it altogether too small. As before stated, when a steamship is being
driven through the water, the water adheres to its sides and is moved
forward by the ship--that is, it has acceleration imparted to it which
exactly corresponds to the power consumed in driving the ship through
the water. This, of course, retards it and we find in a well-designed
ship, not run above its natural speed, that about 80 per cent. of the
power of the engine is consumed in skin friction, or in imparting a
forward motion to the water. Suppose that we should take such a ship,
remove the screw, and tow it through the water with a very long wire
rope at a speed of, say, 20 miles an hour; we should find that the water
at the stern of the ship was moving forward at a velocity of fully 6
miles an hour--that is, travelling in the same direction as the ship. By
replacing the screw, and applying engine power sufficient to give the
ship the same speed of 20 miles an hour, identical results would be
produced. The skin friction still impels the water forward, so that the
screw, instead of running in stationary water, is actually running in
water moving in the same direction as the ship at a velocity of 6 miles
an hour. If the slip of the screw should only be equal to this forward
motion, the apparent slip would be nothing; in fact, the ship would be
moving just as fast as it would move if the screw were running in a
solid nut instead of in the yielding water. Curiously enough there have
been cases of negative slip in which the actual slip of the screw in the
water was less than the forward movement of the water, and in such cases
a ship is said to have negative slip. A very noticeable case of this
kind occurred in the Royal Navy in the sixties.[1] I was at the time
engaged in a large shipbuilding establishment in New York, and remember
distinctly the interest that the case created amongst the draughtsmen
and engineers of that establishment. Of course, this apparently
impossible phenomenon created a great deal of discussion on both sides
of the Atlantic. It appears that this ship had been built under an
Admiralty Specification which called for a screw of a certain diameter
and pitch with a specified number of revolutions per minute, and for a
certain number of knots per hour, also that the boiler pressure should
not go above a certain number of pounds per square inch. When the ship
was finished and went on her trial trip, it was found impossible to make
the full number of turns called for in the specification with the boiler
pressure allowable; nevertheless, the speed was greater than the
specification called for, and as speed was the desideratum, and not the
number of revolutions, the contractors thought their ship should be
accepted. Then arose a discussion as to the diameter and pitch of the
screw. It was claimed that a mistake must have occurred. A careful
measurement was made in the dry dock, and all was found correct. Once
more the ship was tried, and again her speed was in excess of the
specification, notwithstanding that it was still impossible to get the
specified number of revolutions per minute. Mathematicians then took the
matter in hand, and it was found that the ship actually travelled faster
than she would have done if the screw had been running in a solid nut.
Instead of a positive slip, the screw had in reality a negative slip;
but this was not believed at the time, and the discussion and
controversy continued. The ship was tried again and again, and always
with the same results. This apparently inexplicable phenomenon was
accounted for in the following manner:--The hull of the ship was said to
be rather imperfect and to cause a considerable drag in the water, so
that, when the ship was moving at full speed, the water at the stern had
imparted to it a forward velocity greater than the actual slip.

  [1] The particulars relating to this event are taken from accounts
  published at the time in American papers.

What is true of ships is true of flying machines. Good results can never
be obtained by placing the screw in front instead of in the rear of the
machine. If the screw is in front, the backwash strikes the machine and
certainly has a decidedly retarding action. The framework, motor, etc.,
offer a good deal of resistance to the passage of the air, and if the
air has already had imparted to it a backward motion, the resistance is
greatly increased. The framework will always require a considerable
amount of energy to drive it through the air, and the whole of this
energy is spent in imparting a forward motion to the air, so if we place
the propelling screw at the rear of the machine in the centre of the
greatest atmospheric resistance, it will recover a portion of the lost
energy, as in the steamship referred to. It will, therefore, be seen
that when the screw is at the rear, it is running in air which is
already moving forward with a considerable velocity, which reduces the
slip of the screw in a corresponding degree. I have made experiments
with a view of proving this, which I shall mention further on, and which
ought to leave no chance for future discussion.

[Illustration: Fig. 21.--Small apparatus for testing fabrics for
aeroplanes, the material being subjected to an air blast in order to
test its lifting effect as compared with its tendency to travel with the
blast.]

My first experiments had shown that wooden aeroplanes did much better
than any of the fabric covered aeroplanes that I was able to make at
that time, but as wood was quite out of the question on my large machine
on account of its weight, it was necessary for me to conduct
experiments with a view of ascertaining the relative values of different
fabrics. For this purposes, I made the little apparatus shown (Fig. 21).
This was connected to a fan blower driven by a steam engine having a
governor that worked directly on the point of cut-off. The speed was,
therefore, quite uniform and the blast of air practically constant. I
had a considerable number of little frames cut out of sheet steel, and
to these I attached various kinds of fabric, such as ordinary satin,
white silk, closely woven silk, linen, various kinds of woollen fabrics,
including some very coarse tweeds, also glass-paper, tracing linen, and
the best quality of Spencer’s balloon fabric. The blast of air was not
large enough to cover the whole surface of the aeroplanes, so that the
character of the back of the frames was of no account. The first object
experimented with was a smooth piece of tin. When this was placed at an
angle of 1 in 14, it was found that the drift or tendency to travel in
the direction of the blast was just one-fourteenth part of the upward
tendency, or lift. This was exactly as it should have been. Upon
changing the angle to 1 in 10, a similar thing occurred; the lift was
ten times the drift. I, therefore, considered the results obtained
with the sheet of tin as unity, and gave to every other material
experimented with, a coefficient of the unity thus established. Upon
testing a frame covered with tightly-drawn white silk, a considerable
amount of air passed through, and with an angle of 1 in 14, the lift was
only about double the drift. A piece of very open fabric, a species of
buckram, was next tried, and with this the lift and drift were about
equal. With closely-woven, shiny satin the coefficient was about ·80;
with a piece of ordinary sheeting the coefficient was ·90; with
closely-woven, rough tweeds, ·70; and with glass-paper about ·75. With a
piece of tracing linen very tightly drawn, results were obtained
identical with those of a sheet of tin, and with Spencer’s balloon
fabric the coefficient was about ·99. I, therefore, decided to cover my
aeroplanes with this material. It will be observed that the apparatus is
so arranged that both the lift and the drift can be easily measured.

[Illustration: Fig. 22.--Apparatus for testing the lifting effect of
aeroplanes and condensers in an air blast. _k_, _k_ show two aeroplanes
in position for being tested.]

[Illustration: Fig. 23.--Apparatus for testing aeroplanes, condensers,
etc., in an air blast. The opening is 3 feet square. Thin brass
sustainers are shown in position for testing.]

[Illustration: Fig. 24.--Cross-sections of bars of wood employed for
ascertaining the coefficient of different forms.]

[Illustration: Fig. 25.--Transverse sections of bars of wood
experimented with for the purpose of ascertaining their coefficients as
relates to a normal plane.]

[Illustration: Fig. 26.--A flat aeroplane placed at different angles.]

In order to ascertain the resistance encountered by various shaped
bodies driven at various speeds through the air, the best form of
aeroplanes, and the efficiency of atmospheric condensers, I made the
apparatus shown in Figs. 22 and 23. The smaller and straight portion of
this apparatus was 12 feet long and exactly 3 feet square inside, and
was connected as shown to a shorter box 4 feet square. Two strongly made
wooden screws _b_, _b_ and _d_, were attached to the same shaft. These
screws had two blades each, and while one pair of blades was in a
vertical position, the other was in a horizontal position. I interposed
between the screws, slats of thin wood arranged in the manner shown at
_d_, _d_; this was to prevent rotation of the air. At _e_ I placed
vertical slats of thin wood, and horizontal slats of the same size at
_f_. At _g_ two wide and thin boards, sharp at both edges and made in
the form of the letter X, were placed in the box as shown in section XY.
An engine of 100 H.P. with an automatic variable cut-off was employed
which gave to the screws a uniform rate of rotation, and as the engine
had no other work to do, the governor could be arranged to give varying
speeds such as were required for the experiments. The objects to be
tested were attached to the movable bars. In the drawing, the aeroplane
_k_, _k_ is shown in position for testing. This apparatus was provided
with a rather complicated set of levers, which permitted not only the
measurement of the lift of the objects experimented with, but also that
of the drift. The principle employed in this apparatus was a
modification of the ordinary weighing apparatus used by grocers, etc.
The first object tested was a bar of wood exactly 2 inches square shown
in Fig. 24. This was placed in such a manner that the wind struck
squarely against the side as shown in the drawing, and with a wind of 49
miles per hour, it was found that the drift or tendency to move with the
air was 5·16 lbs.; at the same time, the wind on my instrument gave a
pressure of 2 lbs. on a normal plane 6 inches square. The velocity of
the wind was ascertained by an anemometer of the best London make. Upon
turning the same bar of wood in the position shown at _b_, the drift
mounted to 5·47 lbs. A round bar of wood, 2 inches in diameter, shown at
_c_, gave a drift of 2·97 lbs. These experiments were repeated with a
wind velocity of 40 miles per hour, when it was found that the drift of
_a_ was 4·56 lbs., and that of the round bar, 2·80 lbs. It will be seen
from these experiments that the power required for driving bars or rods
through the air is considerably greater than one would have supposed.
The next object experimented with was _a_, Fig. 25. When this was
subject to a wind of 40 miles an hour, the drift was 0·78 lb. Upon
reversing this bar--that is, putting the thin edge instead of the thick
edge next to the wind--the drift mounted to 1·22 lbs.; _b_ showed a
drift of 0·28 lb. with the thick edge to the wind, and 0·42 lb. with the
thin edge to the wind; _c_ showed a drift of 0·23 lb. with the thick
edge to the wind, and 0·59 lb. with the thin edge to the wind; and _d_,
which was the same thickness as the others and 12 inches wide, both
edges being alike, showed a drift of only 0·19 lb. These experiments
show in a most conclusive manner the shapes that are most advantageous
to use in constructing the framework of flying machines. Aeroplane _e_,
Fig. 26, when placed on the machine in a horizontal position showed
neither lift nor drift, but upon placing it at an angle of 1 in 20, as
shown at _f_, the lift was 3·98 lbs. and the drift 0·30 lb. with a wind
velocity of 40 miles per hour. At this low angle the blade trembled
slightly. Upon placing the same plane at an angle of 1 in 16 as shown at
_g_, the lift was 4·59 lbs. and the drift 0·53 lb. It will be observed
that the underneath side of this plane is perfectly flat. The next
experiment was with planes slightly curved, as shown in Fig. 27. The
aeroplane _a_ was 16 inches wide, very thin, and only slightly curved.
When set at a very low angle, it vibrated so as to make the readings
very uncertain, but when set at an angle of 1 in 10 it lifted 9·94 lbs.
with a drift of 1·12 lbs. By slightly changing the angle it was made to
lift 10·34 lbs. with a drift of 1·23 lbs., the wind velocity being 41
miles per hour. Aeroplane _b_, 12 inches wide, Fig. 27, when placed at
an angle of 1 in 14 with an air blast of 41 miles per hour, gave a lift
of 5·28 lbs. with a drift of 0·44 lb.; at an angle of 1 in 12 the lift
was 5·82 lbs. and the drift 0·5 lb.; at an angle of 1 in 10 the lift was
6·75 lbs. and the drift 0·73 lb.; with an angle of 1 in 8 the lift was
7·75 lbs. and the drift 1 lb.; with an angle of 1 in 7 the lift was 8·5
lbs. and the drift 1·25 lbs.; at an angle of 1 in 6 the lift was 9·87
lbs. and the drift 1·71 lbs. Aeroplane _c_, Fig. 27, which had more
curvature than _b_, when run in a horizontal position, gave a
considerable lift, and when raised to an angle of 1 in 12 it gave a lift
of 6·12 lbs. with a drift of 0·54 lb. In another experiment at the same
angle, it gave a lift of 6·41 lbs. with a drift of 0·56 lb.; at an angle
of 1 in 16 it gave a lift of 5·47 lbs. with a drift of 0·37 lb.; at an
angle of 1 in 10 it gave a lift of 6·97 lbs. and a drift of 0·70 lb.; at
an angle of 1 in 8 it gave a lift of 8·22 lbs. with a drift of 1·08
lbs.; at an angle of 1 in 7 it gave a lift of 9·94 lbs. with a drift of
1·45 lbs.; at an angle of 1 in 6 it gave a lift of 10·34 lbs. and a
drift of 1·75 lbs. This plane was then carefully set so that both the
forward and aft edges were exactly the same height, and with a wind
blast of 41 miles per hour it gave a lift of 2·09 lbs. with a drift of
0·21 lb. It was then pitched 1 in 18 in the wrong direction, and at this
point, the lifting effect completely disappeared, while the drift was
practically nothing.

[Illustration: Fig. 27.--Group of aeroplanes used in experimental
research. Although shown the same size in the drawing, aeroplane _a_ was
16 inches wide, and _b_ and _c_, 12 inches wide.]

[Illustration: Fig. 28.--An 8-inch aeroplane which did very well. This
aeroplane gave decided lifting effect when the bottom side was placed
dead level, as shown at _a_.]

When the aeroplane _a_ (Fig. 28) was placed in a horizontal position,
and the apparatus carefully balanced, it showed at a wind velocity of 40
miles an hour, a lift of 1·56 lbs., and a drift of 0·08 lb.; at an angle
of 1 in 20, a lift of 3·62 lbs. and a drift of 0·21 lb.; at an angle of
1 in 16, a lift of 4·09 lbs. with a drift of 0·26 lb.; at an angle of 1
in 14, a lift of 4·5 lbs. and a drift of 0·33 lb.; at an angle of 1 in
12, a lift of 5 lbs. and a drift of 0·43 lb.; at an angle of 1 in 10, a
lift of 5·75 lbs. and a drift of 0·60 lb.; at an angle of 1 in 8, a lift
of 6·75 lbs. and a drift of 0·86 lb. The blast was then increased to a
velocity of 47·33 miles an hour, when it was found that the lift at an
angle of 1 in 16 was 5 lbs. and the drift 0·33 lb. It will be observed
that this aeroplane was only 8 inches wide, while the others were 12
inches or more. They were all rather more than 3 feet long, but the
width of the blast to which they were subjected was exactly 3 feet, and
they were placed as near to the end of the trunk as possible.

[Illustration: Fig. 29.--Resistance due to placing objects in close
proximity to each other.]

The next experiments were made with the view of ascertaining what effect
would be produced when objects were placed near to each other (see Fig.
29). Two bars of wood 2 inches thick, and shaped as shown in the
drawing, were placed on the machine and subjected to a blast of 41 miles
per hour; the drift at various distances from center to center was as
follows:--

  24 inches centers,   drift 6     ozs.
  22    „      „         „   6      „
  20    „      „         „   6      „
  18    „      „         „   6-1/8  „
  16    „      „         „   6-1/8  „
  14    „      „         „   6-1/4  „
  12    „      „         „   6-1/2  „
  10    „      „         „   7      „
   8    „      „         „   7-3/4  „
   6    „      „         „   8-1/2  „
   4    „      „         „   9-1/4  „

It will be seen by this that the various members constituting the frame
of a flying machine should not be placed in close proximity to each
other.

A bar of wood similar in shape to _d_ (Fig. 25), but being 9 inches wide
instead of 12 inches, was mounted in a wind blast of 41 miles an hour,
with the front edge 3·31 inches above the rear edge, and this showed a
lift of 7·08 lbs. and a drift of 3·23 lbs. When the angle was reduced to
2·31 inches, it gave a lift of 4·53 lbs. with a drift of 0·78 lb., and
with the angle reduced to 1·31 inches, the lift was 3·37 lbs. and the
drift 0·5 lb. It will, therefore, be seen that even objects rounded on
both sides give a very fair lift, and in designing the framework of
machines advantage should be taken of this knowledge. The bar of wood
_c_ (Fig. 25) was next experimented with. With the sharp edge to the
wind, and with the front edge 2 inches higher than the rear edge, the
lift was 2·54 lbs. and the drift 0·76 lb. By turning it about so that
the wind struck the thick edge, the lift was 4·45 lbs. and the drift
0·47 lb. This seemed rather remarkable, but, as it actually occurred, I
mention it for other people to speculate upon. It, however, indicates
that we should take advantage of all these peculiarities of the air in
constructing the framework of a machine, which in itself is extremely
important, as I find that a very large percentage of the energy derived
from the engines is consumed in forcing the framework through the air.
It is quite true that a certain amount of this energy may be recovered
by the screw, provided that the screw runs in the path occupied by the
framework. Still, it is much better that the framework should be so
constructed as to offer the least possible resistance to the air, and,
as far as possible, all should be made to give a lifting effect.

[Illustration: Fig. 30.--Cross-section of condenser tube, made in the
form of Philipps’ sustainers, in which _c_ is the steam passage.]

[Illustration: Fig. 31.--The grouping of condenser tubes, made in the
form of Philipps’ sustainers. This arrangement is very effective,
condenses the steam or cools the water, and gives a lifting effect at
the same time. The shape and arrangement of tubes shown at _b_, _b_,
although effective as a condenser, produce no lifting effect, but a
rather heavy drift.]

Having ascertained the lifting effect of wooden aeroplanes of various
forms and at varying velocities of the wind, and, also, the resistance
offered by various bodies driven through the air, I next turned my
attention to the question of condensation. I wished to recover as much
water as possible from my exhaust steam. I had already experimented with
Mr. Horatio Philipps’ sustainers, and I found that their lifting effect
was remarkable. A curious thing about these aeroplanes was that they
gave an appreciable lift when the front edge was rather lower than the
rear. I therefore determined to take advantage of this peculiar
phenomenon, and to make my condenser tubes as far as possible in the
shape of Mr. Philipps’ sustainers. Fig. 30 shows a section of one of
these tubes, in which _a_, _a_ is the top surface, _b_ a soldered joint,
and _c_ the steam space. These were mounted on a frame as shown at _a_
(Fig. 31). I had already found that bodies placed near to each other
offered an increased resistance to the air, but by placing these
sustainers in the manner shown this was avoided, as the air had
sufficient space to pass through without being either driven forward or
compressed. It was found by experiment that the arrangement of tubes or
sustainers, shown at _d_, _d_ (Fig. 31), was very efficient as a
condenser, but it gave a very heavy drift and no lifting effect at all;
whereas, on the other hand, the arrangement shown at _a_ was equally
efficient, and, at the same time, gave a decided lifting effect. When
twelve of these tubes or sustainers were placed at an angle of 1 in 12,
the lifting effect was 12·63 lbs. and the drift 2·06 lbs. It was found,
however, that a good deal of the drift was due to the wind getting at
the framework that was used for holding the sustainers in position. With
a wind velocity of 40 miles an hour and a temperature of 62° F., 2·25
lbs. of water were condensed in five minutes, and, while running, the
back edge of the sustainers was quite cool. At another trial of the same
arrangement under the same conditions, the lift was 11 lbs. and the
drift 2·63 lbs. It is quite possible on this occasion that the metal was
so extremely thin that the angles were not always maintained;
consequently, that no two readings would be alike. It was found at this
point that the belt was slipping, and a larger pulley was put on the
driving shaft of the screws; and under these conditions, with a wind of
49 miles per hour and an angle of 1 in 8, the lifting effect ran up to
14·87 lbs. with a drift of 2·44 lbs., and the condenser delivered 2·87
lbs. of water from dry steam in five minutes. The weight of metal in
this condenser was extremely small, the thickness being only about 1/500
of an inch. This condenser delivered the weight of the sustainers in
water every five minutes. They should, however, have been twice as
heavy. Cylinder oil was now introduced with the steam in order to
ascertain what effect this would have. After seven minutes’ steaming,
2·25 lbs. of water were condensed in five minutes. It will be seen from
these experiments that an atmospheric condenser, if properly
constructed, is fairly efficient. Roughly speaking, it requires 2,400
times as much air in volume as of water to use as a cooling agent. With
the steam engine condenser only a relatively small amount of water is
admitted, and this is found to be sufficient; but in an atmospheric
condenser working in the atmosphere, it must be as open as possible, so
that no air which has struck one heated surface can ever come in contact
with another.



CHAPTER V.

EXPERIMENTS WITH APPARATUS ATTACHED TO A ROTATING ARM.


From what information I have at hand, it appears that Prof. Langley made
his first experiments with a small apparatus, using aeroplanes only a
few inches in dimensions which travelled round a circle perhaps 12 feet
in diameter. With this little apparatus, he was able to show that the
lifting effect of aeroplanes was a great deal more than had previously
been supposed. After having made these first experiments, he seems to
have come to the conclusion that Newton’s law was erroneous. Shortly
after Langley had made these experiments on what he called a whirling
table, which, however, was not a very appropriate name, I made an
apparatus myself, but very much larger than that employed by Prof.
Langley. I reckoned the size of my aeroplanes in feet, where he had
reckoned his in inches. The circumference of the circle around which my
aeroplanes were driven was exactly 200 feet, and shortly after this
Langley constructed another apparatus, the same dimensions as my own.
From an engraving which I have before me, it appears that he constructed
an extremely large wooden scale beam supported by numerous braces, but
free to be tilted in a vertical direction after the manner of all other
scale beams. As this apparatus was of great weight and offered enormous
resistance to the air, I do not understand how it was possible to obtain
any very correct readings, especially as it was in the open and subject
to every varying current of air.

[Illustration: Fig. 32.--Machine with a rotating arm, 31·8 feet long, to
which is attached the object to be experimented with. Professor Langley
had a similar machine and called it a “whirling table.”]

[Illustration: Fig. 33.--A screw and fabric covered aeroplane in
position for testing.]

[Illustration: Fig. 34.--The rotating arm of the machine with a screw
and aeroplane attached.]

In constructing my apparatus, which is shown in the photographs, and
also in a side elevation (Fig. 32), I aimed at making the apparatus very
light and strong, avoiding as far as possible atmospheric resistance. In
the drawing, _a_, is a thick seamless steel pipe 6 inches diameter; _b_,
is a cast-iron pedestal firmly bolted to _d_, and connected to a large
cast-iron spider embedded in hydraulic cement; by this means great
rigidity and stiffness were obtained. _n_, _n_ was formed of strong
Georgia pine planks 2 inches thick, and strongly bolted together. The
two members of the long radial arm _h_, _h_, were made of Honduras
mahogany, an extremely strong wood, and had their edges tapered off as
shown at _y_, _y_. The power was transmitted from a small steam engine
provided with a sensitive governor through the shaft _f_, _f_. In the
base _c_, of the casting _b_, was placed a pair of tempered steel bevel
gears, giving to the vertical shaft a high velocity. From a pulley on
the top of this shaft, the belt _i_, was run through the arms _h_, _h_,
as shown in section _y_, _y_. This gave a rapid rotation to the screw
shaft in a very simple manner. The operation of the machine was as
follows:--the aeroplane _g_, to be tested was secured to a sort of
weighing apparatus which is shown in detail (Fig. 36), and the screw
attached to the shaft. Upon starting the engine, a very rapid rotation
was given to the screw which caused the radial arm to travel at a high
velocity, the whole weight resting on a ball bearing at _w_. The radial
arms and all of their attachments were balanced by a cigar-shaped lead
weight _s_, which was secured to a sliding bar so as to make it easily
adjustable. The thrust of the screw caused the screw shaft to travel
longitudinally, and this was opposed by a spring connected by a very
thin and light wire to the pointer of the index _o_. As the apparatus
rotated rather slowly on account of its great diameter, it was quite
possible to observe the lift while the machine was running at its
highest speed. The aeroplanes were mounted after the manner of the tray
of a grocer’s scales (see Fig. 36), and the lift of the aeroplane was
determined by what it would lift at _r_--that is, while the machine was
running at a given speed, iron or lead weights were placed in the pail
_r_, until the lift of the aeroplane was exactly balanced, and then, in
order to ascertain exactly what the lift was, the aeroplane was placed
under what might be called a small crane, and a cord, running over a
pulley, attached. The amount of weight necessary to lift the plane into
the same position that it occupied while running was taken as its true
lift. In order to facilitate experiments the gauge _p_, was provided.
This gauge consisted of a large glass tube and the index _p_, with a
quantity of red water at _q_. The centrifugal force of rotation caused
the red water to rise in the tube. This was easily seen, so that if
experiments were being tried, we will say at 50 miles an hour, it was
always possible to turn on steam until the red liquid mounted to 50.
This device was very simple and effective, and saved a great deal of
time. In order to prevent the twisting of the radial arm, a piece of
stiff oval steel tube 12 feet long was secured between the arms at _j_,
and on each end of this tube were attached the wires _u_, _u_. This not
only effectually supported the end of the arm, but at the same time
prevented twisting and made everything extremely stiff. Of course, while
the machine was running at a high velocity, centrifugal force had to be
dealt with, and in order to prevent this from causing friction in the
articulated joints of the weighing apparatus (Fig. 36), thin steel wires
_k_, _k_ were provided. As this apparatus was in the open, it was found
that the slightest movement of the air greatly interfered with its
action. On one occasion when a fabric covered aeroplane, 4 feet long by
3 feet wide, was placed in position, the four corners being held down by
the wires _v_, _v_, and the apparatus driven at a high velocity, a
sudden gust of wind snapped two of the wires, broke the aeroplane, and
the flying fragments smashed the screw, and this notwithstanding that
each of the four wires was supposed to be strong enough to resist at
least four times any possible lifting that the whole aeroplane might be
subjected to.

[Illustration: Fig. 35.--The little steam engine used by me in my
rotating arm experiments; the tachometer and dynamometer are distinctly
shown.]

In order to ascertain the force and direction of the wind, I made an
extremely simple and effective apparatus which is fully shown (see Fig.
38). Whilst conducting these experiments it occurred to me, when a large
aeroplane was used, that after it had been travelling for a considerable
time, it would impart to the air in the path of its travel, a downward
motion, and that the lifting effect would be greatly reduced on this
account. In order to test this, I provided four light brass screws and
mounted them, as shown at _x_, on a hardened polished steel point much
above their centre of gravity, so that they balanced themselves. On
account of the absence of friction, they were easily rotated, and
responded to the least breath of air that might be moving. One morning
when there was a dead calm, I placed four of these screws equidistant
around the whole circle. Some of them rotated very slowly in one
direction and some in another; some alternated, but all their motions
were extremely slow. However, upon setting the machine going with a
large aeroplane and a powerful screw, I found after a few turns that the
air was moving downwards around the whole circle at a velocity of about
2 miles an hour, but as the screw was a considerable distance below the
aeroplane, I estimated that the actual downward velocity of the air in
which the aeroplane was travelling was about 4 miles an hour. The result
of my experiments are clearly shown in an unpublished paper which I
wrote at the time, and as it is of considerable historical interest, I
have placed it in the appendix, notwithstanding that there may be
certain repetitions.

[Illustration: Fig. 36.--The machine attached to the end of the rotating
shaft--_a_, _a_, the body of the machine; _b_, _b_, four-legged spider
secured to _a_, _a_; _c_, _c_, parallel bars; _d_, _d_, vertical member
to which the aeroplane _g_, _g_ is attached; _h_, _h_, the screw; _f_,
_f_, wires for preventing distortion of the aeroplane.]

[Illustration: Fig. 37.--Marking off the dynamometer. In order to
ascertain the actual amount of power consumed in driving the propeller,
a brake was put on in place of the screw, a weight applied, and the
engine run at full speed. In this way all the uncertain and unknowable
factors were eliminated.]

In Fig. 36, _a_, _a_ is the body of the apparatus, partly of gunmetal
and partly of wood. It is provided with a steel shaft to which the screw
_h_, is attached, and also with a cylindrical pulley for taking the
belt. The thrust of the screw pushes the shaft inwards and records the
lift at _o_ (Fig. 32). The corners of the aeroplane _g_, _g_, are
attached by wires to the steel plate _e_. _b_, _b_, is a four-arm
spider for holding the ends of the parallel bars _c_, _c_, and _d_, _d_,
show vertical steel bars to which all devices to be tested are attached.
In testing aeroplanes, weights may be placed at _e_, sufficient to
balance the lifting effect, and then by adding the weight to the upward
pull of the aeroplane, the true lift of the aeroplane is obtained. It is
also possible to attach an aeroplane at _e_, that is, the machine was
made to test superposed aeroplanes if required. In these experiments, I
naturally assumed that the best position for a screw was at the rear and
in the path of the greatest resistance, but as some experimenters with
navigable balloons placed the screw in front in order to pull the
apparatus along instead of to push it, I made experiments to see what
the relative difference might be. In order to do this, I wound a large
amount of rope one-half inch in diameter around the whole apparatus
forward of the screw, converting it into an irregular mass well
calculated to offer atmospheric resistance. Upon starting the engine, I
was rather surprised to see how little retardation these ropes gave to
the apparatus. It appeared to me that nearly all of the energy consumed
in driving the ropes through the air was recovered by the screw. I then
removed the right-hand screw and replaced it by a left-hand screw of the
same pitch and dimensions (Fig. 37_a_). I then found that the blast of
the screw blowing against the tangle of ropes greatly retarded the
travel; in fact, with the same number of revolutions per minute, the
velocity fell off 60 per cent. I think that these experiments ought to
show that there is but one place for the screw, and that is at the
stern, and in the direct path of the greatest atmospheric resistance.

[Illustration: Fig. 37_a_.--Right- and left-hand four-blade screws used
in my experiments for ascertaining the comparative efficiency between
screws placed in front and in the rear of the machine.]

[Illustration: Fig. 38.--Apparatus for indicating the force and velocity
of the wind direct without any timing, counting, or mathematical
calculations.]

Fig. 38 shows an original apparatus which I designed and made for my own
use; with ordinary anemometers it is necessary to count the number of
turns per minute in order to ascertain the velocity of the wind. I
wanted something that would indicate the velocity and the direction of
the wind without any figures or formulæ. I therefore made the apparatus
shown in the drawing, in which _a_, _a_, is a metallic disc 13·54 inches
in diameter, giving it an area of exactly 1 square foot. This is
attached to the horizontal bar _b_, and the whole mounted on two bell
crank levers as shown. When the wind is not blowing, the long arms of
these two levers assume a vertical position, and the spiral spring _h_,
is in exact line with the pivots on which these levers are mounted, and
has no effect except to hold the levers in a vertical position. As the
spring has very little tension in this position, and as it requires a
considerable movement in order to give it tension, the arms _c_, _c_,
and the bar _b_, _b_, are very easily pushed backwards, but as the
distance through which they travel increases, the angle of the lever
changes and the tension of the spring increases at the same time, so
that when the disc is pushed backwards to any considerable distance, a
strong resistance is encountered. Had I made this apparatus so that the
pressure acted directly on the spiral spring, the spaces on the index
indicating low velocities would have been very near together, while
those indicating high velocities would have been widely separated, but
with this device properly designed, the spacing on the index became
regular and even. The index being very large enabled one to read it at a
considerable distance, and at the same time, it acted as a tail and kept
the apparatus face to the wind. The spaces of the dial were not laid off
with a pair of dividers, but each particular division was marked by an
actual pull on the bar _b_, through the agency of a cord and easily
running pulley and weight. The markings, however, were not correct,
because Haswell’s formula was employed in which the pressure of the wind
against the normal plane is considerably greater than with the more
recent formula, which is now known to be correct. Haswell’s formula was
V² × ·005 = P, and the recent formula P = 0·003 × V², where P = pressure
in lbs. per square foot and V = velocity in miles per hour. In my
experiments, I also employed a very well made and delicate anemometer by
Negretti & Zambra.

[Illustration: Fig. 39.--Apparatus for testing the lifting effect of
aeroplanes at a low angle and extremely high velocity. _a_, _a_, the
aeroplane; _b_, lead weight; _c_, long and slender pine rod; _d_, tail
for keeping the apparatus head on and ensuring its travelling straight
through the air; _e_, the point of suspension, also the centre of
gravity. When this apparatus was travelling at the rate of 80 miles an
hour, it gave a lifting effect of about 36 lbs., which is about 7 lbs.
per square foot.]


CRYSTAL PALACE EXPERIMENTS.

Having fully satisfied myself that aeroplanes flying around a circle 200
feet in circumference had their lifting effect reduced to no
insignificant degree by constantly engaging air which had already had
imparted to it a downward movement by a previous revolution, I
determined to make some experiments where this trouble could not occur,
but the opportunity did not present itself until after the large
roundabout, erroneously described as “a captive flying machine,” was put
up at the Crystal Palace. This presented a fine opportunity for making
experiments at an extremely high velocity around a very large circle. I
will only refer to a few of these experiments. To a prolongation of one
of the long arms, I attached a thin steel wire rope about 60 feet above
the platform; I then attached to this wire rope the little device shown
(Fig. 39), in which _a_, is an aeroplane, 5 feet long and 1 foot wide,
placed at an inclination of 1 in 20. Great care was used in preparing
this aeroplane to see that it was free from blemish, smooth, and without
any irregularities. Both edges were sharp and the curvature was about
one-eighth of an inch on the underneath side. It was made relatively
thick in the middle where it was attached to the bar _c_, and thinner
at the ends. _b_, shows a lump of lead just heavy enough to balance the
bar _c_, and the tail; _d_, was a light but strong wooden frame, all the
edges being thin and sharp, and covered with a special silk that Mr.
Cody had found to be best for such purposes. The wire rope _e_, was
attached to the long arm which I referred to. The great length of the
bar _c_, and the accuracy with which the whole was made and balanced
caused the aeroplane to travel straight through the air adjusting itself
to all the shifting currents. Upon starting the machine on a very calm
day, this apparatus mounted as high as the point of support, sometimes
going 10 or more feet higher and sometimes 8 or 10 feet lower. However,
as a rule, it carried its own weight at a velocity of 80 miles an hour
around a circle 1,000 feet in circumference. Under these conditions, of
course, there could be no downward motion of the air as all the air
affected would be removed long before it could be struck the second time
by the aeroplane. I had no means of ascertaining exactly how much this
plane did actually lift, because the air was always moving to some
extent, and it was not an easy matter to ascertain whether it was above
or below the point of support. I am sure, however, that it was as much
as 36 lbs., or rather more than 7 lbs. to the square foot, and this is
just what it should have lifted, providing that we consider the results
obtained by smaller planes placed in an air blast of 40 miles an hour
and at the same angle. When these experiments were finished, I made a
very small apparatus having only about 25 square feet of lifting
surface, and this carried the weight of a man, in fact several gentlemen
came up from London and went round on it themselves. I saw, however,
that it was a dangerous practice, because if the wind was blowing at
all, the apparatus would mount very much above the point of support
while travelling against the wind, only to drop much below the point of
support on the other side of the circle where it was travelling with the
wind; in fact, on one occasion the apparatus shown (Fig. 39) mounted in
a high wind fully 20 feet above the point of support and came down with
such a crash on the other side that it broke the wire rope. In
connection with this, it is interesting to note that when I erected the
first so-called “captive flying machine” on my own grounds at Thurlow
Park, I intended that instead of ordinary boats such as were ultimately
employed, each particular boat should be fitted with an aeroplane, that
the engine should be of 200 H.P., and that the passengers should
actually be running on the air, each boat being provided with a powerful
electric motor in addition to the motive power that drove the shaft. Had
this been carried out as was originally designed, it would have removed
the apparatus altogether from the category of vulgar merry-go-rounds,
but such was not to be. Unforeseen circumstances were against me. I had
some of these boats fitted up with aeroplanes and running on my grounds,
and two of the engineers of the London County Council came out to see
the apparatus before it was put up for public use. On that occasion the
wind was blowing a perfect gale of 40 miles an hour, and as the boats
travelled at the rate of 35 miles an hour, they, of course, encountered
a wind of 75 miles an hour when passing against the wind, and a minus
velocity of 5 miles an hour when travelling with the wind on the other
side of the circle. The aeroplanes, although of considerable size, were
small in relation to weight. I had neglected to put any weight in the
boats, and when three of us were studying the eccentric path through
which the boats were travelling, suddenly one of them in passing to the
windward, raised very much above the point of support and plunged down
with great force on the other side; in fact, the shock was so great that
it made everything rattle, but nothing was broken. Nevertheless, the
engineers said at once, it would not do to run the boats with those
aeroplanes; it was too dangerous. This would not, however, have occurred
if the boats had been loaded, or the velocity of the wind had been less.
It, however, demonstrated what a tremendous lift may be obtained from a
well-made aeroplane passing at a high velocity through the wind at a
sharp angle. These aeroplanes were only about 12 feet long and 5 feet
wide, having, therefore, 60 square feet of surface. They were, however,
strong, well-made, and perfectly smooth, both top and bottom. I would
say right here that I am not a success as a showman--previous long years
of rubbing up against honest men have disqualified me altogether for
such a profession. I was extremely anxious to go on with my experiments.
I appreciated fully that I had made a machine that lifted 2,000 lbs.
more than its own weight, and I knew for a dead certainty if I took the
matter up again, got rid of my boiler and water tank, and used an
internal combustion engine, such as I thought I could produce, that
mechanical flight would soon be a _fait accompli_. I had already spent
more than £20,000, and was looking about for some means of making the
thing self-supporting. I believed that the so-called “captive flying
machine” would be very popular, and bring in a lot of money, and it
would have done so, if it had been put up as originally designed. I
proposed to use my share of the profits for experimental work on real
flying machines. That I was not far wrong in believing that such a
machine would be a success, is witnessed by the fact that just about the
same time, an American inventor thought of the same thing, put up some
three or four machines the first year, and the next year about 50. They
were highly profitable, and there are fully 140 of them running at the
present time in the U.S.A. It is a fact that nothing in the way of
side-shows at exhibitions or public resorts has ever had the success of
this machine in the U.S.A., and even the little machine at Earl’s Court
took £325 10s. in one day and £7,500 in a season. However, this little
attempt to make one hand wash the other cost me no less than £10,400,
not to mention more than a year of very hard work. This sum would have
been amply sufficient to have enabled me to continue my experiments
until success was assured.



CHAPTER VI.

HINTS AS TO THE BUILDING OF FLYING MACHINES.


[Illustration: Fig. 40.--Front elevation of proposed aeroplane
machine--_a_, _a_, the aeroplanes; _g_, _g_, condenser; _f_, the engine;
_q_, guard for screw; _k_, _k_, support for wheels.]

[Illustration: Fig. 41.--Side elevation of proposed superposed aeroplane
machine--_a_, _a_, main aeroplanes; _b_, _b_, rear aeroplanes; _c_,
vertical rudder; _d_, horizontal front rudder; _e_, screw; _f_, motor;
_g_, condenser; _h_, steering gear; _i_ and _j_, pneumatic buffers; _k_
and _l_, wheels; _m_, point at which _k_ is pivoted to the main frame;
_n_, handle of the steering gear.]

[Illustration: Fig. 42.--Plan of proposed aeroplane machine, in which
_a_, _a_ are the proposed superposed main aeroplanes; _b_, _b_, the
after superposed aeroplanes; _c_, _c_, the forward horizontal rudder;
_d_, platform; _e_, screw; _h_, _h_, and _i_, _i_, pulleys used in
communicating motion from the steering gear, _f_, to the rudder, _j_;
_g_, lever attached to the aeroplane or rudder, _c_, _c_, and connected
to the steering gear, _f_.]

For those who really wish to build a flying machine that will actually
fly with very little experimental work, I have given an outline sketch
sufficiently explicit to enable a skilful draughtsman to make a working
drawing in which Fig. 40 is a front elevation, Fig. 41 a side elevation,
and Fig. 42 a plan. Fig. 41, _a_, _a_, shows the two forward or main
aeroplanes; _b_, _b_, the two after aeroplanes, which are smaller and
shorter; _c_, the rudder; _d_, the forward horizontal rudder; _e_, the
screw; _f_, the motor; _g_, the condenser or cooler; _h_, the steering
gear; _i_, and _j_, atmospheric buffers; _k_ and _l_, wheels attached to
a lever pivoted to the body of the machine; _q_, a shield for protecting
the screw. It will be observed that the framework is extremely long,
and, consequently, the distance between the aeroplanes is very great;
but it should be borne in mind that the longer the machine, the less any
change of center of lifting effect, as relates to the center of gravity,
will be felt. Moreover, it is much easier to manœuvre a machine of
great length than one which is very short, because it gives one more
time to think and act. If the length was infinitely great the tendency
to pitch would be infinitely small. I have shown a steering gear
consisting of a lever with a handle _n_, arranged in such a manner that
it moves both the vertical rudder _c_, and the horizontal rudder _d_, so
that the man who steers the machine has nothing to think of except to
point the lever _n_, _p_, in the direction that he wishes the machine to
go. This lever is mounted on a universal joint at _h_, and is connected
with suitable wires to the two rudders. In order to prevent shock when
the machine alights, it is necessary to provide something that is strong
and, at the same time, yielding, and able to travel through a
considerable distance before the machine comes to a state of rest. In
the machines which I have seen on the Continent, a very elaborate
apparatus is employed, which is not only very heavy, but also offers a
considerable resistance to the forward motion of the machine through the
air. It consists of many tubes, very long levers and heavy spiral
springs, etc. In the device which I am recommending, all this is
dispensed with, and something very much simpler, cheaper, and lighter is
substituted. Moreover, with my proposed apparatus a certain amount of
lifting effect is produced. The levers _k_, _k_, to which the wheels are
attached, should be of thin wood, light and strong, and say about a foot
wide, strongly pivoted to the frame and held in position by an
atmospheric buffer made of strong and thin steel tubing, shown in
section (Fig. 51). These pneumatic cylinders may be pumped up to any
degree, so as to support the weight of the machine, and then, as it
comes down, the compression and escape of air arrest its motion. The
condenser _g_, is placed in such a position that it will act even while
the machine is on the ground and the propellers working. In Continental
machines, very small screw propellers are used. These screws have
probably been made small because the experimenters have found that they
encounter a good deal of friction in the atmosphere, but this is caused
by imperfect shape and the rib of steel at the back of the blades. In
order to use a small screw, experimenters have been forced to use a very
quick-running engine which makes it necessary to have the cylinders very
short, so, in order to get the necessary power, they are obliged to use
no less than eight cylinders. However, by increasing the diameter of the
screw and making it of such a form that very little or no atmospheric
skin friction is encountered, a much better and cheaper engine of a
totally different type may be employed. There is no reason why more than
four cylinders should be used, but the stroke of the piston and diameter
of the cylinder should be increased. Doubtless Continental experimenters
have an idea that, as the engine cannot be provided with a flywheel, it
must have a very large number of cylinders in order to give a steady
pull completely around the circle, and thus avoid so-called “dead
centers”; but, when we consider the enormously high velocity of the
periphery of the screw, and also take into consideration that the
momentum is in proportion to the square of the velocity, it is quite
obvious that there can be no slowing up between strokes even if only
one cylinder should be employed working on the four-cycle principle, in
which work is only done one stroke in four. Then, again, I find that the
weight of these Continental engines can be greatly reduced, providing
that they are made with the same degree of refinement that I employed in
building my steam engines.

Recently there has been a great deal of discussion in _Engineering_ and
other journals regarding the comparative merits of the aeroplane system
and the hélicoptère. Some condemn both systems and pin their faith to
flapping wings. It has been contended that the screw propeller is
extremely wasteful in energy, and that in all Nature neither fish nor
bird propels itself by means of a screw. As we do not find a screw in
Nature, why then should we employ it in a machine for performing
artificial flight?

Why not stick to Nature? In reply to this, I would say that even Nature
has her limits, beyond which she cannot go. When a boy was told that
everything was possible with God, he asked; “Could God make a two-year
old calf in five minutes?” He was told that God certainly could. “But,”
said the boy, “would the calf be two years old?” It appears to me that
there is nothing in Nature which is more efficient, or gets a better
grip on the water than a well-made screw propeller, and no doubt there
would have been fish with screw propellers, providing that Dame Nature
could have made an animal in two pieces. It is very evident that no
living creature could be made in two pieces, and two pieces are
necessary if one part is stationary and the other revolves; however, the
tails and fins very often approximate to the action of the propeller
blades; they turn first to the right and then to the left, producing a
sculling effect which is practically the same. This argument might also
be used against locomotives. In all Nature, we do not find an animal
travelling on wheels, but it is quite possible that a locomotive might
be made that would walk on legs at the rate of two or three miles an
hour. But locomotives with wheels are able to travel at least three
times as fast as the fleetest animal with legs, and to continue doing so
for many hours at a time, even when attached to a very heavy load. In
order to build a flying machine with flapping wings, to exactly imitate
birds, a very complicated system of levers, cams, cranks, etc., would
have to be employed, and these of themselves would weigh more than the
wings would be able to lift. However, it is quite possible to approach
very closely to the motion of a bird’s wings with no reciprocating or
vibrating parts, and without flapping at all.

[Illustration: Fig. 43.--Plan of a hélicoptère machine showing position
of the screws. Owing to the tilting of the shaft forward, the blades
present no angle when at _d_, _d_, but 10° at _c_, _c_, while at _f_,
_f_ their angle above the horizontal is 5°. The horizontal arrows show
the direction of the wind against the machine.]

[Illustration: Fig. 44.--Shows the position of the blades of a
hélicoptère as they pass around a circle, when the angle of the shaft
and the angle of the blades are the same.]

In Fig. 43, I have shown a plan of a hélicoptère machine in which two
screws are employed rotating in opposite directions, _a_, _a_, being the
port screw; _b_, _b_, the starboard screw; and _d_, _d_, the platform
for the machinery and operator. The screws should be 20 feet in diameter
and made of wood. Suppose now that the pitch of these screws is such
that the extremities of the blades have an angle of 5°; if now we tilt
the shaft forward in the direction of flight to the extent of 5°, we
shall completely wipe out the angle of inclination of the blades when at
_b_ (Fig. 44), whereas it will be observed that the pitch as regards the
horizontal will be increased to 10° at _a_, on the outer side, and
remain unchanged at _c_, and _d_. If the peripheral velocity of the
blades is, say, four times the velocity at which the machine is expected
to travel, the blades will get a good grip on the air at _c_, _d_, but
when they travel forward and encounter air which is travelling at a high
velocity in the opposite direction, they assume the position shown at
_b_. If the pitch of the screw blades was a little more than the angle
of the shaft, the blades at _b_ would also produce a lifting effect, and
as the velocity with which they pass through the air is extremely high,
a very strong lifting effect would be produced even if the angle was not
more than 1 in 40. By tracing the path and noting the position of the
ends of the blades as they pass completely around the circle as shown
(Fig. 44), it will be observed that they very closely resemble the
motion of a bird’s wing. I have no doubt that a properly made machine on
this plan would be highly satisfactory, but one should not lose sight of
the fact that even with a machine of this type, well designed and
sufficiently light to sustain itself in the air while flying, it would
still be necessary for it to move along rapidly when starting in order
to get the necessary grip on the air. Upon starting the engine, in a
machine of this kind, a very strong downward draught of air would be
produced, and the whole power of the engines would be used in
maintaining this downward blast, but if the machine should at the same
time be given a rapid forward motion sufficiently great to bring the
blades into contact with new air, the inertia of which had not been
disturbed, and which was not moving downwards, the lifting effect would
be increased sufficiently to lift the machine off the ground. It would,
therefore, work very much like an aeroplane machine. It would also be
possible to provide a third screw of less dimensions and running at a
less velocity, to push the machine forward, so as not to render it
necessary to give such a decided tilt to the shafts.

As before stated, great care should be taken in designing and making the
framework of flying machines, and no stone should be left unturned in
order to arrive at the greatest degree of lightness without diminishing
the strength too much; then, again, elasticity should be considered. If
we use a thin tube all the material is at the surface, far from the
neutral centre, and great stiffness is obtained, but such a tube will
not stand so much deflection as a piece of wood; then, again, wood is
cheaper than steel, and in case of an accident, repairs are very quickly
and easily made. Wood, however, cannot be obtained in long lengths
absolutely free from blemishes. It therefore becomes necessary to find
some way of making these long members of flying machines of such wood as
may be found suitable in the following table.

  +---------------------+-------------+-------------+----------+
  |                     |  Strength   | Weight of a |          |
  |                     | per Sq. In. |  Cube Foot  | Relative |
  |                     |   in Lbs.   |   in Lbs.   |  Value.  |
  +---------------------+-------------+-------------+----------+
  | Alder,              |     ...     |    50       |    ...   |
  | Apple,              |     ...     |    49·562   |    ...   |
  | Ash, English,       |    16,000   |    52·812   |   302·9  |
  | Ash, White,         |    14,000   |    43·125   |   324·6  |
  | Bamboo,             |     6,300   |    25       |   252    |
  | Beech, English,     |    11,500   |    53·25    |   215·9  |
  | Birch,              |    15,000   |    45       |   333·3  |
  | Box, African,       |    23,000   |      ...    |    ...   |
  | „    France,        |      ...    |    83       |    ...   |
  | Cedar, American,    |    11,600   |    35·062   |   330·8  |
  | Deal, Christiania,  |    12,400   |      ...    |    ...   |
  | Ebony,              |    27,000   |    83·187   |   324·6  |
  | Elm,                |     6,000   |    35·625   |   168·4  |
  |  „   Rock,          |    13,000   |    50       |   260    |
  | Fir, Norway Spruce, |      ...    |    32       |    ...   |
  |  „   Dantzic,       |      ...    |    36·375   |    ...   |
  | Hackmatack,         |    12,000   |    37       |   324·3  |
  | Hickory,            |    11,000   |    49·5     |   222·2  |
  | Ironwood,           |      ...    |    61·875   |    ...   |
  | Juniper,            |      ...    |    36·375   |    ...   |
  | Lance,              |    23,000   |    45       |   511·1  |
  | Lignum-Vitæ,        |    11,800   |    83·312   |   141·6  |
  | Lime,               |      ...    |    50·25    |    ...   |
  | Locust,             |    20,500   |    45·5     |   450·5  |
  | Mahogany, Honduras, |    21,000   |    35       |   600    |
  |     „     Spanish,  |    12,000   |    53·25    |   225·3  |
  | Maple,              |      ...    |    46·875   |    ...   |
  | Oak, African,       |     9,500   |    51·437   |   184·7  |
  |  „   Canadian,      |      ...    |    54·5     |    ...   |
  |  „   Dantzic,       |     4,200   |    47·437   |    88·5  |
  |  „   English,       |     7,571   |    53·625   |   141·2  |
  |  „   Live,          |    16,380   |    66·75    |   245·4  |
  |  „   Pa, seasoned,  |    20,333   |      ...    |    ...   |
  |  „   White,         |    16,500   |    53·75    |   306·9  |
  |  „   Va,            |    25,222   |      ...    |    ...   |
  | Pine, Norway,       |    14,000   |    46·25    |   302·7  |
  |   „   Pitch,        |      ...    |    41·25    |    ...   |
  |   „   Red,          |    13,000   |    36·875   |   352·5  |
  |   „   White,        |    11,800   |    34·625   |   340·8  |
  |   „   Yellow,       |    13,000   |    28·812   |   451·2  |
  |   „   Va,           |    19,200   |      ...    |    ...   |
  | Poplar,             |     7,000   |    23·937   |   292·4  |
  |   „     White,      |      ...    |    33·062   |    ...   |
  | Redwood, Cal,       |    10,833   |      ...    |    ...   |
  | Spruce,             |    12,400   |    31·25    |   396·8  |
  | Sycamore,           |    13,000   |    38·937   |   333·8  |
  | Tamarack,           |      ...    |    23·937   |    ...   |
  | Teak, African,      |    21,000   |    61·25    |   342·8  |
  |   „   Indian,       |    15,000   |    41·062   |   365·3  |
  | Walnut,             |      ...    |    41·937   |    ...   |
  |    „    Black,      |    16,633   |    31·25    |   532·2  |
  |    „    Michigan,   |    17,500   |      ...    |    ...   |
  | Willow,             |    13,000   |    36·562   |   355·5  |
  +---------------------+-------------+-------------+----------+

The relative value of different kinds of wood is shown in this table,
and it will be observed that some are much more suitable for the purpose
than others. The true value of a wood to be used in flying machines is
only ascertained by considering its strength in comparison with its own
weight--that is, the wood which is strongest in proportion to its weight
is the best. It will be seen that Honduras mahogany stands at the head
of the list, but American white pine is very good for certain purposes,
as it is light, strong, easily obtained, and takes the glue very well
indeed. In Fig. 45, I have shown a good system of producing the long
members necessary in flying machines. I will admit that it costs
something to fit up and produce the kind of joints which I have shown,
but when the members are once made, they are exceedingly strong and
stiff. Fig. 46 shows sections of the struts, and these may be made of
either straight-grained Honduras mahogany or of lance wood; either
answers the purpose very well, because being very strong and
straight-grained, permits the struts to be made of such a shape and size
as to offer very little resistance in cutting their way through the air.
The framework of the aeroplane unless carefully designed will offer
great resistance to being driven through the air. Suppose that the
bottom member of the truss (Fig. 47) is straight, and the top one curved
in the direction shown; no matter how taut the cloth may be drawn, the
pressure of the air will cause it to bag upwards between the different
trusses, so as to present very nearly the correct curve which is
necessary to produce the maximum lifting effect, and without offering
too much resistance to the air; however, one must not forget for a
single moment that the air flows over both sides of the aeroplane. When
the aeroplane is made very thick in the middle and sharp at the edges
(Fig. 48), with the bottom side dead level, it produces a decided
lifting effect no matter which way it is being propelled through the
air. This is not because the bottom side produces any lifting effect of
itself, but because the air running over the top follows the surface.
The aeroplane encounters air which is not moving at all. The air is
first moved upwards slightly, but it also has to run down the incline to
the rear edge of the aeroplane, so that, when it is discharged, it has a
decided downward trend; therefore, the air passing over the top side
instead of under the bottom side, produces the lifting effect, showing
that the top side of an aeroplane as well as the lower side should be
considered. The top side should, therefore, be free from all
obstructions.

[Illustration: Fig. 45.--System of splicing and building up wooden
members. When they have to be curved and to keep their shape, they
should be bent at the curve at the time of being glued together, and
joined in the middle as at _d_.]

[Illustration: Fig. 46.--Cross-section of struts.]

[Illustration: Fig. 47.--Truss suitable for use with flying machines,
having aeroplanes about 6 feet to 8 feet wide.]

The top of the aeroplane as well as the bottom should be covered with
some light material, if the very best results are to be obtained. In
another chapter I have shown a form of fabric-covered aeroplane, made by
myself, that was not distorted in the least by the air pressure, and
produced just as good effects as it would have done if it had been
carefully carved out of a piece of wood. On more than one occasion Lord
Kelvin came to my place; he said that my workshop was a perfect museum
of invention. At the Oxford Meeting of the British Association for the
Advancement of Science, Lord Salisbury in the chair, I was much
gratified when Lord Kelvin said that he had examined my work, and found
that it was beautifully designed and splendidly executed. He
complimented me very highly indeed. While at my place, he said that the
most ingenious thing that he had seen was the way I had prevented my
aeroplanes from being distorted by the air. He spoke of this several
times with great admiration, and, I think, if the fabric-covered
aeroplane is to be used at all, that my particular system will be found
altogether the best.

[Illustration: Fig. 48.--The paradox aeroplane that lifts no matter in
which direction it is being driven.]

[Illustration: Fig. 49.--The Antoinette motor.]

Regarding the motors now being employed, I think that there is still
room for a great deal of improvement in the direction of greater
lightness, higher efficiency and reliability. At the present time,
flying machine motors have such small cylinders, the rotation is so
rapid, and the cooling appliances so imperfect, that the engine soon
becomes intensely heated, and then its efficiency is said to fall off
about 40 or 50 per cent., some say even 60 per cent. This is probably on
account of the high temperature of the cylinder, piston, and air inlet.
The heat expands the air as it enters, so that the actual weight of air
in the cylinder is greatly reduced, and the engine power reduced in a
corresponding degree. There is no trouble about cooling the motor, and a
condenser of high efficiency may be made that will cool the water
perfectly, and, at the same time, lift a good deal more than its own
weight. All the conditions are favourable for using a very effective
atmospheric condenser (see Figs. 30 and 31).

Water may be considered as 2400 times as efficient as air, volume for
volume, in condensing steam. When a condenser is made for the purpose of
using water as a cooling agent, a large number of small tubes may be
closely grouped together in a box, and the water pumped in at one end of
the box and discharged at the other end through relatively small
openings; but when air is employed, the tubes or condensing surfaces
must be widely distributed, so that a very large amount of air is
encountered, and air which has struck one tube and become heated must
never touch a second tube (see Figs. 30 and 31, also Appendix).

[Illustration: Fig. 50.--Section showing the Antoinette motor, such as
used in the Farman and De la Grange machines.]

Fig. 51 shows a pneumatic buffer which I have designed, in which _a_,
_a_, is a steel tube highly polished on the inside; _b_, a nozzle for
connecting the air-pump, which is of the bicycle variety; _c_, a nipple
to which is attached a strong india-rubber bulb; _d_, a piston which is
made air-tight by a leather cup; and _f_, the connection to the lever
carrying the wheels on which the machine runs. While the machine is at a
state of rest on the ground, the piston-rod _d_, is run out to its full
extent, and supports the weight of the machine--the pressure being about
150 lbs. to the square inch. When, however, the machine comes violently
down to the earth, the piston is pushed inward, compressing the air,
and by the time it has travelled, say, one-half the stroke, the air
pressure will have mounted to 300 lbs. to the square inch. At this
point, the rubber bulb _c_, ought to burst and allow the compressed air
to escape under a high pressure. Air escaping through a relatively small
hole absorbs the momentum of the descent and brings the machine to a
state of rest without a destructive shock. It is, of course, necessary
for the navigator to select a broad and level field for descent, and
then to approach it from the leeward and slow up his machine as near the
ground as possible, tilting the forward end upwards in order to arrest
its forward motion, and touching the ground while still moving against
the wind at a fairly high velocity. If all these points are studied, and
well carried out, very little danger will result; then, again, the
aeroplanes _b_, _b_, and the forward rudder _d_ (Fig. 41), should be so
arranged that, in case of an accident, their outward sides may be
instantly turned upwards, in such a manner as to prevent the machine
from plunging, and keep it on an even keel while the engines are not
running.

[Illustration: Fig. 51.--Pneumatic buffer--_a_, _a_, cylinder; _b_,
attachment for pumping up; _c_, air outlet, covered with a rubber
thimble made to burst under about 300 lbs. pressure; _d_, the piston.]


STEERING BY MEANS OF A GYROSCOPE.

A ship at sea has only to be steered in a horizontal direction; the
water in which it is floated assures its stability in a vertical
direction; but when a flying machine is once launched in the air, it has
to be steered in two directions--that is, the vertical and the
horizontal. Moreover, it is constantly encountering air currents that
are moving with a much higher velocity than any water currents that have
ever to be encountered. It is, therefore, evident that, as far as
vertical steering is concerned, it should be automatic. Some have
suggested shifting weights, flowing mercury, and swinging pendulums; but
none of these is of the least value, on account of the swaying action
which always has to be encountered. A pendulum could not be depended
upon for working machinery on board a ship, and the same laws apply to
an airship. We have but one means at our disposal, and that is the
gyroscope. When a gyroscope is spun at a very high velocity on a
vertical axis, with the point of support very much above the center of
gyration, it has a tendency to maintain a vertical axis; a horizontal or
swinging motion of its support will not cause it to swing like a
pendulum. It therefore becomes possible by its use to maintain an
airship on an even keel. In a steam steering apparatus, such as is used
on shipboard, it is not sufficient to apply steam-power to move the
rudders, unless some means are provided whereby the movement of the
rudder closes off the steam, otherwise the rudder might continue to
travel after the effect had been produced, and ultimately be broken; and
so it is with steering a flying machine in a vertical direction.
Whenever the fore and aft rudders respond to the action of the gyroscope
and are set in motion, they must at once commence to shut off the power
that works them, otherwise they would continue to travel. In the
photograph (Fig. 52) I have shown an apparatus which I constructed at
Baldwyn’s Park. It will be seen that the gyroscope is enclosed in a
metal case; a tangent screw, just above the case, rotates a pointer
around a small disc, which admits of the speed of the gyroscope being
observed. Steam is admitted through a universal joint, descends through
the shaft and escapes through a series of small openings placed at a
tangent, so as to give rotation to the wheel after the manner of a
Barker’s mill. The casing about the rotating wheel is extremely light as
relates to the wheel, so that, when the gyroscope is once spun on a
vertical axis, the rest of the apparatus may be tilted in any direction,
while the gyroscope and its attachments maintain a vertical axis. The
gyroscope and its attachments are suspended from a long steel tube,
which in reality is a steam cylinder. The sleeve which supports the
gyroscope moves freely in a longitudinal direction, and the whole is
held in position by a triple-threaded screw on the small tube above the
cylinder. The steam is admitted through a piston value operated by a
species of link motion, as shown. The piston-rod extends to each end of
the cylinder, and regulates the rudders by pulling a small wire rope,
the travel of the piston being about 8 feet. At the end of the cylinder
(not shown) the piston-rod is provided with an arm and a nut which
engages the small top tube--this tube being provided with a long
spiral--so that, as the piston moves, the top tube is rotated, and
thereby slides the gyroscope’s support, and changes its position as
relates to the piston valve. It will, therefore, be seen that the action
is the same as with the common steam steering gear used on shipboard. A
little adjusting screw at the right hand of the print is shown. The
upward projecting arm of the bell crank lever is for the purpose of
attaching the wooden handle, making it possible to move the
connecting-rod instantly into a position where the steam piston will
move the rudders into the position shown (Fig. 56).

I copy the following from a description which I wrote of this apparatus
at the time:--

“GYROSCOPE APPARATUS FOR AUTOMATICALLY STEERING MACHINE IN A VERTICAL
DIRECTION.

“This apparatus consists of a long steam cylinder which is provided with
a piston, the piston-rod extending beyond the cylinder at each end; the
ropes working the fore and aft rudders are attached to the ends of this
piston-rod, and steam is supplied through an equilibrium valve. The
gyroscope is contained in a gunmetal case, and is driven by a jet of
steam entering through the trunnions. When the gyroscope is spinning at
a high velocity, the casing holding it becomes very rigid and is not
easily moved from its vertical position. If the machine rears or
pitches, the cylinder and valve are moved with the machine while the
gyroscope remains in a vertical position. This causes the steam valve to
be moved so as to admit steam into the cylinder and move the piston in
the proper direction to instantly bring the machine back into its normal
position. As the fore and aft rudders are moved, the long tubular shaft
immediately over the steam cylinder is rotated in such a manner as to
move the whole gyroscope in the proper direction to close off the steam.
The apparatus may be made to regulate at any angle by adjusting the
screw which regulates the position of the tubular shaft. The link that
suspends the end of the steam valve connecting-rod is supported by a
bell crank lever, and while the machine is moving ahead, the lever
occupies the position shown in the photograph (Fig. 52); but if the
machinery and engine stop, the bell crank lever may be moved so as to
throw the connecting-rod below the centre, when the steam will move the
piston in the proper direction to throw both the rudders into the
falling position, as shown in Fig. 56.”

[Illustration: Fig. 52.--Gyroscope, used for the control of the fore and
aft horizontal rudders, thus keeping the machine on an even keel while
in the air.]

[Illustration: Fig. 53.--In order to adjust the lifting effect so that
it was directly over the centre of gravity, and to test the action of my
fore and aft horizontal rudders, I ran the machine along the steel rail
_i_, _i_, and adjusted my weights and aeroplanes in such a manner that,
when the machine was run at a speed of 30 miles an hour along the track,
with the rudders adjusted in the manner shown, the front wheel _j_, was
raised from the steel track and the small wheel _m_, brought into
contact with the upper track _h_. When the rudder _b_, _b_, is in this
position, it produces a strong lifting effect, while the rudder _c_,
_c_, does not lift at all.]

[Illustration: Fig. 54.--This shows the rudders placed in such a
position that _b_, _b_, does not lift at all, while _c_, is placed at
such an angle as to produce a strong lifting effect, especially so as it
is in the blast of the screws _d_, _d_. With the rudders in this
position, and at a speed of 30 miles an hour, I was able to lift the
rear wheels _k_, _k_, off the steel rails and to bring the small wheel
_l_, in contact with the upper track _h_. These experiments showed that
the machine could be tilted in either direction by changing the position
of the rudder.]

[Illustration: Fig. 55.--When the rudders were placed in the position
shown, and the machine was run over the track at a rate of 40 miles an
hour, all the weight was lifted off the wheels, _j_, and _k_, and both
the small wheels _m_, and _l_, engaged the upper track.]

[Illustration: Fig. 56.--In case of a breakdown or failure of the
engines when the machine is in flight, it is necessary to place the
rudders in the position shown, in order to prevent the machine from
diving to the earth. When the rudders are in this position, a rapid and
destructive descent is not possible, as the machine will preserve an
even keel while falling.]



CHAPTER VII.

THE SHAPE AND EFFICIENCY OF AEROPLANES.


In Prof. Langley’s lifetime, we had many discussions regarding the width
and shape of aeroplanes. The Professor had made many experiments with
very small and narrow planes, and was extremely anxious to obtain some
data regarding the effect that would be produced by making the planes of
greater width. He admitted that by putting some two or three aeroplanes
tandem, and all at the same angle, the front aeroplane _a_ (Fig. 57),
would lift a great deal more than _b_, and that _c_, would lift still
less. He suggested the arrangement shown at _a′_, _b′_, _c′_, in which
_b′_ is set at such an angle as to give as much additional acceleration
to the air as it had received in the first instance by passing under
_a′_, and that _c′_, should also increase the acceleration to the same
extent. With this arrangement, the lifting effect of the three
aeroplanes ought to be the same, but I did not agree with this theory.
It seemed to me that it would only be true if it dealt with the volume
of air represented between _j_, and _k_, and that he did not take into
consideration the mass of air between _k_, and _l_, that had to be dealt
with, and which would certainly have some effect in buoying up the
stream of air, _j_, _k_. Prof. Langley admitted the truth of this, and
said that nothing but experiment would demonstrate what the real facts
were. But it was a matter which I had to deal with. I did not like the
arrangement _a′_, _b′_, _c′_, as the angle was so sharp, especially at
_c′_, that a very large screw thrust would be necessary. I therefore
made a compromise on this system which is shown at _a′′_, _b′′_, _c′′_.
In this case _a′′_, has an inclination of 1 in 10, _b′′_ an inclination
of 1 in 6, and _c′′_ an inclination of 1 in 5. It will be seen that this
form, which is shown as one aeroplane at _a′′′_, _b′′′_, _c′′′_, is a
very good shape. It is laid out by first drawing the line _c_, _d_,
dropping the perpendicular equal to one-tenth of the distance between
_c_ and _d_, and then drawing a straight line from _c_, through _e_, to
_f_, where another perpendicular is dropped, and half the distance
between _d_ and _e_ laid off, and another straight line drawn from _e_,
through _g_, to _h_, and the perpendicular _h_, _i_, laid off the same
as _f_, _g_. We then have four points, and by drawing a curve through
these, we obtain the shape of the aeroplane shown above, which is an
exceedingly good one. This shape, however, is only suitable for
velocities, up to 40 miles per hour; at higher velocities, the curvature
would be correspondingly reduced.

[Illustration: Fig. 57.--Diagram showing the evolution of a wide
aeroplane.]


THE ACTION OF AEROPLANES AND THE POWER REQUIRED EXPRESSED IN THE
SIMPLEST TERMS.

In designing aeroplanes for flying machines, we should not lose sight of
the fact that area alone is not sufficient. Our planes must have a
certain length of entering edge--that is, the length of the front edge
must bear a certain relation to the load lifted. An aeroplane 10 feet
square will not lift half as much for the energy consumed as one 2 feet
wide and 50 feet long; therefore, we must have our planes as long as
possible from port to starboard. At all speeds of 40 miles per hour or
less, there should be at least 1 foot of entering edge for every 4 lbs.
carried. However, at higher speeds, the length may be reduced as the
square of the speed increases. An aeroplane 1 foot square will not lift
one-tenth as much as one that is 1 foot wide and 10 feet long. This is
because the air slips off at the ends, but this can be prevented by a
thin flange, or _à la_ Hargrave’s kites. An aeroplane 2 feet wide and
100 feet long placed at an angle of 1 in 10, and driven edgewise through
the air at a velocity of 40 miles per hour, will lift 2·5 lbs. per
square foot. But as we find a plane 100 feet in length too long to deal
with, we may cut it into two or more pieces and place them one above the
other--superposed. This enables us to reduce the width of our machine
without reducing its lifting effect; we still have 100 feet of entering
edge, we still have 200 feet of lifting surface, and we know that each
foot will lift 2·5 lbs. at the speed we propose to travel. 200 × 2·5 =
500; therefore our total lifting effect is 500 lbs., and the screw
thrust required to push our aeroplane through the air is one-tenth of
this, because the angle above the horizontal is 1 in 10. We, therefore,
divide what Prof. Langley has so aptly called the “lift” by 10; 500/10
= 50. It will be understood that the vertical component is the lift, and
the horizontal component the drift, the expression “drift” also being a
term first applied by Prof. Langley. Our proposed speed is 40 miles per
hour, or 3,520 feet in a minute of time. If we multiply the drift in
pounds by the number of feet travelled in a minute of time, and divide
the product thus obtained by 33,000, we ascertain the H.P. required--

  50 × 3,520
  ---------- = 5·33.
    33,000

It therefore takes 5·33 H.P. to carry a load of 500 lbs. at a rate of 40
miles per hour, allowing nothing for screw slip or atmospheric
resistance due to framework and wires. But we find we must lift more
than 500 lbs., and as we do not wish to make our aeroplanes any longer,
we add to their width in a fore and aft direction--that is, we place
another similar aeroplane, also 2 feet wide, just aft of our first
aeroplane. This will, of course, have to engage the air discharged from
the first, and which is already moving downwards. It is, therefore, only
too evident that if we place it at the same angle as our first
one--viz., 1 in 10--it will not lift as much as the first aeroplane, and
we find that if we wish to obtain a fairly good lifting effect, it must
be placed at an angle of 1 in 6. Under these conditions, the screw
thrust for this plane will be 1/6th part of the lift, or 8·88 H.P.
against 5·33 H.P. with our first aeroplane. In order to avoid confusion,
we will call our first plane _a′′_, our second plane _b′′_, and the
third _c′′_, the same as in Fig. 57. Still we are not satisfied, we want
more lift, we therefore add still another aeroplane as shown (_c′′_,
Fig. 57). This one has to take the air which has already been set in
motion by the two preceding planes _a′′_ and _b′′_, so in order to get a
fair lifting effect, we have to place our third plane at the high angle
of 1 in 5. At this angle, our thrust has to be 1/5th of the lifting
effect, and the H.P. required is twice as much per pound carried as with
the plane _a′′_, where the angle was 1 in 10; therefore, it will take
10·66 H.P. to carry 500 lbs. As there is no reason why we should have
three aeroplanes placed tandem where one would answer the purpose much
better, we convert the whole of them into one, as shown (_a′′′_, _b′′′_,
_c′′′_, Fig. 57), and by making the top side smooth and uniform, we get
the advantage of the lifting effect due to the air above the aeroplane
as well as below it. The average H.P. is therefore 5·33 + 8·88 + 10·66 ÷
3 = 8·29 H.P. for each plane, or 25 H.P. for the whole, which is at the
rate of 60 lbs. to the H.P., all of which is used to overcome the
resistance due to the weight and the inclination of the aeroplanes, and
which is about half the total power required. We should allow as much
more for loss in screw slip and atmospheric resistance due to the motor,
the framework, and the wires of the machine. If, however, the screw is
placed in the path of the greatest resistance, it will recover a portion
of the energy imparted to the air. We shall, however, require a 50 H.P.
motor, and thus have 30 lbs. to the H.P.

From the foregoing it will be seen that at a speed of 40 miles an hour,
the weight per H.P. is not very great. If we wish to make a machine more
efficient, we must resort to a multitude of very narrow superposed
planes, or sustainers, as Mr. Philipps calls them, or we must increase
the speed. If an aeroplane will lift 2·5 lbs. per square foot placed at
an angle of 1 in 10, and driven at a velocity of 40 miles an hour, the
same aeroplane will lift 1·25 lbs. if placed at an angle of 1 in 20, and
as the lifting effect varies as the square of the velocity, the same
plane will lift as much more at 60 miles per hour, as 60² is greater
than 40²--that is, 2·81 lbs. per square foot instead of 1·25 lbs. At
this high speed, providing that the width of the plane is not more than
3 feet, it need be only slightly curved and have a mean angle of 1 in
20.

An aeroplane 100 feet long and 3 feet wide would have 300 square feet of
lifting surface, each of which would lift 2·81 lbs., making the total
lifting effect 843 lbs. 843 ÷ 20 = 42·15, which is the screw thrust that
would be necessary to propel such a plane through the air at a velocity
of 60 miles per hour. 60 miles per hour is 5,280 feet in a minute,
therefore the H.P. required is 42·15 × 5,280 ÷ 33,000 = 6·7 H.P.
Dividing the total lifting effect 843 by 6·7, we have 843 ÷ 6·7 = 125·8,
the lift per H.P. If we allow one-half for loss in friction, screw slip,
etc., we shall be carrying a load of 843 lbs. with 13·4 H.P. It will,
therefore, be seen that a velocity of 60 miles an hour is much more
economical in power than the comparatively low velocity of 40 miles an
hour; moreover, it permits of a considerable reduction in the size and
weight of the machine, and this diminishes the atmospheric resistance.

[Illustration: Fig. 58.--In a recently published mathematical treatise
on Aerodynamics, an illustration is shown, representing the path that
the air takes on encountering a rapidly moving curved aeroplane. It will
be observed that the air appears to be attracted upwards before the
aeroplane reaches it, exactly as iron filings would be attracted by a
magnet, and that the air over the top of the aeroplane is thrown off at
a tangent, producing a strong eddying effect at the top and rear. Just
why the air rises up before the aeroplane reaches it is not plain, and
as nothing could be further from the facts, mathematical formulas
founded on such a mistaken hypothesis can be of but little value to the
serious experimenter on flying machines.]

[Illustration: Fig. 59.--An illustration from another scientific
publication also on the Dynamics of Flight. It will be observed that the
air in striking the underneath side of the aeroplane is divided into two
streams, a portion of it flowing backwards and over the top of the edge
of the aeroplane where it becomes compressed. An eddy is formed on the
back and top of the aeroplane, and the air immediately aft the aeroplane
is neither rising nor falling. Just how these mathematicians reason out
that the air in striking the front of the aeroplane would jump backwards
and climb up over the top and leading edge against the wind pressure is
not clear.]

[Illustration: Fig. 60.--This shows another illustration from the same
mathematical work, and represents the direction which the air is
supposed to take on striking a flat aeroplane. With this, the air is
also divided, a portion moving forward and over the top of the aeroplane
where it is compressed, leaving a large eddy in the rear, and, as the
dotted lines at the back of the aeroplane are horizontal, it appears
that the air is not forced downwards by its passage. Here, again,
formula founded on such hypothesis is misleading in the extreme.]

[Illustration: Fig. 61.--This shows the shape and the practical angle of
an aeroplane. This angle is 1 in 10, and it will be observed that the
air follows both the upper and the lower surface, and that it leaves the
plane in a direction which is the resultant of the top and bottom
angle.]

[Illustration: Fig. 62.--This shows an aeroplane of great thickness,
placed at the highest angle that will ever be used--1 in 4--and even
with this the air follows the upper and lower surfaces. No eddies are
formed, and the direction that the air takes after leaving the aeroplane
is the resultant of the top and bottom angles.]

[Illustration: Fig. 63.--Section of a screw blade having a rib on the
back. The resistance caused by this rib is erroneously supposed to be
skin friction.]

[Illustration: Fig. 64.--Shows a flat aeroplane placed at an angle of
45°, an angle which will never be used in practical flight, but at this
angle the momentum of the approaching air and the energy necessary to
give it an acceleration sufficiently great to make it follow the back of
the aeroplane are equal, and at this point, the wind may either follow
the surface or not. Sometimes it does and sometimes it does not. See
experiments with screws.]

[Illustration: Fig. 65.--The aeroplane here shown is a mathematical
paradox. This aeroplane lifts, no matter in which direction it is
driven. It encounters air which is stationary and leaves it with a
downward trend; therefore it must lift. However, if we remove the
section _b_, and only subject _a_ to the blast, as shown at Fig. 66, no
lifting effect is produced. On the contrary, the air has a tendency to
press _a_, downwards. The path which the air takes is clearly shown;
this is most important, as it shows that the shape of the top side is a
factor which has to be considered. All the lifting effect in this case
is produced by the top side.]

[Illustration: Fig. 66.]

[Illustration: Fig. 67.--In this drawing _a_ represents an aeroplane, or
a bird’s wing. Suppose that the wind is blowing in the direction of the
arrows; the real path of the bird as relates to the air is from _i_ to
_j_,--that is, the bird is falling as relates to the air although moving
on the line _c_, _d_, against the wind. In some cases, a bird is able to
travel along the line _g_, _h_, instead of in a horizontal direction,
thus rising and apparently flying into the teeth of the wind at the same
time.]


SOME RECENT MACHINES.

Professor S. P. Langley, of the Smithsonian Institute, Washington, D.C.,
made a small flying model in 1896. This, however, only weighed a few
pounds; but as it did actually fly and balance itself in the air, the
experiment was of great importance, as it demonstrated that it was
possible to make a machine with aeroplanes so adjusted as to steer
itself automatically in a horizontal direction. In order to arrive at
this result, an innumerable number of trials were made, and it was only
after months of careful and patient work that the Professor and his
assistants succeeded in making the model fly in a horizontal direction
without rearing up in front, and then pitching backwards, or plunging
while moving forward.

The Wright Brothers of Dayton, Ohio, U.S.A., often referred to as “the
mysterious Wrights,” commenced experimental work many years ago. The
first few years were devoted to making gliding machines, and it appears
that they attained about the same degree of success as many others who
were experimenting on the same lines at the same time; but they were not
satisfied with mere gliding machines, and so turned their attention in
the direction of motors. After some years of experimental work, they
applied their motor to one of their large gliding machines, and it is
said that with this first machine they actually succeeded in flying
short distances. Later on, however, with a more perfect machine, they
claim to have made many flights, amongst which I will mention three: 12
miles in 20 minutes, on September 29th, 1905; 20·75 miles in 33 minutes,
on October 4th; and 24·2 miles in 38 minutes, on October 5th of the same
year. As there seems to be much doubt regarding these alleged flights,
we cannot refer to them as facts until the Wright Brothers condescend to
show their machine and make a flight in the presence of others;
nevertheless, I think we are justified in assuming that they have met
with a certain degree of success which may or may not be equal to the
achievements of Messrs Farman and De la Grange. It is interesting to
note in this connection that all flying machines that have met with any
success have been made on the same lines; all have superposed
aeroplanes, all have fore and aft horizontal rudders, and all are
propelled with screws; and in this respect they do not differ from the
large machine that I made at Baldwyn’s Park many years ago. I have seen
both the Farman and the De la Grange machines; they seem to be about the
same in size and design, and what is true of one is equally true of the
other; I will, therefore, only describe the one that seems to have done
the best--the De la Grange. The general design of this machine is
clearly shown in the illustrations (Figs. 68 and 69). The dimensions are
as follows: The two main aeroplanes are 32·8 feet long and 4·9 feet
wide; the tail or after rudder is made in the form of a Hargrave’s box
kite, the top and bottom sides of the box being curved and covered with
balloon fabric, thus forming aeroplanes. This box is 9·84 feet long from
port to starboard, and 6·56 feet wide in a fore and aft direction. The
diameter of the screw is 7·2 feet and it has a mean pitch of 5·7 feet.
The screw blades are two in number and are extremely small, being only
6·3 inches wide at the outer end and 3·15 inches at the inner end, their
length being 2·1 feet. The space between the fore and aft aeroplanes is
4·9 feet. The total weight is about 1,000 lbs. with one man on board.
The speed of this machine through the air is not known with any degree
of certainty; it is, however, estimated to be 32 to 40 miles per hour.
When the screw is making 1,100 revolutions per minute, the motor is said
to develop 50 H.P.

[Illustration: Fig. 68.--The De la Grange machine on the ground and
about to make a flight.]

[Illustration: Fig. 69.--The De la Grange machine in full flight and
very near the ground.]

In the following calculations, I have assumed that the machine has the
higher speed--40 miles per hour. I have been quite unable to obtain any
reliable data regarding the angle at which the aeroplanes are set, but
it would appear that the angle is about 1 in 10. The total area of the
two main aeroplanes is 321·4 square feet. A certain portion of the lower
main aeroplane is cut away, but this is compensated for by the forward
horizontal rudder placed in the gap thus formed. The two rear aeroplanes
forming the tail of the machine have an area of 128·57 square feet. The
area of all the aeroplanes is, therefore, 450 square feet. As the weight
of the machine is 1,000 lbs., the lift per square foot is 2·2 lbs.
Assuming that the angle of the aeroplanes is 1 in 10, the screw thrust
would be 100 lbs., providing, however, that the aeroplanes were perfect
and no friction of any kind was encountered. Forty miles per hour is at
the rate of 3,520 feet in a minute of time, therefore, (3,520 ×
100)/33,000 = 10·66 H.P. If we allow another 10 H.P. for atmospheric
resistance due to the motor, the man, and the framework of the machine,
it would require 20·66 H.P. to propel the machine through the air at the
rate of 40 miles per hour. If the motor actually develops 50 H.P., 29
H.P. will be consumed in screw slip and overcoming the resistance due
to the imperfect shape of the screw. The blades of the De la Grange
screw propeller are extremely small, and the waste of energy is,
therefore, correspondingly great--their projected area being only 1·6
square feet for both blades. Allowing 200 lbs. for screw thrust, we have
the following: 200/1·60 = 125 lbs. pressure per square foot on the
blades. If we multiply the pitch of the screw in feet by the number of
revolutions per minute, we find that if it were travelling in a solid
nut it would advance over 70 miles an hour. By the Eiffel tower formula
P = 0·003 V², a wind blowing at a velocity of 70 miles per hour produces
a pressure of 14·7 lbs. per square foot on a normal plane; therefore,
assuming that the projected area of the screw blades is 1·6, we have 1·6
× 14·7 = 23·52 lbs., which is only one-fifth part of what the pressure
really is when the screws are making 1,100 turns a minute. It is
interesting to note that the ends of the screw blades travel at a
velocity of 414 feet per second, which is about one-half the velocity of
a cannon ball fired from an old-fashioned smooth bore.

[Illustration: Fig. 70.--Farman’s machine in flight].

A flying machine has, of course, to be steered in two directions at the
same time--the vertical and the horizontal. In the Farman and De la
Grange machines, the horizontal steering is effected by a small windlass
provided with a hand wheel, the same as on a steam launch, and the
vertical steering is effected by a longitudinal motion of the shaft of
the same windlass. As the length of the machine is not very great, it
requires very close attention on the part of the man at the helm to keep
it on an even keel; if one is not able to think and act quickly,
disaster is certain. On one occasion, the man at the wheel pushed the
shaft of the windlass forward when he should have pulled it back, and
the result was a plunge and serious damage to the machine; happily no
one was injured, though some of the bystanders were said to have had
very narrow escapes. The remedy for this is to make all hand-steered
machines of great length, which gives more time to think and act; or,
still better, to make them automatic by the use of a gyroscope.

[Illustration: Fig. 71.--Bleriot’s machine. This machine raised itself
from the ground, but as the centre of gravity was very little, if any,
above the centre of lifting effect, it turned completely over in the
air.]

[Illustration: Fig. 72.--Santos Dumont’s flying machine.]

VELOCITY AND PRESSURE OF THE WIND.

  The pressure varies as the square of the velocity or P ∝ V². The old
  formula for wind blowing against a normal plane was P = 0·005 × V².
  The latest or Eiffel Tower formula gives a much smaller value, being P
  = 0·003 × V², where V represents the velocity in miles per hour, and P
  the pressure in pounds per square foot.

  +---------------------------------+---------+----------------------+
  |          VELOCITY.              |Pressure |                      |
  |                                 |  on a   |Character of the Wind.|
  +---------+-----------+-----------+Sq. Foot.|                      |
  |Per Hour.|Per Minute.|Per Second.|         |                      |
  +---------+-----------+-----------+---------+----------------------+
  | Miles.  |   Feet.   |    Feet.  |   Lbs.  |                      |
  |   1     |     88    |     1·5   |    ·003 |  Barely observable.  |
  |   2     |    176    |     2·9   |    ·012 |} Just                |
  |   3     |    264    |     4·4   |    ·027 |} perceptible.        |
  |   4     |    352    |     5·9   |    ·048 |  Light breeze.       |
  |   5     |    440    |     7·3   |    ·075 |} Gentle,             |
  |   6     |    528    |     8·8   |    ·108 |} pleasant            |
  |   8     |    704    |    11·7   |    ·192 |} wind.               |
  |  10     |    880    |    14·7   |    ·3   |  Fresh breeze.       |
  |  15     |  1,320    |    22     |    ·675 |  Brisk breeze.       |
  |  20     |  1,760    |    29·4   |   1·2   |  Stiff breeze.       |
  |  25     |  2,200    |    36·7   |   1·875 |  Very brisk breeze.  |
  |  30     |  2,640    |    44     |   2·7   |} High                |
  |  35     |  3,080    |    51·3   |   3·675 |} wind.               |
  |  40     |  3,520    |    58·7   |   4·8   |  Very high wind.     |
  |  45     |  3,960    |    66     |   6·075 |  Gale.               |
  |  50     |  4,400    |    73·4   |   7·5   |  Storm.              |
  |  60     |  5,280    |    88     |  10·8   |} Great               |
  |  70     |  6,160    |   102·7   |  14·7   |} storm.              |
  |  80     |  7,040    |   117·2   |  19·2   |  Hurricane.          |
  |  90     |  7,920    |   132     |  24·3   |}                     |
  | 100     |  8,800    |   146·7   |  30     |} Tornado.            |
  | 110     |  9,680    |   161·2   |  36·3   | }                    |
  | 120     | 10,560    |   176     |  43·2   | } “Washoe            |
  | 130     | 11,440    |   191     |  50·7   | } zephyrs.”[2]       |
  | 140     | 12,320    |   205·3   |  58·8   | }                    |
  | 150     | 13,200    |   220     |  67·5   | }                    |
  +---------+-----------+-----------+---------+----------------------+

  [2] With apologies to Mark Twain.

[Illustration: Fig. 72_a_.--Angles and degrees compared. It will be
observed that an angle of 1 in 4 is practically 14°.]

TABLE OF EQUIVALENT INCLINATIONS.

  +------------+----------------+-------------------+
  |   Rise.    | Sine of Angle. | Angle in Degrees. |
  +------------+----------------+-------------------+
  |  1 in 30,  |      ·0333     |        1·91       |
  |  1  „ 25,  |      ·04       |        2·29       |
  |  1  „ 20,  |      ·05       |        2·87       |
  |  1  „ 18,  |      ·0555     |        3·18       |
  |  1  „ 16,  |      ·0625     |        3·58       |
  |  1  „ 14,  |      ·0714     |        4·09       |
  |  1  „ 12,  |      ·0833     |        4·78       |
  |  1  „ 10,  |      ·1        |        5·73       |
  |  1  „  9,  |      ·1111     |        6·38       |
  |  1  „  8,  |      ·125      |        7·18       |
  |  1  „  7,  |      ·143      |        8·22       |
  |  1  „  6,  |      ·1667     |        9·6        |
  |  1  „  5,  |      ·2        |       11·53       |
  |  1  „  4,  |      ·25       |       14·48       |
  |  1  „  3,  |      ·3333     |       19·45       |
  +------------+----------------+-------------------+

TABLE OF EQUIVALENT VELOCITIES.

  +-----------+-------------+-------------+-------------+-------------+
  |           |             |             |             |             |
  |   Miles   |     Feet    |     Feet    |    Metres   |    Metres   |
  | per Hour. | per Second. | per Minute. | per Minute. | per Second. |
  +-----------+-------------+-------------+-------------+-------------+
  |           |             |             |             |             |
  |     1,    |      1·5    |        88   |      26·8   |      ·447   |
  |     2,    |      2·9    |       176   |      53·6   |      ·894   |
  |     3,    |      4·4    |       264   |      80·5   |     1·341   |
  |     4,    |      5·9    |       352   |     107·3   |     1·788   |
  |     5,    |      7·3    |       440   |     134·1   |     2·235   |
  |     6,    |      8·8    |       528   |     160·9   |     2·682   |
  |     8,    |     11·7    |       704   |     214·6   |     3·576   |
  |    10,    |     14·7    |       880   |     268·2   |     4·470   |
  |    15,    |     22      |     1,320   |     402·3   |     6·705   |
  |    20,    |     29·4    |     1,760   |     536·4   |     8·940   |
  |    25,    |     36·7    |     2,200   |     670·5   |    11·176   |
  |    30,    |     44      |     2,640   |     804·6   |    13·411   |
  |    35,    |     51·3    |     3,080   |     938·8   |    15·646   |
  |    40,    |     58·7    |     3,520   |   1,072·9   |    17·881   |
  |    45,    |     66      |     3,960   |   1,207     |    20·116   |
  |    50,    |     73·4    |     4,400   |   1,341·1   |    22·352   |
  |    60,    |     88      |     5,280   |   1,609·2   |    26·822   |
  |    70,    |    102·7    |     6,160   |   1,877·5   |    31·292   |
  |    80,    |    117·2    |     7,040   |   2,145·8   |    35·763   |
  |    90,    |    132      |     7,920   |   2,414     |    40·233   |
  |   100,    |    146·7    |     8,800   |   2,682·2   |    44·704   |
  |   110,    |    161·2    |     9,680   |   2,950·2   |    49·174   |
  |   120,    |    176      |    10,560   |   3,218·4   |    53·644   |
  |   130,    |    191      |    11,440   |   3,486·6   |    58·115   |
  |   140,    |    205·3    |    12,320   |   3,755·1   |    62·585   |
  |   150,    |    220      |    13,200   |   4,023·3   |    67·056   |
  +-----------+-------------+-------------+-------------+-------------+

To convert feet per minute into metres per second, multiply by ·00508.

TABLE SHOWING VELOCITY AND THRUST CORRESPONDING WITH VARIOUS
HORSE-POWERS.

  +---------+-------------------------------------------------+
  |Velocity |                 Horse-Power.                    |
  |in Miles +------+-------+-------+--------+--------+--------+
  |per Hour.|   1  |   10  |   20  |   30   |   40   |   50   |
  |         +------+-------+-------+--------+--------+--------+
  |         |               Thrust in Pounds.                 |
  +---------+------+-------+-------+--------+--------+--------+
  |    1,   |375   |3,750  |7,500  |11,250  |15,000  |18,750  |
  |    5,   | 75   |  750  |1,500  | 2,250  | 3,000  | 3,750  |
  |   10,   | 37·5 |  375  |  750  | 1,125  | 1,500  | 1,875  |
  |   15,   | 25   |  250  |  500  |   750  | 1,000  | 1,250  |
  |   20,   | 18·8 |  187·5|  375  |   562·5|   750  |   937·5|
  |   25,   | 15   |  150  |  300  |   450  |   600  |   750  |
  |   30,   | 12·5 |  125  |  250  |   375  |   500  |   625  |
  |   35,   | 10·7 |  107·1|  214·3|   321·4|   428·6|   535·7|
  |   40,   |  9·4 |   93·8|  187·5|   281·3|   375  |   468·8|
  |   45,   |  8·3 |   83·3|  166·7|   250  |   333·3|   416·7|
  |   50,   |  7·5 |   75  |  150  |   225  |   300  |   375  |
  |   60,   |  6·3 |   62·5|  125  |   187·5|   250  |   312·5|
  |   70,   |  5·4 |   53·6|  107·1|   160·7|   214·3|   267·9|
  |   80,   |  4·7 |   46·9|   93·8|   140·6|   187·5|   234·4|
  |   90,   |  4·2 |   41·7|   83·3|   125  |   166·7|   208·3|
  |  100,   |  3·75|   37·5|   75  |   112·5|   150  |   187·5|
  +---------+------+-------+-------+--------+--------+--------+

  +---------+--------------------------------------------+
  |Velocity |                Horse-Power.                |
  |in Miles +--------+--------+--------+--------+--------+
  |per Hour.|   60   |   70   |   80   |   90   |   100  |
  |         +--------+--------+--------+--------+--------+
  |         |             Thrust in Pounds.              |
  +---------+--------+--------+--------+--------+--------+
  |    1,   |22,500  |26,250  |30,000  |33,750  |37,500  |
  |    5,   | 4,500  | 5,250  | 6,000  | 6,750  | 7,500  |
  |   10,   | 2,250  | 2,625  | 3,000  | 3,375  | 3,750  |
  |   15,   | 1,500  | 1,750  | 2,000  | 2,250  | 2,500  |
  |   20,   | 1,125  | 1,312·5| 1,500  | 1,687·5| 1,875  |
  |   25,   |   900  | 1,050  | 1,200  | 1,350  | 1,500  |
  |   30,   |   750  |   875  | 1,000  | 1,125  | 1,250  |
  |   35,   |   642·8|   750  |   857·1|   964·3| 1,071·4|
  |   40,   |   562·5|   656·3|   750  |   843·8|   937·5|
  |   45,   |   500  |   583·3|   666·7|   750  |   833·3|
  |   50,   |   450  |   525  |   600  |   675  |   750  |
  |   60,   |   375  |   437·5|   500  |   562·5|   625  |
  |   70,   |   321·4|   375  |   428·6|   482·1|   535·7|
  |   80,   |   281·3|   328·2|   375  |   421·9|   468·8|
  |   90,   |   250  |   291·7|   333·3|   375  |   416·7|
  |  100,   |   225  |   262·5|   300  |   337·5|   375  |
  +---------+--------+--------+--------+--------+--------+

[Illustration: Fig. 72_b_.--When an aeroplane is driven through the air,
it encounters stationary air and leaves it with a downward trend. With a
thick curved aeroplane, as shown, the air follows both the top and the
bottom surfaces, and the direction that the air takes is the resultant
of these two streams of air. It will be seen that the air takes the same
direction that it would take if the plane were flat, and raised from _a_
to _c_, which would be substantially the same as shown at _f_, _h_, _g_.
It has, however, been found by actual experiment that the curved plane
is preferable, because the lifting effect is more evenly distributed,
and the drift is less in proportion to the lift.]

[Illustration: Fig. 72_c_.--Aeroplanes experimented with by Mr. Horatio
Philipps. In the published account which is before me, the angles at
which these planes were placed are not given, but, by comparing the lift
with the drift, we may assume that it was about 1 in 10.

Fig. 5 seems to have been the best shape, and I find that this plane
would have given a lifting effect of 2·2 lbs. per square foot at a
velocity of 40 miles per hour.]

PHILIPPS’ EXPERIMENTS.

  +----------+----------+------------+------------+------------+------+
  | DESCRIP- | SPEED OF | DIMENSIONS |    LIFT.   |   DRIFT.   |      |
  |  TION    |   AIR    |    OF      |            |            | LIFT |
  |   OF     | CURRENT. | AEROPLANES.|            |            |  DI- |
  |  FORM.   +----+-----+            +------+-----+------+-----+ VIDED|
  |          |Feet|Miles|            |Whole | Lbs.|Whole | Lbs.|  BY  |
  |          |per | per |            |Plane.| per |Plane.| per |DRIFT.|
  |          |sec.|hour.|            | Ozs. | sq. | Ozs. | sq. |      |
  |          |    |     |            |      | ft. |      | ft. |      |
  +----------+----+-----+------------+------+-----+------+-----+------+
  |Plane sur-|    |     |            |      |     |      |     |      |
  |faces,    | 39 |26·59| 16" × 5"   |   9  |1·013|  2   |0·225|  4·5 |
  |Fig. 1,   | 60 |40·91| 16" × 1·25"|   9  |4·05 | 0·87 |0·392| 10·3 |
  |  „  2,   | 48 |32·73| 16" × 3"   |   9  |1·688| 0·87 |0·163| 10·3 |
  |  „  3,   | 44 |30   | 16" × 3"   |   9  |1·688| 0·87 |0·163| 10·3 |
  |  „  4,   | 44 |30   | 16" × 5"   |   9  |1·013| 0·87 |0·098| 10·3 |
  |  „  5,   | 39 |26·59| 16" × 5"   |   9  |1·013| 0·87 |0·098| 10·3 |
  |  „  6,   | 27 |18·41| 16" × 5"   |   9  |1·013| 2·25 |0·253|  4   |
  |          |    |     |Area sq. ft.|      |     |      |     |      |
  |Rook’s    |    |     |            |      |     |      |     |      |
  |wing,     | 39 |26·59|      0·5   |   8  |1·0  |  1·0 |0·125|  8   |
  +----------+----+-----+------------+------+-----+------+-----+------+



CHAPTER VIII.

BALLOONS.


As far as the actual navigation of the air is concerned, balloonists
have had everything to themselves until quite recently, but we find that
at the present moment, experimenters are dividing their attention about
equally between balloons or machines lighter than the air, and true
flying machines or machines heavier than the air. In all Nature, we do
not find any bird or insect that does not fly by dynamic energy alone,
and I do not believe that the time is far distant when those now
advocating machines lighter than the air, will join the party advocating
machines heavier than the air, and, eventually, balloons will be
abandoned altogether. No matter from what standpoint we examine the
subject, the balloon is unsuitable for the service, and it is not
susceptible of much improvement. On the other hand, the flying machine
is susceptible of a good deal of improvement; there is plenty of scope
for the employment of a great deal of skill, both mechanical and
scientific, for a good many years to come.

I do not know that I can express myself better now than I did when I
wrote an article for the Engineering Supplement of the _Times_, from
which I quote the following:--

“The result of recent experiments must have convinced every thinking man
that the day of the balloon is past. A balloon, from the very nature of
things, must be extremely bulky and fragile.

“It has always appeared to the writer that it would be absolutely
impossible to make a dirigible balloon that would be of any use, even in
a comparatively light wind. Experiments have shown that only a few
hundred feet above the surface of the earth, the air is nearly always
moving at a velocity of at least 15 miles an hour, and more than
two-thirds of the time at a velocity considerably greater than this. In
order to give a balloon sufficient lifting power to carry two men and a
powerful engine, it is necessary that it should be of enormous bulk.
Considered as a whole, including men and engine, it must have a mean
density less than the surrounding air, otherwise it will not rise.
Therefore, not only is a very large surface exposed to the wind, but the
whole thing is so extremely light and fragile as to be completely at the
mercy of wind and weather. Take that triumph of engineering skill, the
‘Nulli Secundus,’ for example. The gas-bag, which was sausage-shaped and
30 feet in diameter, was a beautiful piece of workmanship, the whole
thing being built up of goldbeater’s skin. The cost of this wonderful
gas-bag must have been enormous. The whole construction, including the
car, the system of suspension, the engine and propellers, had been well
thought out and the work beautifully executed; still, under these most
favourable conditions, only a slight shower of rain was sufficient to
neutralise its lifting effect completely--that is, the gas-bag and the
cordage about this so-called airship absorbed about 400 lbs. of water,
and this was found to be more than sufficient to neutralise completely
the lifting effect. A slight squall which followed entirely wrecked the
whole thing, and it was ignominiously carted back to the point of
departure.

“We now learn that the War Office is soon to produce another airship
similar to the ‘Nulli Secundus,’ but with a much greater capacity and a
stronger engine. In the newspaper accounts it is said that the gas-bag
of this new balloon would be sausage-shaped and 42 feet in diameter,
that it is to be provided with an engine of 100 horse-power, which it is
claimed will give to this new production a speed of 40 miles an hour
through the air, so that, with a wind of 20 miles an hour, it will still
be able to travel by land 20 miles an hour against the wind. Probably
the writer of the article did not consider the subject from a
mathematical point of view. As the mathematical equation is an extremely
simple one, it is easily presented so as to be understood by any one
having the least smattering of mathematical or engineering knowledge.
The cylindrical portion of the gas-bag is to be 42 feet in diameter; the
area of the cross-section would therefore be 1,385 feet. If we take a
disc 42 feet in diameter and erect it high in the air above a level
plain, and allow a wind of 40 miles an hour, which is the proposed speed
of the balloon, to blow against it, we should find that the air pressure
would be 11,083 lbs.--that is, a wind blowing at a velocity of 40 miles
an hour would produce a pressure of 8 lbs. to every square foot of the
disc.[3] Conversely, if the air were stationary, it would require a push
of 11,083 lbs. to drive this disc through the air at the rate of 40
miles an hour.

  [3] Haswell gives the pressure of the wind at 40 miles an hour as 8
  lbs. per square foot, and this is said to have been verified by the
  United States Coast Survey. Molesworth makes it slightly less; but the
  new formula, according to most recent experiments (Dr. Stanton’s
  experiments at the National Physical Laboratory and M. Eiffel’s at
  Eiffel Tower), is P = 0·003 V², which would make the pressure only 4·8
  lbs. per square foot, and which would reduce the total H.P. required
  from 472 to 283, where P represents pounds per square foot and V miles
  per hour.

“A speed of 40 miles an hour is at the rate of 3,520 feet in a minute of
time. We therefore have two factors--the pounds of resistance
encountered, and the distance through which the disc travels in one
minute of time. By multiplying the total pounds of pressure on the
complete disc by the number of feet it has to travel in one minute of
time, we have the total number of foot-pounds required in a minute of
time to drive a disc 42 feet in diameter through the air at a speed of
40 miles an hour. Dividing the product by the conventional horse-power
33,000, we shall have 1,181 horse-power as the energy required to propel
the disc through the air. However, the end of the gas-bag is not a flat
disc, but a hemisphere, and the resistance to drive a hemisphere through
the air is much less than it would be with a normal plane or flat disc.
In the ‘Nulli Secundus’ we may take the coefficient of resistance of the
machine, considered as a whole, as 0·20--that is, that the resistance
will be one-fifth as much as that of a flat disc. This, of course,
includes not only the resistance of the balloon itself, but also that of
the cordage, the car, the engine, and the men.

“Multiplying 1,181 by the coefficient ·20, we shall have 236; therefore,
if the new balloon were attached to a long steel wire and drawn by a
locomotive through the air, the amount of work or energy required would
be 236 horse-power--that is, if the gas-bag would stand being driven
through the air at the rate of 40 miles an hour, which is extremely
doubtful. Under these conditions, the driving wheels of the locomotive
would not slip, and therefore no waste of power would result, but in the
dirigible balloon we have a totally different state of affairs. The
propelling screws are very small in proportion to the airship, and their
slip is fully 50 per cent.--that is, in order to drive the ship at the
rate of 40 miles an hour, the screws would have to travel at least 80
miles an hour. Therefore, while 236 horse-power was imparted to the ship
in driving it forward, an equal amount would have to be lost in slip,
or, in other words, in driving the air rearwards. It would, therefore,
require 472 horse-power instead of 100 to drive the proposed new balloon
through the air at the rate of 40 miles an hour.

“It will be seen from this calculation that the new airship will still
be at the mercy of the wind and weather. Those who pin their faith on
the balloon as the only means of navigating the air may dispute my
figures. However, all the factors in the equation are extremely simple
and well known, and no one can dispute any of them except the assumed
coefficient of resistance, which is given here as ·20. The writer feels
quite sure that, after careful experiments are made, it will be found
that this coefficient is nearer ·40 than ·20, especially so at high
speeds when the air pressure deforms the gas-bag. Only a slight bagging
in the front end of the balloon would run the coefficient up to fully
·50, and perhaps even more.”--_Times_, Feb. 26, 1908.

[Illustration: Fig. 73.--The enormous balloon, “Ville de Paris,” of the
French Government. This balloon is a beautiful piece of workmanship, and
is said to be the most practical balloon ever invented, not excepting
the balloon of Count Zeppelin. Some idea of its size may be obtained by
comparing it with the size of the men who are standing immediately
underneath.]

Since writing the _Times_ article, a considerable degree of success has
been attained by Count Zeppelin. According to newspaper accounts, his
machine has a diameter of about 40 feet, and a length of no less than
400 feet. It appears that this balloon consists of a very light
aluminium envelope, which is used in order to produce a smooth and even
surface, give rigidity, and take the place of the network employed in
ordinary balloons. It seems that the gas is carried in a large number of
bags fitted in the interior of this aluminium envelope. However, by
getting a firm and smooth exterior and by making his apparatus of very
great length as relates to its diameter, he has obtained a lower
coefficient of resistance than has ever been obtained before, and as his
balloon is of great volume, he is able to carry powerful motors and use
screw propellers of large diameter. It appears that he has made a
circuit of considerable distance, and returned to the point of departure
without any accident. A great deal of credit is, therefore, due to him.
His two first balloons came to grief very quickly; he was not
discouraged, but stuck to the job with true Teutonic grit, and has
perhaps attained a higher degree of success than has ever been attained
with a balloon. However, some claim that the French Government balloon,
“La Patrie” is superior to the Zeppelin balloon at all points. When we
take into consideration the fact that the Zeppelin machine is 400 feet
long and lighter than the same volume of air, it becomes only too
obvious that such a bulky and extremely delicate and fragile affair will
easily be destroyed. Of course ascensions will only be made in very
favourable weather, but squalls and sudden gusts of wind are liable to
occur. It is always possible to start out in fine weather if one waits
long enough, but if a flight of 24 hours or even 12 hours is to be
attempted, the wind may be blowing very briskly when we return, and an
ordinary wind will not only prevent the housing of Count Zeppelin’s
balloon, but will be extremely liable to reduce it to a complete wreck
in a few minutes.[4]

  [4] Shortly after this was written, the Zeppelin machine was
  completely demolished by a gust of wind.

I am still strongly of the opinion that the ultimate mastery of the air
must be accomplished by machines heavier than the air.



APPENDIX I.


MAJOR BADEN-POWELL’S DEMAND.

(_From our own Correspondent._)

  BERLIN, Friday.

Germany’s fleet of “air cruisers,” or dirigible airships, will, it is
proudly announced to-day, presently number six:--

  Count Zeppelin’s III., rigid type.

  Count Zeppelin’s IV., rigid type, which has done a twelve-hour flight
  and will be taken over by the Government, with No. III., for £100,000,
  after a twenty-four-hour test.

  Major Gross’s Army airship, half rigid.

  Motor Airship Study Society’s old airship, non-rigid.

  Major von Parseval’s non-rigid ship building for the above society.

  New airship, of which details are kept secret, nearly ready at the
  works of the Siemens-Schuckert Electric Company.

The first announcement of the last-named airship was given in _The Daily
Mail_ several months ago. The company has engaged a celebrated military
aeronaut, Captain von Krogh, as commander of the vessel. The Study
Society’s new non-rigid ship will be sold to the War Office as soon as
she has completed her trial trips.

The Army will then possess three dirigibles, each representing one of
the three opposed types of construction--rigid, half-rigid, and
non-rigid--with a view to arriving at a conclusion on their merits.

       *       *       *       *       *

“Only a year or so ago, our authorities were talking of aerial
navigation in its relation to war as ‘an interesting and instructive
study.’ Now we must reckon it as the gravest problem of the moment. The
cleverest aeronauts in England should be called upon at once to design
an airship, not only as efficient as that of Count Zeppelin’s, but
possessed of even greater speed. (His average was said to be about 34
miles an hour.) In speed will lie the supremacy of the air when it comes
to actual warfare. Of two opposing airships, the faster will be able to
outmanœuvre its adversary and hold it at its mercy.”--_Daily Mail_, July
11, 1908.


COMMAND OF THE AIR.

GERMANY AS THE AERIAL POWER.

TEUTONIC VISION.

A LANDING OF 350,000 MEN.

Herr Rudolph Martin, author of books on war in the air and “Is a
World-War Imminent?” points out how England is losing her insular
character by the development of airships and aeroplanes.

“In a world-war,” he said to me, “Germany would have to spend two
hundred millions sterling in motor airships, and a similar amount in
aeroplanes, to transport 350,000 men in half an hour during the night
from Calais to Dover. Even to-day the landing of a large German army in
England is a mere matter of money. I am opposed to a war between Germany
and England, but should it break out to-day, it would last at least two
years, for we would conclude no peace until a German army had occupied
London.

“In my judgment it would take two years for us to build motor airships
enough simultaneously to throw 350,000 men into Dover _via_ Calais.
During the same night, of course, a second transport of 350,000 men
could follow. The newest Zeppelin airship can comfortably carry fifty
persons from Calais to Dover. The ships which the Zeppelin works in
Friedrichshafen will build during the next few months are likely to be
considerably larger than IV., and will carry one hundred persons. There
is no technical reason against the construction of Zeppelin airships of
1,100,000 or even 1,700,000 cubic feet capacity, or twice or three times
the capacity of IV. (500,000 cubic feet).

“I am at present organising a German ‘Air Navy League,’ to establish
air-traffic routes in Germany. Aluminium airships could carry on regular
traffic between Berlin and London, Paris, Cologne, Munich, Vienna,
Moscow, Copenhagen, and Stockholm. In war time these ships would be at
the disposal of the German Empire.

“The development of motor airship navigation will lead to a perpetual
alliance between England and Germany. The British fleet will continue to
rule the waves, while Germany’s airships and land armies will represent
the mightiest Power on the Continent of Europe.”--_Daily Mail_, July 11,
1908.

It is needless to say that the above was written before the wreck of
Zeppelin’s machine.

       *       *       *       *       *

For many years scientific mechanicians and mathematicians have told us
that the navigation of the air was quite possible. They have said it is
only a question of motive power; “Give us a motor that is sufficiently
light and strong, and we will very soon give you a practical flying
machine.” A domestic goose weighs about 12 lbs., and it has been
estimated that it only exerts about one-twelfth part of a horse-power in
flying--that is, it is able to exert one man-power with a weight of only
12 lbs., which seems to be a very good showing for the goose. However,
at the present moment, we are able to make motors which develop the
power of ten men--that is, one horse-power--with less than the weight of
a common barnyard fowl. Under these conditions it is quite evident that
if a machine can be so designed that it will not be too wasteful in
power, it must be a success. It is admitted by scientific men that all
animals, such as horses, deer, dogs, and also birds, are able to develop
much more dynamic energy for the carbon consumed than is possible with
any thermodynamic machine that we are able to make. It may be said that
many animals are able to develop the full dynamic energy of the carbon
they consume, whereas the best of our motors do not develop more than 10
per cent. of the energy contained in the combustibles that they consume;
but, as against this, it must be remembered that birds feed on grass,
fruit, fish, etc., heavy and bulky materials containing only a small
percentage of carbon, whereas with a motor we are able to use a pure
hydrocarbon that has locked up in its atoms more than twenty times as
much energy per pound as in the ordinary food consumed by birds. I
think, in fact I assert, that the time has now arrived, having regard to
the advanced state of the art in building motors, when it will be quite
a simple and safe affair to erect works and turn out successful flying
machines at less cost than motor cars; in fact, there is nothing that
stands in the way of success to-day. The value of a successful flying
machine, when considered from a purely military standpoint, cannot be
over-estimated. The flying machine has come, and come to stay, and
whether we like it or not, it is a problem that must be taken into
serious consideration. If we are laggards we shall, unquestionably, be
left behind, with a strong probability that before many years have
passed over our heads, we shall have to change the colouring of our
school maps.

       *       *       *       *       *

As the newspaper accounts that we receive from the Continent give all
weights and measures in the metric system, it is convenient to have some
simple means at hand to convert their values into English weights and
measures. I therefore give the following, which will greatly simplify
matters both for French and English measurements:--

    One metre      =      39·37    inches.
     „  decimetre  =       3·937      „
     „  centimetre =        ·3937  inch.
     „  millimetre =        ·03937   „

  In order to convert

    Metres into inches, multiply by                         39·37.
          „     feet,      „      „                          3·28.
          „     yards,     „      „                          1·09.
          „     miles,     „      „                           ·00062138.

    Cubic metres into cubic yards, multiply by               1·30802.
         „         „        feet,      „    „               35·31658.

    Miles per hour into feet per minute, multiply by        88.
         „        „       „      second,    „     „          1·46663.
         „        „  kilometres per hour,   „     „          1·6093.
         „        „  metres per second,     „     „           ·44702.

    Miles per minute into feet per second,  „     „         88.

    Pounds into grammes, multiply by                       453·5926.
      „     „   kilogrammes, „    „                           ·45359.

    Pounds pressure per sq. inch into atmospheres, multiply
    by                                                     ·06804.

    British thermal units into
      Pounds of water, 1° C., multiply by                  ·55556.
      Kilogramme-calories,       „     „                   ·252
      Joules (mechanical equivalent), multiply by      1047·96.
      Foot-pounds, multiply by                          778.

    Pounds of water into pints, multiply by                ·8.
      „      „       „   cubic feet,    „                  ·016046.
      „      „       „   litres,        „                  ·454587.
      „      „       „   cubic centimetres, multiply by 454·656.

    Gallons of water into pounds, multiply by            10.
      „      „       „  cubic feet,  „     „               ·16057.
      „      „       „  kilogrammes, „     „              4·5359.
      „      „       „  litres,      „     „              4·54586.

    Litres of water into cubic inches, multiply by       61·0364.
      „      „       „   pounds,          „     „         2·20226.
      „      „       „   gallons,         „     „          ·21998.

    Air, 1 cubic foot weighs at 62°                     532·5 grains.

    Air, cubic feet into pounds, 32° F., multiply by       ·08073.

    Pounds of dry air into cubic feet,      „     „      13·145.

    Kilogramme-calories into British thermal units,
                                        multiply by       3·9683.
      „        „       „  gramme-calories, „     „     1000.
      „        „       „  mechanic equivalent in
                            foot-lbs., multiply by     3065·7.



APPENDIX II.


RECAPITULATION OF EARLY EXPERIMENTS.

[Illustration: Fig. 74.--Photograph of a model of my machine, showing
the fore and aft horizontal rudders and the superposed aeroplanes.]

In my early “whirling table”[5] experiments, the aeroplanes used were
from 6 inches to 4 feet in width. They were for the most part made of
thin pine, being slightly concave on the underneath side and convex on
the top, both the fore and aft edges being very sharp. I generally
mounted them at an angle of 1 in 14[6]--that is, in such a position that
in advancing 14 feet they pressed the air down 1 foot. With this
arrangement, I found that with a screw thrust of 5 lbs. the aeroplane
would lift 5 × 14, or 70 lbs., while if the same plane was mounted at an
angle of 1 in 10, the lifting effect was almost 50 lbs. (5 × 10). This
demonstrated that the skin friction on these very sharp, smooth and
well-made aeroplanes was so small a factor as not to be considered.
When, however, there was the least irregularity in the shape of the
aeroplane, the lifting effect, when considered in terms of screw thrust,
was greatly diminished. With a well-made wooden plane placed at an angle
of 1 in 14, I was able to carry as much as 113 lbs. to the H.P., whereas
with an aeroplane consisting of a wooden frame covered with a cotton
fabric (Fig. 75), I was only able to carry 40 lbs. to the H.P.[7]

  [5] A name given by Professor Langley to an apparatus consisting of a
  long rotating arm to which objects to be tested are attached.

  [6] I found it more convenient to express the angle in this manner
  than in degrees.

  [7] The actual power consumed by the aeroplane itself was arrived at
  as follows:--The testing machine was run at the desired speed without
  the aeroplane, and the screw thrust and the power consumed carefully
  noted. The aeroplane was then attached and the machine again run at
  the same speed. The difference between the two readings gave the power
  consumed by the aeroplane.

[Illustration: Fig. 75.--The fabric-covered aeroplane experimented with.
The efficiency of this aeroplane was only 40 per cent. of that of a
well-made wooden aeroplane.]

[Illustration: Fig. 76.--The forward rudder of my large machine, showing
the fabric attached to the lower side. The top was also covered with
fabric. This rudder considered as an aeroplane had a very high
efficiency and worked very well indeed.]

These facts taken into consideration with my other experiments with
large aeroplanes, demonstrated to my mind that it would not be a very
easy matter to make a large and efficient aeroplane. If I obtained the
necessary rigidity by making it of boards, it would be vastly too heavy
for the purpose, while if I obtained the necessary lightness by making
the framework of steel and covering it with a silk or cotton fabric in
the usual way, the distortion would be so great that it would require
altogether too much power to propel it through the air. I therefore
decided on making a completely new form of aeroplane. I constructed a
large steel framework arranged in such a manner that the fore and aft
edges consisted of tightly drawn steel wires. This framework was
provided with a number of light wooden longitudinal trusses, similar to
those shown in Fig. 76. The bottom side was then covered with balloon
fabric secured at the edges, and also by two longitudinal lines of
lacing through the centre. It was stretched very tightly and slightly
varnished, but not sufficiently to make it absolutely air-tight. The top
of this framework was covered with the same kind of material, but
varnished so as to make it absolutely airtight. The top and bottom were
then laced together forming very sharp fore and aft edges, and the top
side was firmly secured to the light wooden trusses before referred to.
Upon running this aeroplane, I found that a certain quantity of air
passed through the lower side and set up a pressure between the upper
and lower coverings. The imprisoned air pressed the top covering upward,
forming longitudinal corrugations which did not offer any perceptible
resistance to the air, whereas the bottom fabric, having practically the
same pressure on both sides, was not distorted in the least. This
aeroplane was found to be nearly as efficient as it would have been had
it been carved out of a solid piece of wood. It will be seen by the
illustration that this large or main aeroplane is practically octagonal
in shape, its greatest width being 50 feet, and the total area 1,500
square feet.


EXPERIMENTS WITH A LARGE MACHINE.

Upon running my large machine over the track (Fig. 77) with only the
main aeroplane in position, I found that a lifting effect of 3,000 to
4,000 lbs. could be obtained with a speed of 37 to 42 miles an hour. It
was not always an easy matter to ascertain exactly what the lifting
effect was at a given speed on account of the wind that was generally
blowing. Early in my experiments, I found if I ran my machine fast
enough to produce a lifting effect within 1,000 lbs. of the total weight
of the machine, that it was almost sure to leave the rails if the least
wind was blowing. It was, therefore, necessary for me to devise some
means of keeping the machine on the track. The first plan tried was to
attach some very heavy cast-iron wheels weighing with their axle-trees
and connections about 1-1/2 tons. These were constructed in such a
manner that the light flanged wheels supporting the machine on the steel
rails could be lifted 6 inches above the track, leaving the heavy wheels
still on the rails for guiding the machine. This arrangement was tried
on several occasions, the machine being run fast enough to lift the
forward end off the track. However, I found considerable difficulty in
starting and stopping quickly on account of the great weight, and the
amount of energy necessary to set such heavy wheels spinning at a high
velocity. The last experiment with these wheels was made when a head
wind was blowing at the rate of about 10 miles an hour. It was rather
unsteady, and when the machine was running at its greatest velocity, a
sudden gust lifted not only the front end, but also the heavy front
wheels completely off the track, and the machine falling on soft ground
was soon blown over by the wind.

I then provided a safety track of 3 × 9 Georgia pine placed about 2 feet
above the steel rails, the wooden track being 30 feet gauge and the
steel rails 9 feet gauge (Fig. 77). The machine was next furnished with
four extra wheels placed on strong outriggers and adjusted in such a
manner that when it had been lifted 1 inch clear of the steel rails,
these extra wheels would engage the upper wooden track.[8]

  [8] Springs were interposed between the machine and the axle-trees.
  The travel of these springs was about 4 inches; therefore, when the
  machine was standing still, the wheels on the outriggers were about 5
  inches below the upper track.

[Illustration: Fig. 77.--View of the track used in my experiments. The
machine was run along the steel railway which was 9 feet gauge, and was
prevented from rising by the wooden track which was 35 feet gauge.]

[Illustration: Fig. 78.--The machine on the track tied up to the
dynamometer.]

[Illustration: Fig. 79.--Two dynagraphs, one for making a diagram of the
lifting effect off the main axle-tree, and the other for making a
diagram of the lift off the front axle-tree. By this arrangement, I was
able to ascertain the exact lifting effect at all speeds, and to arrange
my aeroplanes in such a manner that the center of lifting effect was
directly over the center of gravity. The paper-covered cylinders made
one rotation in 2,000 feet.]

When fully equipped, my large machine had five long and narrow
aeroplanes projecting from each side. Those that are attached to the
sides of the main aeroplanes are 27 feet long, thus bringing the total
width of the machine up to 104 feet. The machine is also provided with a
fore and an aft rudder made on the same general plan as the main
aeroplane. When all the aeroplanes are in position, the total lifting
surface is brought up to about 6,000 square feet. I have, however, never
run the machine with all the planes in position. My late experiments
were conducted with the main aeroplane, the fore and aft rudders, and
the top and bottom side planes in position, the total area then being
4,000 square feet. With the machine thus equipped, with 600 lbs. of
water in the tank and boiler and with the naphtha and three men on
board, the total weight was a little less than 8,000 lbs. The first run
under these conditions was made with a steam pressure of 150 lbs. to the
square inch, in a dead calm, and all four of the lower wheels remained
constantly on the rails, none of the wheels on the outriggers touching
the upper track. The second run was made with 240 lbs. steam pressure to
the square inch. On this occasion, the machine seemed to vibrate between
the upper and lower tracks. About three of the top wheels were engaged
at the same time, the weight on the lower steel rails being practically
nil. Preparations were then made for a third run with nearly the full
power of the engines. The machine was tied up to a dynamometer (Fig.
78), and the engines were started with a pressure of about 200 lbs. to
the square inch. The gas supply was then gradually turned on with the
throttle valves wide open; the pressure soon increased, and when 310
lbs. was reached, the dynamometer showed a screw thrust of 2,100
lbs.,[9] but to this must be added the incline of the track which
amounts to about 64 lbs. The actual thrust was therefore 2,164 lbs. In
order to keep the thrust of the screws as nearly constant as possible, I
had placed a small safety valve--3/4-inch--in the steam pipe leading to
one of the engines. This valve was adjusted in such a manner that it
gave a slight puff of steam at each stroke of the engine with a
pressure of 310 lbs. to the square inch, and a steady blast at 320 lbs.
to the square inch. As the valves and steam passages of these engines
were made very large, and as the piston speed was not excessive, I
believed if the steam pressure was kept constant that the screw thrust
would also remain nearly constant, because as the machine advances and
the screws commence to run slightly faster, an additional quantity of
steam will be called for and this could be supplied by turning on more
gas. When everything was ready, with careful observers stationed on each
side of the track, the order was given to let go. The enormous screw
thrust started the machine so quickly that it nearly threw the engineers
off their feet, and the machine bounded over the track at a great rate.
Upon noticing a slight diminution in the steam pressure, I turned on
more gas, when almost instantly the steam commenced to blow a steady
blast from the small safety valve, showing that the pressure was at
least 320 lbs. in the pipes supplying the engines with steam. Before
starting on this run, the wheels that were to engage the upper track
were painted, and it was the duty of one of my assistants to observe
these wheels during the run, while another assistant watched the
pressure gauges and dynagraphs (Fig. 79). The first part of the track
was up a slight incline, but the machine was lifted clear of the lower
rails and all of the top wheels were fully engaged on the upper track
when about 600 feet had been covered. The speed rapidly increased, and
when 900 feet had been covered, one of the rear axle-trees, which were
of 2-inch steel tubing, doubled up (Fig. 80), and set the rear end of
the machine completely free. The pencils ran completely across the
cylinders of the dynagraphs and caught on the underneath end. The rear
end of the machine being set free, raised considerably above the track
and swayed. At about 1,000 feet, the left forward wheel also got clear
of the upper track and shortly afterwards, the right forward wheel tore
up about 100 feet of the upper track. Steam was at once shut off and
the machine sank directly to the earth imbedding the wheels in the soft
turf (Figs. 81 and 82) without leaving any other marks, showing most
conclusively that the machine was completely suspended in the air before
it settled to the earth. In this accident, one of the pine timbers
forming the upper track went completely through the lower framework of
the machine and broke a number of the tubes, but no damage was done to
the machinery except a slight injury to one of the screws (Fig. 83).

  [9] The quantity of water entering the boiler at this time was so
  great as to be beyond the range of the feed-water indicator.

[Illustration: Fig. 80.--The outrigger wheel that gave out and caused an
accident with the machine.]

[Illustration: Fig. 81.--Shows the broken planks and the wreck that they
caused. It will be observed that the wheels sank directly into the
ground without leaving any track.]

[Illustration: Fig. 82.--The condition of the machine after the
accident. One of the broken planks that formed the upper track is shown.
It will be observed that the wheels have sunk directly into the ground
without leaving any tracks, showing that the machine did not run along
the ground, but came directly down when it stopped.]

In my experiments with the small apparatus for ascertaining the power
required to perform artificial flight, I found that the most
advantageous angle for my aeroplane was 1 in 14, but when I came to make
my large machine, I placed my aeroplanes at an angle of 1 in 8 so as to
be able to get a great lifting effect at a moderate speed with a short
run. In the experiments which led to the accident above referred to, the
total lifting effect upon the machine must have been at least 10,000
lbs. All the wheels which had been previously painted and which engaged
the upper track were completely cleaned of their paint and had made an
impression on the wood, which clearly indicated that the load which they
had been lifting was considerable.[10] Moreover, the strain necessary to
double up the axle-trees was fully 1,000 lbs. each, without considering
the lift on the forward axle-trees which did not give way but broke the
upper track.

  [10] The latest form of outrigger wheels for engaging the upper track
  is shown in Fig. 84.

[Illustration: Fig. 83.--This shows the screw damaged by the broken
planks; also a hole in the main aeroplane caused by the flying
splinters.]

The advantages arising from driving the aeroplanes on to new air, the
inertia of which has not been disturbed, are clearly shown in these
experiments. The lifting effect of the planes was 2·5 lbs. per square
foot. A plane loaded at this rate will fall through the air with a
velocity of 22·36 miles per hour, according to the formula √(200 × P) =
V. But as the planes were set at an angle of 1 in 8, and as the machine
travelled at the rate of 40 miles an hour, the planes only pressed the
air downwards 5 miles an hour (40 ÷ 8 = 5). A fall of 5 miles an hour
without advancing would only exert a pressure of ·125 lb. per square
foot, according to the formula (V² × ·005 = P).[11]

  [11] This is the old formula used by Haswell. The account of this
  experimental work was written in the autumn of 1894 and Haswell’s
  formula was used. I have thought best to make no changes.

[Illustration: Fig. 84.--This shows a form of outrigger wheels which
were ultimately used.]

Engineers and mathematicians who have written to prove that flying
machines were impossible have generally computed the efficiency of
aeroplanes moving through the air, on the basis that the lifting effect
would be equal to a wind blowing against the plane at the rate at which
the air was pressed down by the plane while being driven through the
air. According to this system of reasoning, my 4,000 square feet of
aeroplanes would have lifted only ·125 lb. per square foot, and in order
to have lifted 10,000 lbs. they would have to have had an area twenty
times as great. This corresponds exactly with the discrepancy which
Professor Langley has found in the formula of Newton.

With aeroplanes of one-half the width of those I employed, and with a
velocity twice as great, the angle could be much less, and the
advantages of continually running on to fresh air would be still more
manifest. With a screw thrust of 2,000 lbs., the air pressure on each
square foot of the projected area of the screw blades is 21·3 lbs.,
while the pressure on the entire discs of the screws is 4 lbs. per
square foot, which would seem to show with screws of this size, that
four blades would be more efficient than two.

[Illustration: Fig. 85.--One pair of my compound engines. This engine
weighed 310 lbs. and developed 180 H.P., with 320 lbs. of steam per
square inch.]

The engines, as before stated, are compound (Fig. 85). The area of the
high-pressure piston is 20 square inches, and that of the low-pressure
piston is 50·26 square inches. Both have a stroke of 12 inches. With a
boiler pressure of 320 lbs., the pressure on the low-pressure piston is
125 lbs. to the square inch. This abnormally high pressure in the
low-pressure cylinder is due to the fact that there is a very large
amount of clearance in the high-pressure cylinder to prevent shock in
case water should go over when the machine pitches; moreover, the steam
in the high-pressure cylinder is cut off at three-quarters stroke, while
the steam in the low-pressure cylinder is cut off at five-eighths
stroke. If we should compute the power of these engines with the steam
entering at full stroke, without any friction, and with no back pressure
on the low-pressure cylinder, the total horse-power would foot up to
461·36 horse-power at the speed at which the engines were run--namely,
375 turns per minute. If we compute the actual power consumed by the
screws, by multiplying their thrust, which is probably 2,000 lbs. while
they are travelling, by their pitch, 16 feet, and this by the number of
turns which they make in a minute, and then divide the product by
33,000,

  2,000 × 16 × 375
  ---------------- = 363·63,
       33,000

we find that we have 363·63 horse-power in actual effect delivered on
the screws of the machine, which shows that there is rather less than 22
per cent. loss in the engines, due to cutting off before the end of the
stroke, to back pressure, and to friction. The actual power applied to
the machine being 363·63 horse-power, it is interesting to know what
becomes of it. When the machine has advanced 40 miles (which it would do
in an hour), the screws have travelled 68·1 miles (375 × 16 × 60/5,280)
= 68·1; therefore, 150 horse-power is wasted in slip, and 213·63
horse-power consumed in driving the machine through the air. Now, as the
planes are set at an angle of 1 in 8, the power actually used in lifting
the machine is 133·33, and the loss in driving the body of the machine,
its framework and wires through the air is 90·30 horse-power.

  Power lost in screw slip,                          150    H.P.
    „    „   driving machinery and framework,         80·30  „
    „   actually consumed in lifting the machine,    133·33  „
                                                     ------
          Total power delivered by the engines,      363·63  „


THE ADVANTAGES AND DISADVANTAGES OF VERY NARROW PLANES.

[Illustration: Fig. 86.--The path that the air has to take in passing
between superposed aeroplanes in close proximity to each other. By this
arrangement the drift is considerably increased.]

My experiments have demonstrated that relatively narrow aeroplanes lift
more per square foot than very wide ones; but as an aeroplane, no matter
how narrow it may be, must of necessity have some thickness, it is not
advantageous to place them too near together. Suppose that aeroplanes
should be made 1/4-inch thick, and be superposed 3 inches apart--that
is, at a pitch of 3 inches--one-twelfth part of the whole space through
which these planes would have to be driven would be occupied by the
planes themselves, and eleven-twelfths would be air space (Fig. 86). If
a group of planes thus mounted should be driven through the air at the
rate of 36 miles an hour,[12] the air would have to be driven forward at
the rate of 3 miles an hour, or else it would have to be compressed, or
spun out, and pass between the spaces at a speed of 39 miles an hour. As
a matter of fact, however, the difference in pressure is so very small
that practically no atmospheric compression takes place. The air,
therefore, is driven forward at the rate of 3 miles an hour, and this
consumes a great deal of power; in fact, so much that there is a
decided disadvantage in using narrow planes thus arranged.

  [12] The arrows in the accompanying drawings show the direction of the
  air currents, the experiments having been made with stationary planes
  in a moving current of air.

In regard to the curvature of narrow aeroplanes, I have found that if
one only desires to lift a large load in proportion to the area, the
planes may be made very hollow on the underneath side; but when one
considers the lift in terms of the screw thrust, I find it advisable
that the planes should be as thin as possible, and the underneath side
nearly flat. I have also found that it is a great advantage to arrange
the planes after the manner shown in Fig. 87. In this manner the sum of
all the spaces between the planes is equal to the whole area occupied by
the planes; consequently, the air neither has to be compressed, spun
out, nor driven forward. I am, therefore, able by this arrangement to
produce a large lifting effect per square foot, and, at the same time,
to keep the screw thrust within reasonable limits.

[Illustration: Fig. 87.--The position of narrow aeroplanes arranged in
such a manner that the air has free passage between them, and this
arrangement has been found superior to arranging one above the other
after the manner of a Venetian blind.]

A large number of experiments with very narrow aeroplanes have been
conducted by Mr. Horatio Philipps at Harrow, in England. Fig. 88 shows a
cross-section of one of Mr. Philipps’ planes. Mr. Philipps is of the
opinion that the air, in striking the top side of the plane, is thrown
upwards in the manner shown, and a partial vacuum is thereby formed over
the central part of the plane, and that the lifting effect of planes
made in this form is therefore very much greater than with ordinary
narrow planes. I have experimented with these “sustainers” (as Mr.
Philipps calls them) myself, and I find it is quite true that they lift
in some cases as much as 8 lbs. per square foot,[13] but the lifting
effect is not produced in the exact manner that Mr. Philipps seems to
suppose. The air does not glance off in the manner shown. As the
“sustainer” strikes the air two currents are formed, one following the
exact contour of the top, and the other that of the bottom. These two
currents join and are thrown downwards, as relates to the “sustainer,”
at an angle which is the resultant of the angles at which the two
currents meet. These “sustainers” may be made to lift when the front
edge is lower than the rear edge, because they encounter still air, and
leave it with a downward motion.

  [13] In my early experiments I lifted as much as 8 lbs. per square
  foot with aeroplanes which were only slightly curved, but very thin
  and sharp.

[Illustration: Fig. 88.--The very narrow aeroplanes, or sustainers,
employed by Mr. Philipps. It has been supposed that the air in striking
at A was deflected in the manner shown, but such is not the case. The
air in reality follows the surface, as shown in the dotted line in the
second illustration.]

In my experiments with narrow superposed planes, I have always found
that with strips of thin metal made sharp at both edges and only
slightly curved, the lifting effect, when considered in terms of screw
thrust, was always greater than with any arrangement of the wooden
aeroplanes used in Philipps’ experiments. It would, therefore, appear
that there is no advantage in the peculiar form of “sustainer” employed
by this inventor.

If an aeroplane be made perfectly flat on the bottom side and convex on
the top, and be mounted in the air so that the bottom side is exactly
horizontal, it produces a lifting effect no matter in which direction it
is run, because, as it advances, it encounters stationary air which is
divided into two streams. The top stream being unable to fly off at a
tangent when turning over the top curve, flows down the incline and
joins the current which is flowing over the lower horizontal surface.
The angle at which the combined stream of air leaves the plane is the
resultant of these two angles; consequently, as the plane finds the air
in a stationary condition, and leaves it with a downward motion, the
plane itself must be lifted. It is true that small and narrow aeroplanes
may be made to lift considerably more per square foot of surface than
very large ones, but they do not offer the same safeguard against a
rapid descent to the earth in case of a stoppage or breakdown of the
machinery. With a large aeroplane properly adjusted, a rapid and
destructive fall to the earth is quite impossible.


THE EFFICIENCY OF SCREW PROPELLERS, STEERING, STABILITY, &c.

Before I commenced my experiments at Baldwyn’s Park, I attempted to
obtain some information in regard to the action of screw propellers
working in the air. I went to Paris and saw the apparatus which the
French Government employed for testing the efficiency of screw
propellers, but the propellers were so very badly made that the
experiments were of no value. Upon consulting an English experimenter,
who had made a “life-long study” of the question, he assured me that I
should find the screw propeller very inefficient and very wasteful of
power, and that all screw propellers had a powerful fan-blower action,
drawing in air at the centre and discharging it with great force at the
periphery. I found that no two men were agreed as to the action of screw
propellers. All the data or formulæ available were so confusing and
contradictory as to be of no value whatsoever. Some experimenters were
of the opinion that, in computing the thrust of a screw, we should only
consider the projected area of the blades, and that the thrust would be
equal to a wind blowing against a normal plane of equal area at a
velocity equal to the slip. Others were of the opinion that the whole
screw disc would have to be considered; that is, that the thrust would
be equal to a wind blowing against a normal plane having an area equal
to the whole disc, and at the velocity of the slip. The projected area
of the two screw blades of my machine is 94 square feet, and the area of
the two screw discs is 500 square feet. According to the first system of
reasoning, therefore, the screw thrust of my large machine, when running
at 40 miles an hour with a slip of 18 miles per hour, would have been,
according to the well-known formula,

  V² × ·005 = P

  18² × ·005 × 94 = 152·28 lbs.

If, however, we should have considered the whole screw disc, it would
have been 18² × ·005 × 500 = 810 lbs. However, when the machine was run
over the track at this rate, the thrust was found to be rather more than
2,000 lbs. When the machine was secured to the track and the screws
revolved until the pitch in feet, multiplied by the turns per minute,
was equal to 68 miles an hour, it was found that the screw thrust was
2,164 lbs. In this case, it was of course, all slip, and when the screws
had been making a few turns they had established a well-defined
air-current, and the power exerted by the engine was simply to maintain
this air current. It is interesting to note that, if we compute the
projected area of these blades by the foregoing formula, the thrust
would be--68² × ·005 × 94 = 2,173·28 lbs., which is almost exactly the
observed screw thrust.

When I first commenced my experiments with a large machine, I did not
know exactly what sort of boiler, gas generator, or burner I should
finally adopt; I did not know the exact size that it would be necessary
to make my engines; I did not know the size, the pitch, or the diameter
of the screws which would be the most advantageous; neither did I know
the form of aeroplane which I should finally adopt. It was, therefore,
necessary for me to make the foundation or platform of my machine of
such a character that it would allow me to make the modifications
necessary to arrive at the best results. The platform of the machine is,
therefore, rather larger than is necessary, and I find if I were to
design a completely new machine, that it would be possible to greatly
reduce the weight of the framework, and, what is still more, to greatly
reduce the force necessary to drive it through the air.

[Illustration: Fig. 89.--One of the large screws being hoisted into
position. Its size may be judged by comparison with the man.]

At the present time, the body of my machine is a large platform, about 8
feet wide and 40 feet long. Each side is formed of very long trusses of
steel tubes, braced in every direction by strong steel wires. The
trusses which give stiffness are all below the platform. In designing a
new machine, I should make the trusses much deeper and at the same time
very much lighter, and, instead of having them below the platform on
which the boiler is situated, I should have them constructed in such a
manner as to completely enclose the boiler and the greater part of the
machinery.[14] I should make the cross-section of the framework
rectangular and pointed at each end. I should cover the outside very
carefully with balloon material, giving it a perfectly smooth and even
surface throughout, so that it might be easily driven through the air.

  [14] This arrangement of the framework is now common to all successful
  machines.

In regard to the screws, I am at the present time able to mount screws
17 feet 10 inches in diameter (Fig. 89). I find, however, that my
machine would be much more efficient if the screws were 24 feet in
diameter and I believe with such very large screws, four blades would be
much more efficient than two.

My machine may be steered to the right or to the left by running one of
the propellers faster than the other. Very convenient throttle valves
have been provided to facilitate this system of steering. An ordinary
vertical rudder placed just after the screws may, however, prove more
convenient if not more efficient.

The machine is provided with fore and aft horizontal rudders, both of
which are connected with the same windlass.

In regard to the stability of the machine, the centre of weight is much
below the centre of lifting effect; moreover, the upper wings are set at
such an angle that whenever the machine tilts to the right or to the
left the lifting effect is increased on the lower side and diminished on
the higher side. This simple arrangement makes it automatic as far as
rolling is concerned. I am of the opinion that whenever flying machines
come into use, it will be necessary to steer in a vertical direction by
means of an automatic steering gear controlled by a gyroscope. It will
certainly not be more difficult to manœuvre and steer such machines than
it is to control completely submerged torpedoes.

When the machine is once perfected, it will not require a railway track
to enable it to get the necessary velocity to rise. A short run over a
moderately level field will suffice. As far as landing is concerned, the
aerial navigator will touch the ground when moving forward, and the
machine will be brought to a state of rest by sliding on the ground for
a short distance. In this manner very little shock will result, whereas
if the machine is stopped in the air and allowed to fall directly to the
earth without advancing, the shock, although not strong enough to be
dangerous to life or limb, might be sufficient to disarrange or injure
the machinery.


THE COMPARATIVE VALUE OF DIFFERENT MOTORS.

So far I have only discussed the navigation of the air by the use of
propellers driven by a steam engine. The engines that I employ are what
is known as compound engines--that is, they have a large and a small
cylinder. Steam at a very high pressure enters the high-pressure
cylinder, expands and escapes at a lower pressure into a larger cylinder
where it again expands and does more work. A compound engine is more
economical in steam than a simple engine, and therefore requires a
smaller boiler to develop the same horse-power, so that when we consider
the weight of water and fuel for a given time, together with the weight
of the boiler and the engine, the engine motor with a compound engine is
lighter than a simple engine. However, if only the weight of the engine
is to be considered then the simple engine will develop more power per
unit of weight than the compound engine. For instance, if, instead of
allowing the steam to enter the small cylinder, and the exhaust from
this cylinder to enter the large or low-pressure cylinder--which
necessitates that the high-pressure piston has to work against a back
pressure equal to the full pressure on the low-pressure cylinder--I
should connect both cylinders direct with the live steam, and allow both
to discharge their exhaust directly into the air, I should then have a
pair of simple engines, and instead of developing 363 H.P. they would
develop fully 500 H.P., or nearly 1 H.P. for every pound of their
weight. I mention this fact to show that the engines are exceedingly
light, and that when compared with simple engines their power should be
computed on the same basis. It will, therefore, be seen that if we do
not take into consideration the steam supply or the amount of fuel and
water necessary, the simple steam engine is an exceedingly light motor.

But, as before stated, great improvements have recently been made in oil
engines. I have thought much on this subject, and am of the opinion that
if one had an unlimited supply of money, a series of experiments could
be very profitably conducted with a view of adapting the oil engine for
use on flying machines. If we use a steam engine, it is necessary to
have a boiler, and at best a boiler is rather a large and heavy object
to drive through the air. If we use an oil engine, no boiler is
necessary, and the amount of heat carried over in the cooling water will
only be one-seventh part of what is carried over in the exhaust from a
steam engine of the same power. Therefore, the condenser only need be
one-seventh part the size, and consequently should be made lighter with
the tubes placed at a greater distance apart, and thus reduce the amount
of power necessary to drive the machine through the air. Moreover, the
supply of water necessary will be greatly reduced, and a cheaper and
heavier oil may be employed, which is not so liable to take fire in case
of an accident. It is then only a question as to whether an oil engine
can be made so light as to keep its weight within that of a steam motor;
that is, an oil engine in order to be available for the purpose must be
as light, including its water supply, as a complete steam motor, which
includes not only the engine, but also the boiler, the feed pumps, the
water supply, the burner, the gas generator, and six-sevenths of the
condenser. It requires a very perfect steam engine and boiler, not using
a vacuum, to develop a horse-power with a consumption of 1-1/2 lbs. of
petroleum per hour; but there are many oil engines which develop a
horse-power with rather less than 1 lb. of oil per hour. It will,
therefore, be seen that, as far as fuel is concerned, the oil engine has
a decided advantage over the more complicated steam motor. Moreover,
with an oil engine, the cooling water is not under pressure, so that the
waste of water would be much less than with a steam engine, where the
pressure is so high as to cause a considerable amount of waste through
joints and numerous stuffing-boxes.

The great advances that have been made of late years in electrical
science and engineering have led many to believe that almost any knotty
scientific question may be solved by the employment of electrical
engineering, and a great deal has been written and said in regard to
navigating the air by flying machines driven by electric motors.

Before I commenced my experiments, I made enquiries of all the prominent
electrical engineering establishments where there was any likelihood of
obtaining light and efficient electric motors, and found that it was
impossible to obtain one that would develop a horse-power for any
considerable time that would weigh less than 150 lbs. Since that time,
notwithstanding that a great deal has appeared in the public prints
about the efficiency and lightness of electric motors, I am unable to
learn of any concern that is ready to furnish a complete motor,
including a primary battery, which would supply the necessary current
for two hours at a time, at a weight of less than 150 lbs. per
horse-power, and as far as I have been able to ascertain from what I
have myself seen, I cannot learn that there are any motors in practical
use which do not weigh, including their storage batteries, at least 300
lbs. per horse-power. The last electric motor which I examined was in a
boat; it was driven by a primary battery which weighed over 1,000 lbs.
to the horse-power. From this I am of the opinion that we cannot at
present look to electricity with any hope of finding a motor which is
suitable for the purpose of aerial navigation.


ENGINES.

There is no question but what birds, and for that matter all animals,
when considered as thermo-dynamic machines, are very perfect motors;
they develop the full theoretical amount of energy of the carbon
consumed. This we are quite unable to do with any artificial machine,
but birds, for the most part, have to content themselves with food which
is not very rich in carbon. It is quite true that a bird may develop
from ten to fifteen times as much power from the carbon consumed as can
be developed by the best steam engine, but, as an off-set against this,
a steam engine is able to consume petroleum, which has at least twenty
times as many thermal units per pound as the ordinary food of birds. The
movement of a bird’s wings, from long years of development, has without
doubt attained a great degree of perfection. Birds are able to scull
themselves through the air with very little loss of energy. To imitate
by mechanical means, the exact and delicate motion of their wings would
certainly be a very difficult task, and I do not believe that we should
attempt it in constructing an artificial flying machine. In Nature it is
necessary that an animal should be made all in one piece. It is,
therefore, quite out of the question that any part or parts should
revolve. For land animals there is no question but what legs are the
most perfect system possible, but in terrestrial locomotion by
machinery, not necessarily in one piece, wheels are found to be much
more effective and efficient. The swiftest animal can only travel for a
minute of time at half the speed of a locomotive, while the locomotive
is able to maintain its much greater speed for many hours at a time. The
largest land animals only weigh about 5 tons, while the largest
locomotives weigh from 60 to 80 tons. In the sea, the largest animal
weighs about 75 tons, while the ordinary Atlantic liner weighs from
4,000 to 14,000 tons. The whale, no doubt, is able to maintain a high
speed for several hours at a time, but the modern steamer is able to
maintain a still higher speed for many consecutive days.

As artificial machines for terrestrial and aquatic locomotion have been
made immensely stronger and larger than land or water animals, so with
flying machines, it will be necessary to construct them much heavier and
stronger than the largest bird. If one should attempt to propel such a
machine with wings, it would be quite as difficult a problem to solve as
it would be to make a locomotive that would walk on legs. What is
required in a flying machine is something to which a very large amount
of power can be directly and continuously applied without any
intervening levers or joints, and this we find in the screw propeller.

       *       *       *       *       *

When the Brayton gas engine first made its appearance, I commenced
drawings of a flying machine, using a modification of the Brayton motor
which I designed expressly for the purpose; but even this was found to
be too heavy, and it was not until after I had abandoned the vertical
screw system that it was possible for me to design a machine which, in
theory, ought to fly. The next machine which I considered was on the
kite or aeroplane system. This was also to be driven by an oil engine.
Oil engines at that time were not so simple as now, and, moreover, the
system of ignition was very heavy, cumbersome, and uncertain. Since that
time, however, gas and oil engines have been very much improved, and the
ignition tube which is almost universally used has greatly simplified
the ignition, so that at the present time, I am of the opinion that an
oil engine might be designed which would be suitable for the purpose.

In 1889 I had my attention drawn to some very thin, strong, and
comparatively cheap tubes which were being made in France, and it was
only after I had seen these tubes that I seriously considered the
question of making a flying machine. I obtained a large quantity of them
and found that they were very light, that they would stand enormously
high pressures, and generate a very large quantity of steam. Upon going
into a mathematical calculation of the whole subject, I found that it
would be possible to make a machine on the aeroplane system, driven by a
steam engine, which would be sufficiently strong to lift itself into the
air. I first made drawings of a steam engine, and a pair of these
engines was afterwards made. These engines are constructed, for the most
part, of a very high grade of cast steel, the cylinders being only 3/32
of an inch thick, the crank shafts hollow, and every part as strong and
light as possible. They are compound, each having a high-pressure piston
with an area of 20 square inches, a low-pressure piston of 50·26 square
inches, and a common stroke of 1 foot. When first finished, they were
found to weigh 300 lbs. each; but after putting on the oil cups,
felting, painting, and making some slight alterations, the weight was
brought up to 320 lbs. each, or a total of 640 lbs. for the two engines,
which have since developed 362 horse-power with a steam pressure of 320
lbs. per square inch. A photograph of one of these engines is shown in
Fig. 85.

       *       *       *       *       *

When first designing this engine, I did not know how much power I might
require from it. I thought that in some cases it might be necessary to
allow the high-pressure steam to enter the low-pressure cylinder direct,
but as this would involve a considerable loss, I constructed a species
of an injector. This injector may be so adjusted that when the steam in
the boiler rises above a certain predetermined point, say 300 lbs. to
the square inch, it opens a valve and escapes past the high-pressure
cylinder instead of blowing off at the safety valve. In escaping through
this valve, a fall of about 200 lbs. pressure per square inch is made to
do work on the surrounding steam and to drive it forward in the pipe,
producing a pressure on the low-pressure piston considerably higher than
the back pressure on the high-pressure piston. In this way a portion of
the work which would otherwise be lost is utilised, and it is possible,
with an unlimited supply of steam, to cause the engines to develop an
enormous amount of power.

       *       *       *       *       *

=Boiler Experiments.=--The first boiler which I made was constructed
something on the Herreshoff principle, but instead of having one simple
pipe in one very long coil, I used a series of very small and light
pipes, connected in such a manner that there was a rapid circulation
through the whole--the tubes increasing in size and number as the steam
was generated. I intended that there should be a pressure of about 100
lbs. more on the feed water end of the series than on the steam end, and
I believed that this difference in pressure would be sufficient to
ensure a direct and positive circulation through every tube in the
series. This first boiler was exceedingly light, but the workmanship, as
far as putting the tubes together was concerned, was very bad, and it
was found impossible to so adjust the supply of water as to make dry
steam without overheating and destroying the tubes.

[Illustration: Fig. 90.--Steam boiler employed in my experiments. With
this boiler, I had no trouble in producing all the steam that I could
possibly use, and at any pressure up to 400 lbs. to the square inch.]

[Illustration: Fig. 91.--The burner employed in my steam experiments.
This produced a dense and uniform blue purple flame 20 inch deep.]

Before making another boiler I obtained a quantity of copper tubes,
about 8 feet long, 3/8 inch external diameter, and 1/50 of an inch
thick. I subjected about 100 of these tubes to an internal pressure of 1
ton per square inch of cold kerosine oil, and as none of them leaked I
did not test any more, but commenced my experiments by placing some of
them in a white-hot petroleum fire. I found that I could evaporate as
much as 26-1/2 lbs. of water per square foot of heating surface per
hour, and that with a forced circulation, although the quantity of water
passing was very small but positive, there was no danger of
over-heating. I conducted many experiments with a pressure of over 400
lbs. per square inch, but none of the tubes failed. I then mounted a
single tube in a white-hot furnace, also with a water circulation, and
found that it only burst under steam at a pressure of 1,650 lbs. per
square inch. A large boiler, having about 800 square feet of heating
surface including the feed-water heater, was then constructed. It is
shown in Fig. 90. This boiler is about 4-1/2 feet wide at the bottom, 8
feet long and 6 feet high. It weighs with the casing, the dome, the
smoke stack and connections, a little less than 1,000 lbs. The water
first passes through a system of small tubes--1/4 inch in diameter and
1/60 inch thick--which were placed at the top of the boiler and
immediately over the larger tubes--not shown in the cut. This feed-water
heater is found to be very effective. It utilises the heat of the
products of combustion after they have passed through the boiler proper
and greatly reduces their temperature, while the feed-water enters the
boiler at a temperature of 250° F. A forced circulation is maintained
in the boiler, the feed-water entering through a spring valve, the
spring valve being adjusted in such a manner that the pressure on the
water is always 30 lbs. per square inch in excess of the boiler
pressure. This fall of 30 lbs. in pressure acts upon the surrounding hot
water which has already passed through the tubes, and drives it down
through a vertical outside tube, thus ensuring a positive and rapid
circulation through all the tubes. This apparatus is found to work
extremely well. A little glass tube at the top provided with a moving
button, indicates exactly how many pounds of water per hour are passing
into the boiler. By this means, the engineer is not only enabled to
ascertain at a glance whether or not the pumps are working, but also to
what degree they are working.

Water may be considered as 2,400 times as efficient as air, volume for
volume, in condensing steam. When a condenser is made for the purpose of
using water as a cooling agent, a large number of small tubes may be
grouped together in a box, and the water may be pumped in at one end of
the box and discharged at the other end through relatively small
openings; but when air is employed, the tubes or condensing surface must
be widely distributed, so that a very large amount of air is
encountered, and the air which has struck one tube and become heated
must never strike a second tube.

In order to accomplish this, I make my condenser something in the form
of a Venetian blind, the tubes being made of very thin copper and each
tube in the form of a small aeroplane. These were driven edgewise
through the air, so that the actual volume of air passing between them
is several thousand times greater than the volume of water passing
through a marine condenser. I find that with such a condenser I can
recover the full weight of the copper tubes in water every five minutes,
and if I use aluminium, in half that time. Moreover, experiments have
shown that a condenser may be made to sustain considerably more than its
own weight and the weight of its contents in the air, and that all the
steam may be condensed into water sufficiently cool to be pumped with
certainty.

I find that the most advantageous position for the condenser is
immediately after the screw propellers. In this case, if the machine is
moving through the air at the rate of 50 miles an hour, and the slip of
the screws is 15 miles an hour, it follows that the air will be passing
through the condenser at the rate of 65 miles an hour. At this velocity,
the lifting effect on the narrow aeroplanes forming the condenser is
very great, and at the same time the steam is very rapidly condensed.
The tubes are placed at such an angle as to keep them completely drained
and prevent the accumulation of oil, the steam entering the higher end
and the water being discharged at the lower end.

.       .       .       .       .       .       .       .       .


EXPERIMENTS WITH SMALL MACHINES ATTACHED TO A ROTATING ARM.

These experiments demonstrated most conclusively that as much as 133
lbs. could be sustained and carried by the expenditure of one
horse-power, and that a screw was a fairly efficient air propeller. They
also demonstrated that a well made aeroplane, placed at an angle of 1 in
14, would lift practically fourteen times the thrust required to drive
it through the air, and that the skin friction on a smooth and well
finished aeroplane or screw was so small as not to be considered. A
large number of aeroplanes were experimented with, and it was found that
those which were slightly concave on the underneath side and convex on
the top, both edges being very sharp and the surface very smooth and
regular, were the most efficient; also that with small screw propellers,
two blades having slightly increasing pitch were the most efficient.

       *       *       *       *       *

Since writing the foregoing, great progress has been made with flying
machines, and great disasters have happened to airships or balloons.
Count Zeppelin’s gigantic airship encountered a squall or thunder
shower, and the work of years, which had cost over £100,000, was reduced
to scrap metal in a few minutes. Similar disasters have happened to
other balloons.

The British Dirigible No. 2 has not attempted a long flight, but the
Wright Brothers, Farman, and De la Grange have all met with a certain
degree of success.

A few months ago, the remarkable feats of the Wright Brothers in the
States were discredited in Europe. It was claimed that “the accounts
were not authentic,” “too good to be true,” etc., but recent events have
shown that the Wright Brothers are able to outdo anything that was
reported in the American Press. On many occasions they have remained in
the air for more than an hour, and have travelled at the rate of 30 to
40 miles an hour; in fact, the remarkable success of the Wright Brothers
has placed the true flying machine in a new category.

It can no longer be ranked with the philosopher’s stone or with
perpetual motion. Success is assured, and great and startling events may
take place within the next few years.

[Illustration: Fig. 92.--Count Zeppelin’s aluminium-covered airship
coming out of its shed on Lake Constance.]

[Illustration: Fig. 93.--Count Zeppelin’s airship in full flight.]

[Illustration: Fig. 94.--The new British war balloon “Dirigible” No 2.]

[Illustration: Fig. 95.--The Wright aeroplane in full flight.]



GENERAL INDEX.


                                                                    PAGE

  Accident to my large machine,                                      138
  Action of aeroplanes and power required,                           100
  Adjustment of birds’ wings,                                         19
  Admiralty specification for a steamship,                            48
  Advantages of driving aeroplanes on to new air,                    140
  Advantages and disadvantages of very narrow planes,                143
  Aeroplanes:--
    Action of,                                               31, 32, 100
    Advantageous angle of,                                           139
    Advantages and disadvantages of very narrow,                     143
         „     arising from driving aeroplanes on to new air,        140
    Curvature of,                                                    145
    Evolution of a wide aeroplane,                                   102
    Experiments with,                                              49-59
    Fabric covered,                                                  131
    Lifting effect of,                                               141
    Lifting surface of,                                              103
    Philipps’ sustainers,                                            146
    Reduction of projected horizontal area,                         3, 4
    Shape and efficiency of,                                          99
    Superposed,                                                 144, 146
    Testing fabrics for,                                              50
    The paradox aeroplane,                                            88
  Air currents and the flight of birds,                               11
       „       Conclusions regarding,                                 21
       „       Alpes Maritimes,                                       17
       „       Mediterranean,                                         18
       „       Mid-Atlantic,                                          16
       „       2,000 feet above the earth’s surface,                  22
       „       witnessed at Cadiz,                                    20
  Angles and degrees compared,                                       115
  Antoinette motor, The,                                              89

  Balloons,                                                          120
      „     spiders,                                                  27
  Birds as thermo-dynamic machines,                                  153
    „   Two classes of,                                               23
  Bleriot’s machine,                                                 113
  Boiler experiments,                                                156
  Brayton’s gas engine,                                              154
  British war balloon,                                          159, 162
  Building up of my large screws,                                     41
  Burner employed in my experiments,                                 157

  Character of text-books recently published,                          1
  Circulation of air produced by differences in temperature,          27
  Cody’s kite,                                                    28, 30
  Comparative value of different motors,                             151
  Conclusions regarding air currents,                                 21
  Condensers, Testing of,                                             52
  Condenser tubes,                                                    60
  Continental flying machines,                                         9
  Crystal Palace experiments,                                      72-76

  Darwin on the flight of condors,                                    11
  Deflection of air coming in contact with aeroplanes,                 2
  De la Grange machine, The,                                         110
  “Dirigible” No 2,                                             159, 162
  Drift at various distances from center to center, Table of,         58
  Dynagraphs,                                                        136
  Dynamic energy of animals,                                         127

  Eagles, Flight of,                                                  19
  Efficiency of screw propellers,                                    147
       „        screws in steamships,                                 47
  Energy developed by a bird,                                         13
  Engines,                                                           153
  Equivalent inclinations,                                           115
       „     velocities,                                             116
  Experiments of Count Zeppelin,                                     124
       „         Horatio Philipps,                      9, 118, 119, 145
       „         Lord Rayleigh in reference to Newton’s Law,           6
       „         Professor Langley,                       9, 62, 99, 109
       „         Wright Bros.,                                       109
  Experiments to show efficiency of Screw propellers,                 33
       „      with apparatus attached to rotating arm,                62
       „       „   boiler,                                           156
       „       „   hard rolled brass aeroplane,                        3
       „       „   my large machine,                             10, 133
       „       „   rotating arm,                                   64-72
       „       „   small machines attached to rotating arm,          159

  Fabric covered screw,                                               40
  Farman’s machine,                                                  110
  Flying of kites,                                            25, 28, 29
  Forced circulation used by me,                                     158
  Formulæ unsupported by facts,                                        3
  French and English measurements,                              128, 129

  Gulls,                                                          20, 21
  Gyroscope apparatus,                                                93
      „     Steering by means of,                                     92

  Hawks and Eagles,                                                   13
  Hélicoptère machine,                                                82
  Hints as to the building of flying machines,                     77-91
  Horizontal movement of the air,                                     14
  Hub for flying machine, New form of,                                45

  Interstellar temperature,                                           15
  Introductory,                                                        1

  Kites,                                          21, 22, 25, 26, 28, 29
    „   Behaviour of,                                                 26
    „   Flying of,                                            25, 28, 29

  Langley, Experiments of,                                9, 62, 99, 109
     „     on the flight of birds,                                11, 22
     „     on the power exercised by birds,                           12
  “La Patrie,”                                                       124
  Lifting effect of aeroplanes,                                        5
     „    surface of aeroplanes,                                     103
  Low temperature of space,                                           15

  Major Baden Powell’s demand,                                       125
  Mistral, The,                                                       21
  Motors, Development of,                                             31
  Motor, The Antoinette,                                              89
  My compound engines,                                               142
   „ experiments with aeroplanes,                                      7
   „         „        large machine,                             10, 133
   „ steam engines,                                                  155

  Newton’s Law,                                                     2, 6
  “Nulli Secundus,”                                              28, 121

  Oil engines,                                                       154

  Philipps’ experiments,                                9, 118, 119, 145
      „     sustainers,                                              145
  Pneumatic buffer,                                                   90
  Position of screw,                                                  49
  Power exerted by a land animal,                                     13
    „   required,                                                    100
  Principally relating to screws,                                     31

  Rayleigh’s experiments in reference to Newton’s law,                 6
  Recapitulation of early experiments,                               130
  Recent machines,                                                   109
  Relative value of woods for flying machines,                        85
  Reserve energy necessary in flying machines,                        30
  Resistance encountered by various shaped bodies,                    52
  Rotating arm experiments,                                        64-72

  Santos Dumont’s flying machine,                                    113
  Screw blade on Farman’s machine,                                    41
    „   blades, Testing of,                                           36
    „      „    used by the French Government,                        39
    „      „    with radial edges,                                    43
    „   Fabric-covered,                                               40
    „   Position of,                                                  49
    „   propeller made of sheet metal,                                41
    „   propellers, Efficiency of,                               33, 147
  Screws,                              8, 31, 35, 36, 40, 41, 46, 47, 49
    „   Building up of my large,                                      40
    „   their efficiency in steamships,                               47
  Shape and efficiency of aeroplanes,                                 99
  Skin friction,                                                  41, 48
  Spider’s webbing down from the sky,                                 27
  Spirit lamp and ice box,                                            62
  Stability of flying machines,                                 147, 150
  Steam engines used by me,                                          155
  Steering,                                                     147, 149
  Superposed aeroplanes,                                             144
  System of splicing and building up wooden members,                  86

  Tables:--
    Equivalent inclinations,                                         115
         „     velocities,                                           116
    French and English measurements,                            128, 129
    Philipps’ experiments,                                           119
    Relative value of different woods,                                85
    Showing the relative power exerted by different birds,            24
    Velocity and pressure of the wind,                               114
        „     „  thrust corresponding with various horse-powers,     117
  Testing aeroplanes, condensers, etc.,                               52
  Teutonic vision of aerial power,                                   126

  Velocity and pressure of wind,                                     114
     „      „  thrust corresponding with various horse-powers,       117
  “Ville de Paris,”                                                  123

  Wright Bros.’ experiments,                               109, 159, 162

  Zeppelin’s experiments,                                  124, 159, 161


BELL AND BAIN, LIMITED, PRINTERS, GLASGOW.



  Transcriber’s Notes:

  Inconsistent spelling and hyphenation in the original work have been
  retained.

  Depending on the hard- and software used to read this text and their
  settings, not all characters and symbols may display as intended.

  Changes made to the text:

  Illustrations and tables have been moved out of the text, and
  footnotes have been moved to directly under the paragraph or table
  they refer to.

  Obvious punctuation and typographical errors have been corrected
  silently.

  In some formulas × has been inserted for consistency; brackets
  have been added for clarity where necessary.

  Page vii: battaillons Kommandeur changed to Bataillonskommandeur

  Page x: 1 in 20 changed to 1 in 10 (as discussed in the remainder of
  the paragraph)

  Page 24, table: ·64 changed to 7·64

  Page 73: all the air effected changed to all the air affected

  Page 110: Hargraves’ changed to Hargrave’s

  Page 128: over estimated changed to over-estimated.





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