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´╗┐Title: The Aeroplane Speaks
Author: Barber, H. (Horatio), 1875-1964
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.

*** Start of this Doctrine Publishing Corporation Digital Book "The Aeroplane Speaks" ***

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THE AEROPLANE SPEAKS

By H. Barber

(Captain, Royal Flying Corps)



DEDICATED TO THE SUBALTERN FLYING OFFICER



MOTIVE

The reasons impelling me to write this book, the maiden effort of
my pen, are, firstly, a strong desire to help the ordinary man to
understand the Aeroplane and the joys and troubles of its Pilot; and,
secondly, to produce something of PRACTICAL assistance to the Pilot
and his invaluable assistant the Rigger. Having had some eight years'
experience in designing, building, and flying aeroplanes, I have hopes
that the practical knowledge I have gained may offset the disadvantage
of a hand more used to managing the "joy-stick" than the dreadful
haltings, the many side-slips, the irregular speed, and, in short, the
altogether disconcerting ways of a pen.

The matter contained in the Prologue appeared in the Field of May 6th,
13th, 20th, and 27th, 1916, and is now reprinted by the kind permission
of the editor, Sir Theodore Cook.

I have much pleasure in also acknowledging the kindness of Mr. C. G.
Grey, editor of the Aeroplane, to whom I am indebted for the valuable
illustrations reproduced at the end of this book.



CONTENTS

  PROLOGUE

  PART
  I.   THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
  II.  THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
  III. THE GREAT TEST
  IV.  CROSS COUNTRY



  CHAPTER
  I.   FLIGHT
  II.  STABILITY AND CONTROL
  III. RIGGING
  IV.  PROPELLERS
  V.   MAINTENANCE


  TYPES OF AEROPLANES

  GLOSSARY



THE AEROPLANE SPEAKS



PROLOGUE



PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES

The Lecture Hall at the Royal Flying Corps School for Officers was
deserted. The pupils had dispersed, and the Officer Instructor, more
fagged than any pupil, was out on the aerodrome watching the test of a
new machine.

Deserted, did I say? But not so. The lecture that day had been upon
the Elementary Principles of Flight, and they lingered yet. Upon the
Blackboard was the illustration you see in the frontispiece.

"I am the side view of a Surface," it said, mimicking the tones of the
lecturer. "Flight is secured by driving me through the air at an angle
inclined to the direction of motion."

"Quite right," said the Angle. "That's me, and I'm the famous Angle of
Incidence."

"And," continued the Surface, "my action is to deflect the air
downwards, and also, by fleeing from the air behind, to create a
semi-vacuum or rarefied area over most of the top of my surface."

"This is where I come in," a thick, gruff voice was heard, and went
on: "I'm the Reaction. You can't have action without me. I'm a very
considerable force, and my direction is at right-angles to you," and
he looked heavily at the Surface. "Like this," said he, picking up the
chalk with his Lift, and drifting to the Blackboard.

"I act in the direction of the arrow R, that is, more or less, for the
direction varies somewhat with the Angle of Incidence and the curvature
of the Surface; and, strange but true, I'm stronger on the top of the
Surface than at the bottom of it. The Wind Tunnel has proved that by
exhaustive research--and don't forget how quickly I can grow! As the
speed through the air increases my strength increases more rapidly than
you might think--approximately, as the Square of the Speed; so you
see that if the Speed of the Surface through the air is, for instance,
doubled, then I am a good deal more than doubled. That's because I am
the result of not only the mass of air displaced, but also the result
of the Speed with which the Surface engages the Air. I am a product of
those two factors, and at the speeds at which Aeroplanes fly to-day,
and at the altitudes and consequent density of air they at present
experience, I increase at about the Square of the Speed.

"Oh, I'm a most complex and interesting personality, I assure you--in
fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr.
Hyde. There's Lift, my vertical part or COMPONENT, as those who prefer
long words would say; he always acts vertically upwards, and hates
Gravity like poison. He's the useful and admirable part of me. Then
there's Drift, my horizontal component, sometimes, though rather
erroneously, called Head Resistance; he's a villain of the deepest dye,
and must be overcome before flight can be secured."

"And I," said the Propeller, "I screw through the air and produce the
Thrust. I thrust the Aeroplane through the air and overcome the Drift;
and the Lift increases with the Speed and when it equals the Gravity of
Weight, then--there you are--Flight! And nothing mysterious about it at
all."

"I hope you'll excuse me interrupting," said a very beautiful young
lady, "my name is Efficiency, and, while no doubt, all you have said is
quite true, and that, as my young man the Designer says, `You can make a
tea-tray fly if you slap on Power enough,' I can assure you that I'm not
to be won quite so easily."

"Well," eagerly replied the Lift and the Thrust, "let's be friends. Do
tell us what we can do to help you to overcome Gravity and Drift with
the least possible Power. That obviously seems the game to play, for
more Power means heavier engines, and that in a way plays into the hands
of our enemy, Gravity, besides necessitating a larger Surface or Angle
to lift the Weight, and that increases the Drift."

"Very well," from Efficiency, "I'll do my best, though I'm so shy, and
I've just had such a bad time at the Factory, and I'm terribly afraid
you'll find it awfully dry."

"Buck up, old dear!" This from several new-comers, who had just
appeared. "We'll help you," and one of them, so lean and long that he
took up the whole height of the lecture room, introduced himself.

"I'm the High Aspect Ratio," he said, "and what we have got to do to
help this young lady is to improve the proportion of Lift to Drift.
The more Lift we can get for a certain area of Surface, the greater
the Weight the latter can carry; and the less the Drift, then the less
Thrust and Power required to overcome it. Now it is a fact that, if the
Surface is shaped to have the greatest possible span, i.e., distance
from wing-tip to wing-tip, it then engages more air and produces both a
maximum Reaction and a better proportion of Lift to Drift.

"That being so, we can then well afford to lose a little Reaction
by reducing the Angle of Incidence to a degree giving a still better
proportion of Lift to Drift than would otherwise be the case; for you
must understand that the Lift-Drift Ratio depends very much upon the
size of the Angle of Incidence, which should be as small as possible
within certain limits. So what I say is, make the surface of Infinite
Span with no width or chord, as they call it. That's all I require, I
assure you, to make me quite perfect and of infinite service to Miss
Efficiency."

"That's not practical politics," said the Surface. "The way you talk one
would think you were drawing L400 a year at Westminster, and working up
a reputation as an Aeronautical Expert. I must have some depth and chord
to take my Spars and Ribs, and again, I must have a certain chord to
make it possible for my Camber (that's curvature) to be just right for
the Angle of Incidence. If that's not right the air won't get a nice
uniform compression and downward acceleration from my underside, and the
rarefied `suction' area over the top of me will not be as even and clean
in effect as it might be. That would spoil the Lift-Drift Ratio more
than you can help it. Just thrust that chalk along, will you? and the
Blackboard will show you what I mean."

"Well," said the Aspect Ratio, "have it your own way, though I'm sorry
to see a pretty young lady like Efficiency compromised so early in the
game."

"Look here," exclaimed a number of Struts, "we have got a brilliant idea
for improving the Aspect Ratio," and with that they hopped up on to the
Spars. "Now," excitedly, "place another Surface on top of us. Now do you
see? There is double the Surface, and that being so, the proportion of
Weight to Surface area is halved. That's less burden of work for the
Surface, and so the Spars need not be so strong and so deep, which
results in not so thick a Surface. That means the Chord can be
proportionately decreased without adversely affecting the Camber.
With the Chord decreased, the Span becomes relatively greater, and so
produces a splendid Aspect Ratio, and an excellent proportion of Lift to
Drift."

"I don't deny that they have rather got me there," said the Drift, "but
all the same, don't forget my increase due to the drift of the Struts
and their bracing wires."

"Yes, I dare say," replied the Surface, "but remember that my Spars are
less deep than before, and consequently I am not so thick now, and
shall for that reason also be able to go through the air with a less
proportion of Drift to Lift."

"Remember me also, please," croaked the Angle of Incidence. "Since the
Surface has now less weight to carry for its area, I may be set at
a still lesser and finer Angle. That means less Drift again. We are
certainly getting on splendidly! Show us how it looks now, Blackboard."
And the Blackboard obligingly showed them as follows:

"Well, what do you think of that?" they all cried to the Drift.

"You think you are very clever," sneered the Drift. "But you are not
helping Efficiency as much as you think. The suction effect on the top
of the lower Surface will give a downward motion to the air above it and
the result will be that the bottom of the top Surface will not secure
as good a Reaction from the air as would otherwise be the case, and that
means loss of Lift; and you can't help matters by increasing the gap
between the surfaces because that means longer Struts and Wires, and
that in itself would help me, not to speak of increasing the Weight. You
see it's not quite so easy as you thought."

At this moment a hiccough was heard, and a rather fast and
rakish-looking chap, named Stagger, spoke up. "How d'ye do, miss," he
said politely to Efficiency, with a side glance out of his wicked old
eye. "I'm a bit of a knut, and without the slightest trouble I can
easily minimize the disadvantage that old reprobate Drift has been
frightening you with. I just stagger the top Surface a bit forward, and
no longer is that suction effect dead under it. At the same time I'm
sure the top Surface will kindly extend its Span for such distance as
its Spars will support it without the aid of Struts. Such extension will
be quite useful, as there will be no Surface at all underneath it to
interfere with the Reaction above." And the Stagger leaned forward and
picked up the Chalk, and this is the picture he drew:

Said the Blackboard, "That's not half bad! It really begins to look
something like the real thing, eh?"

"The real thing, is it?" grumbled Drift. "Just consider that contraption
in the light of any one Principle, and I warrant you will not find
one of them applied to perfection. The whole thing is nothing but a
Compromise." And he glared fixedly at poor Efficiency.

"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What
WILL the Designer say?"

"Never mind, my dear," said the Lift-Drift Ratio, consolingly. "You are
improving rapidly, and quite useful enough now to think of doing a job
of work."

"Well, that's good news," and Efficiency wiped her eyes with her Fabric
and became almost cheerful. "Suppose we think about finishing it now?
There will have to be an Engine and Propeller, won't there? And a body
to fix them in, and tanks for oil and petrol, and a tail, and," archly,
"one of those dashing young Pilots, what?"

"Well, we are getting within sight of those interesting Factors," said
the Lift-Drift Ratio, "but first of all we had better decide upon the
Area of the Surfaces, their Angle of Incidence and Camber. If we are
to ascend as quickly as possible the Aeroplane must be SLOW in order to
secure the best possible Lift-Drift Ratio, for the drift of the struts
wires, body, etc., increases approximately as the square of the speed,
but it carries with it no lift as it does in the case of the Surface.
The less speed then, the less such drift, and the better the Aeroplane's
proportion of lift to drift; and, being slow, we shall require a LARGE
SURFACE in order to secure a large lift relative to the weight to be
carried. We shall also require a LARGE ANGLE OF INCIDENCE relative to
the horizontal, in order to secure a proper inclination of the Surface
to the direction of motion, for you must remember that, while we shall
fly upon an even keel and with the propeller thrust horizontal (which is
its most efficient attitude), our flight path, which is our direction of
motion, will be sloping upwards, and it will therefore be necessary to
fix the Surface to the Aeroplane at a very considerable angle relative
to the horizontal Propeller Thrust in order to secure a proper angle
to the upwards direction of motion. Apart from that, we shall require a
larger Angle of Incidence than in the case of a machine designed purely
for speed, and that means a correspondingly LARGE CAMBER.

"On the other hand, if we are thinking merely of Speed, then a SMALL
SURFACE, just enough to lift the weight off the ground, will be best,
also a SMALL ANGLE to cut the Drift down and that, of course, means a
relatively SMALL CAMBER.

"So you see the essentials for CLIMB or quick ascent and for SPEED are
diametrically opposed. Now which is it to be?"

"Nothing but perfection for me," said Efficiency. "What I want is
Maximum Climb and Maximum Speed for the Power the Engine produces."

And each Principle fully agreed with her beautiful sentiments, but work
together they would not.

The Aspect Ratio wanted infinite Span, and hang the Chord.

The Angle of Incidence would have two Angles and two Cambers in one,
which was manifestly absurd; the Surface insisted upon no thickness
whatever, and would not hear of such things as Spars and Ribs; and the
Thrust objected to anything at all likely to produce Drift, and very
nearly wiped the whole thing off the Blackboard.

There was, indeed, the makings of a very pretty quarrel when the Letter
arrived. It was about a mile long, and began to talk at once.

"I'm from the Inventor," he said, and hope rose in the heart of each
heated Principle. "It's really absurdly simple. All the Pilot has to do
is to touch a button, and at his will, VARY the area of the Surface, the
Angle of Incidence, and the Camber! And there you are--Maximum Climb or
Maximum Speed as required! How does that suit you?"

"That suits us very well," said the Surface, "but, excuse me asking, how
is it done without apparatus increasing the Drift and the Weight out
of all reason? You won't mind showing us your Calculations, Working
Drawings, Stress Diagrams, etc., will you?"

Said the Letter with dignity, "I come from an Inventor so brilliantly
clever as to be far above the unimportant matters you mention. He is no
common working man, sir! He leaves such things to Mechanics. The point
is, you press a button and----"

"Look here," said a Strut, rather pointedly, "where do you think you are
going, anyway?"

"Well," from the Letter, "as a matter of fact, I'm not addressed yet,
but, of course, there's no doubt I shall reach the very highest quarters
and absolutely revolutionize Flight when I get there."

Said the Chalk, "I'll address you, if that's all you want; now drift
along quickly!" And off went the Letter to The Technical Editor, "Daily
Mauler," London.

And a League was formed, and there were Directors with Fees, and several
out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine
fat Guinea-pig, and all the rest of them. And the Inventor paid his
Tailor and had a Hair-Cut, and is now a recognized Press Expert--but he
is still waiting for those Mechanics!

"I'm afraid," said the Slide-rule, who had been busy making those
lightning-like automatic calculations for which he is so famous, "it's
quite impossible to fully satisfy all of you, and it is perfectly plain
to me that we shall have to effect a Compromise and sacrifice some of
the Lift for Speed."

Thud! What was that?

Efficiency had fainted dead away! The last blow had been too much for
her. And the Principles gathered mournfully round, but with the aid of
the Propeller Slip[1] and a friendly lift from the Surface she was at
length revived and regained a more normal aspect.

Said the Stagger with a raffish air, "My dear young lady, I assure
you that from the experiences of a varied career, I have learned that
perfection is impossible, and I am sure the Designer will be quite
satisfied if you become the Most Efficient Compromise."

"Well, that sounds so common sense," sighed Efficiency, "I suppose it
must be true, and if the Designer is satisfied, that's all I really care
about. Now do let's get on with the job."

So the Chalk drew a nice long slim body to hold the Engine and the
tanks, etc., with room for the Pilot's and Passenger's seats, and placed
it exactly in the middle of the Biplane. And he was careful to make its
position such that the Centre of Gravity was a little in advance of the
Centre of Lift, so that when the Engine was not running and there was
consequently no Thrust, the Aeroplane should be "nose-heavy" just to the
right degree, and so take up a natural glide to Earth--and this was to
help the Pilot and relieve him of work and worry, should he find himself
in a fog or a cloud. And so that this tendency to glide downwards should
not be in evidence when the Engine was running and descent not desired,
the Thrust was placed a little below the Centre of Drift or Resistance.
In this way it would in a measure pull the nose of the Aeroplane up and
counterbalance the "nose-heavy" tendency.

And the Engine was so mounted that when the Propeller-Thrust was
horizontal, which is its most efficient position, the Angle of Incidence
and the Area of the surfaces were just sufficient to give a Lift a
little in excess of the Weight. And the Camber was such that, as far as
it was concerned, the Lift-Drift Ratio should be the best possible for
that Angle of Incidence. And a beautifully simple under-carriage was
added, the outstanding features of which were simplicity, strength,
light-weight, and minimum drift. And, last of all, there was the
Elevator, of which you will hear more by-and-by. And this is what it
looked like then:

And Efficiency, smiling, thought that it was not such a bad compromise
after all and that the Designer might well be satisfied.

"Now," said she, "there's just one or two points I'm a bit hazy about.
It appears that when the Propeller shaft is horizontal and so working
in its most efficient attitude, I shall have a Lift from the Surfaces
slightly in excess of the Weight. That means I shall ascend slightly,
at the same time making nearly maximum speed for the power and thrust.
Can't I do better than that?"

"Yes, indeed," spoke up the Propeller, "though it means that I must
assume a most undignified attitude, for helicopters[2] I never
approved of. In order to ascend more quickly the Pilot will deflect the
Elevator, which, by the way, you see hinged to the Tail. By that
means he will force the whole Aeroplane to assume a greater Angle of
Incidence. And with greater Angle, the Lift will increase, though I'm
sorry to say the Drift will increase also. Owing to the greater Drift,
the Speed through the air will lessen, and I'm afraid that won't be
helpful to the Lift; but I shall now be pointing upwards, and besides
overcoming the Drift in a forward direction I shall be doing my best
to haul the Aeroplane skywards. At a certain angle known as the Best
Climbing Angle, we shall have our Maximum Margin of Lift, and I'm hoping
that may be as much as almost a thousand feet altitude a minute."

"Then, if the Pilot is green, my chance will come," said the Maximum
Angle of Incidence. "For if the Angle is increased over the Best
Climbing Angle, the Drift will rush up; and the Speed, and with it the
Lift, will, when my Angle is reached, drop to a point when the latter
will be no more than the Weight. The Margin of Lift will have entirely
disappeared, and there we shall be, staggering along at my tremendous
angle, and only just maintaining horizontal flight."

"And then with luck I'll get my chance," said the Drift. "If he is a bit
worse than green, he'll perhaps still further increase the Angle. Then
the Drift, largely increasing, the Speed, and consequently the Lift,
will become still less, i.e., less than the Weight, and then--what price
pancakes,[3] eh?"

"Thank you," from Efficiency, "that was all most informing. And now will
you tell me, please, how the greatest Speed may be secured?"

"Certainly, now it's my turn," piped the Minimum Angle of Incidence. "By
means of the Elevator, the Pilot places the Aeroplane at my small Angle,
at which the Lift only just equals the Weight, and, also, at which we
shall make greater speed with no more Drift than before. Then we get our
greatest Speed, just maintaining horizontal flight."

"Yes; though I'm out of the horizontal and thrusting downwards,"
grumbled the Propeller, "and that's not efficient, though I suppose it's
the best we can do until that Inventor fellow finds his Mechanics."

"Thank you so much," said Efficiency. "I think I have now at any rate
an idea of the Elementary Principles of Flight, and I don't know that I
care to delve much deeper, for sums always give me a headache; but isn't
there something about Stability and Control? Don't you think I ought to
have a glimmering of them too?"

"Well, I should smile," said a spruce Spar, who had come all the way
from America. "And that, as the Lecturer says, `will be the subject of
our next lecture,' so be here again to-morrow, and you will be glad to
hear that it will be distinctly more lively than the subject we have
covered to-day."



PART II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE
JOB

Another day had passed, and the Flight Folk had again gathered together
and were awaiting the arrival of Efficiency who, as usual, was rather
late in making an appearance.

The crowd was larger than ever, and among the newcomers some of the most
important were the three Stabilities, named Directional, Longitudinal,
and Lateral, with their assistants, the Rudder, Elevator, and Ailerons.
There was Centrifugal Force, too, who would not sit still and created a
most unfavourable impression, and Keel-Surface, the Dihedral Angle, and
several other lesser fry.

"Well," said Centrifugal Force, "I wish this Efficiency I've heard so
much about would get a move on. Sitting still doesn't agree with me
at all. Motion I believe in. There's nothing like motion--the more the
better."

"We are entirely opposed to that," objected the three Stabilities, all
in a breath. "Unless it's in a perfectly straight line or a perfect
circle. Nothing but perfectly straight lines or, upon occasion, perfect
circles satisfy us, and we are strongly suspicious of your tendencies."

"Well, we shall see what we shall see," said the Force darkly. "But who
in the name of blue sky is this?"

And in tripped Efficiency, in a beautifully "doped" dress of the latest
fashionable shade of khaki-coloured fabric, a perfectly stream-lined
bonnet, and a bewitching little Morane parasol,[4] smiling as
usual, and airily exclaiming, "I'm so sorry I'm late, but you see the
Designer's such a funny man. He objects to skin friction,[5] and
insisted upon me changing my fabric for one of a smoother surface, and
that delayed me. Dear me, there are a lot more of us to-day, aren't
there? I think I had better meet one at a time." And turning to
Directional Stability, she politely asked him what he preferred to do.

"My purpose in life, miss," said he, "is to keep the Aeroplane on its
course, and to achieve that there must be, in effect, more Keel-Surface
behind the Vertical Turning Axis than there is in front of it."

Efficiency looking a little puzzled, he added: "Just like a weathercock,
and by Keel-Surface I mean everything you can see when you view the
Aeroplane from the side of it--the sides of the body, struts, wires,
etc."

"Oh, now I begin to see light," said she: "but just exactly how does it
work?"

"I'll answer that," said Momentum. "When perhaps by a gust of air the
Aeroplane is blown out of its course and points in another direction, it
doesn't immediately fly off on that new course. I'm so strong I pull it
off the new course to a certain extent, and towards the direction of the
old course. And so it travels, as long as my strength lasts, in a more
or less sideways position."

"Then," said the Keel-Surface, "I get a pressure of air all on one side,
and as there is, in effect, most of me towards the tail, the latter
gets pressed sideways, and the Aeroplane thus tends to assume its first
position and course."

"I see," said Efficiency, and, daintily holding the Chalk, she
approached the Blackboard. "Is this what you mean?"

"Yes, that's right enough," said the Keel-Surface, "and you might
remember, too, that I always make the Aeroplane nose into the gusts
rather than away from them."

"If that was not the case," broke in Lateral Stability, and affecting
the fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible
affair! If there were too much Keel-Surface in front, then that gust
would blow the Aeroplane round the other way a very considerable
distance. And the right-hand Surface being on the outside of the turn
would have more speed, and consequently more Lift, than the Surface
on the other side. That means a greater proportion of the Lift on that
side, and before you could say Warp to the Ailerons over the Aeroplane
would go--probable result a bad side-slip"

"And what can the Pilot do to save such a situation as that?" said
Efficiency.

"Well," replied Lateral Stability, "he will try to turn the Aeroplane
sideways and back to an even keel by means of warping the Ailerons or
little wings which are hinged on to the Wing-tips, and about which you
will hear more later on; but if the side-slip is very bad he may not be
able to right the Aeroplane by means of the Ailerons, and then the only
thing for him to do is to use the Rudder and to turn the nose of the
Aeroplane down and head-on to the direction of motion. The Aeroplane
will then be meeting the air in the direction it is designed to do
so, and the Surfaces and also the controls (the Rudder, Ailerons, and
Elevator) will be working efficiently; but its attitude relative to
the earth will probably be more or less upside-down, for the action
of turning the Aeroplane's nose down results, as you will see by the
illustration B, in the right wing, which is on the outside of the
circle. travelling through the air with greater speed than the left-hand
wing. More Speed means more Lift, so that results in overturning the
Aeroplane still more; but now it is, at any rate, meeting the air as it
is designed to meet it, and everything is working properly. It is then
only necessary to warp the Elevator, as shown in illustration C, in
order to bring the Aeroplane into a proper attitude relative to the
earth."

"Ah!" said the Rudder, looking wise, "it's in a case like that when I
become the Elevator and the Elevator becomes me."

"That's absurd nonsense," said the Blackboard, "due to looseness of
thought and expression."

"Well," replied the Rudder, "when 'the Aeroplane is in position A and I
am used, then I depress or ELEVATE the nose of the machine; and, if the
Elevator is used, then it turns the Aeroplane to right or left, which is
normally my function. Surely our roles have changed one with the other,
and I'm then the Elevator and the Elevator is me!"

Said Lateral Stability to the Rudder, "That's altogether the wrong way
of looking at it, though I admit"--and this rather sarcastically--"that
the way you put it sounds rather fine when you are talking of your
experiences in the air to those 'interested in aviation' but knowing
little about it; but it won't go down here! You are a Controlling
Surface designed to turn the Aeroplane about its vertical axis, and the
Elevator is a Controlling Surface designed to turn the Aeroplane about
its lateral axis. Those are your respective jobs, and you can't possibly
change them about. Such talk only leads to confusion, and I hope we
shall hear no more of it."

"Thanks," said Efficiency to Lateral Stability. "And now, please, will
you explain your duties?"

"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.
First of all, I sometimes arrange with the Rigger to wash-out, that is
decrease, the Angle of Incidence on one side of the Aeroplane, and to
effect the reverse condition, if it is not too much trouble, on the
other side."

"But," objected Efficiency, "the Lift varies with the Angle of
Incidence, and surely such a condition will result in one side of the
Aeroplane lifting more than the other side?'

"That's all right," said the Propeller, "it's meant to off-set the
tendency of the Aeroplane to turn over sideways in the opposite
direction to which I revolve."

"That's quite clear, though rather unexpected; but how do you counteract
the effect of the gusts when they try to overturn the Aeroplane
sideways?" said she, turning to Lateral Stability again.

"Well," he replied, rather miserably, "I'm not nearly so perfect as the
Longitudinal and Directional Stabilities. The Dihedral Angle--that is,
the upward inclination of the Surfaces towards their wing-tips--does
what it can for me, but, in my opinion, it's a more or less futile
effort. The Blackboard will show you the argument." And he at once
showed them two Surfaces, each set at a Dihedral Angle like this:

"Please imagine," said the Blackboard, "that the top V is the front
view of a Surface flying towards you. Now if a gust blows it into the
position of the lower V you see that the horizontal equivalent of the
Surface on one side becomes larger, and on the other side it becomes
smaller. That results in more Lift on the lower side and less on the
higher side, and if the V is large enough it should produce such a
difference in the Lift of one side to the other as to quickly turn the
Aeroplane back to its former and normal position."

"Yes," said the Dihedral Angle, "that's what would happen if they would
only make me large enough; but they won't do it because it would too
greatly decrease the horizontal equivalent, and therefore the Lift, and
incidentally it would, as Aeroplanes are built to-day, produce an excess
of Keel Surface above the turning axis, and that in itself would spoil
the Lateral Stability. The Keel Surface should be equally divided above
and below the longitudinal turning axis (upon which the Aeroplane rolls
sideways), or the side upon which there is an excess will get blown over
by the gusts. It strikes me that my future isn't very promising, and
about my only chance is when the Junior Draughtsman makes a mistake, as
he did the other day. And just think of it, they call him a Designer now
that he's got a job at the Factory! What did he do? Why, he calculated
the weights wrong and got the Centre of Gravity too high, and they
didn't discover it until the machine was built. Then all they could do
was to give me a larger Angle. That dropped the bottom of the V lower
down, and as that's the centre of the machine, where all the Weight is,
of course that put the Centre of Gravity in its right place. But now
there is too much Keel Surface above, and the whole thing's a Bad
Compromise, not at all like Our Efficiency."

And Efficiency, blushing very prettily at the compliment, then asked,
"And how does the Centre of Gravity affect matters?"

"That's easy," said Grandfather Gravity. "I'm so heavy that if I am too
low down I act like a pendulum and cause the Aeroplane to roll about
sideways, and if I am too high I'm like a stick balanced on your finger,
and then if I'm disturbed, over I go and the Aeroplane with me; and, in
addition to that, there are the tricks I play with the Aeroplane when
it's banked up,[6] i.e., tilted sideways for a turn, and Centrifugal
Force sets me going the way I'm not wanted to go. No; I get on best with
Lateral Stability when my Centre is right on the centre of Drift, or,
at any rate, not much below it." And with that he settled back into the
Lecturer's Chair and went sound asleep again, for he was so very, very
old, in fact the father of all the Principles.

And the Blackboard had been busy, and now showed them a picture of
the Aeroplane as far as they knew it, and you will see that there is
a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel
Surface or fin, as is very often the case in order to ensure the greater
effect of such surface being behind the vertical turning axis.

But Efficiency, growing rather critical with her newly gained knowledge,
cried out: "But where's the horizontal Tail Surface? It doesn't look
right like that!"

"This is when I have the pleasure of meeting you, my dear," said
Longitudinal Stability. "Here's the Tail Surface," he said, "and in
order to help me it must be set IN EFFECT at a much less Angle of
Incidence than the Main Surface. To explain we must trouble the
Blackboard again," and this was his effort:

"I have tried to make that as clear as possible," he said. "It may
appear a bit complicated at first, but if you will take the trouble to
look at it for a minute you will find it quite simple. A is the normal
and proper direction of motion of the Aeroplane, but, owing to a gust of
air, it takes up the new nose-down position. Owing to Momentum, however,
it does not fly straight along in that direction, but moves more or less
in the direction B, which is the resultant of the two forces, Momentum
and Thrust. And so you will note that the Angle of Incidence, which
is the inclination of the Surfaces to the Direction of Motion, has
decreased, and of course the Lift decreases with it. You will also
see, and this is the point, that the Tail Surface has lost a higher
proportion of its Angle, and consequently its Lift, than has the Main
Surface. Then, such being the case, the Tail must fall and the Aeroplane
assume its normal position again, though probably at a slightly lower
altitude."

"I'm afraid I'm very stupid," said Efficiency, "but please tell me why
you lay stress upon the words 'IN EFFECT.'"

"Ah! I was wondering if you would spot that," he replied. "And there is
a very good reason for it. You see, in some Aeroplanes the Tail Surface
may be actually set at the same Angle on the machine as the Main
Surface, but owing to the air being deflected downwards by the front
Main Surface it meets the Tail Surface at a lesser angle, and indeed in
some cases at no angle at all. The Tail is then for its surface getting
less Lift than the Main Surface, although set at the same angle on
the machine. It may then be said to have IN EFFECT a less Angle of
Incidence. I'll just show you on the Blackboard."

"And now," said Efficiency, "I have only to meet the Ailerons and the
Rudder, haven't I?"

"Here we are," replied the Ailerons, or little wings. "Please hinge us
on to the back of the Main Surfaces, one of us at each Wing-tip, and
join us up to the Pilot's joystick by means of the control cables. When
the Pilot wishes to tilt the Aeroplane sideways, he will move the stick
and depress us upon one side, thus giving us a larger Angle of Incidence
and so creating more Lift on that side of the Aeroplane; and, by means
of a cable connecting us with the Ailerons on the other side of the
Aeroplane, we shall, as we are depressed, pull them up and give them a
reverse or negative Angle of Incidence, and that side will then get
a reverse Lift or downward thrust, and so we are able to tilt the
Aeroplane sideways.

"And we work best when the Angle of Incidence of the Surface in front
of us is very small, for which reason it is sometimes decreased or
washed-out towards the Wing-tips. The reason of that is that by the
time the air reaches us it has been deflected downwards--the greater the
Angle of Incidence the more it is driven downwards--and in order for
us to secure a Reaction from it, we have to take such a large Angle of
Incidence that we produce a poor proportion of Lift to Drift; but the
smaller the Angle of the Surface in front of us the less the air is
deflected downwards, and consequently the less Angle is required of us,
and the better our proportion of Lift to Drift, which, of course, makes
us much more effective Controls."

"Yes," said the Lateral and Directional Stabilities in one voice,
"that's so, and the wash-out helps us also, for then the Surfaces
towards their Wing-tips have less Drift or 'Head-Resistance,' and
consequently the gusts will affect them and us less; but such decreased
Angle of Incidence means decreased Lift as well as Drift, and the
Designer does not always care to pay the price."

"Well," said the Ailerons, "if it's not done it will mean more work for
the Rudder, and that won't please the Pilot."

"Whatever do you mean?" asked Efficiency. "What can the Rudder have to
do with you?"

"It's like this," they replied: "when we are deflected downwards we gain
a larger Angle of Incidence and also enter an area of compressed air,
and so produce more Drift than those of us on the other side of the
Aeroplane, which are deflected upwards into an area of rarefied air due
to the SUCTION effect (though that term is not academically correct) on
the top of the Surface. If there is more Drift, i.e., Resistance, on
one side of the Aeroplane than on the other side, then of course it will
turn off its course, and if that difference in Drift is serious, as it
will very likely be if there is no wash-out, then it will mean a good
deal of work for the Rudder in keeping the Aeroplane on its course,
besides creating extra Drift in doing so."

"I think, then," said Efficiency, "I should prefer to have that
wash-out,[7] and my friend the Designer is so clever at producing
strength of construction for light weight, I'm pretty sure he won't
mind paying the price in Lift. And now let me see if I can sketch the
completed Aeroplane."

"Well, I hope that's all as it should be," she concluded, "for to-morrow
the Great Test in the air is due."



PART III. THE GREAT TEST

It is five o'clock of a fine calm morning, when the Aeroplane is wheeled
out of its shed on to the greensward of the Military Aerodrome. There
is every promise of a good flying day, and, although the sun has not
yet risen, it is light enough to discern the motionless layer of fleecy
clouds some five thousand feet high, and far, far above that a few filmy
mottled streaks of vapour. Just the kind of morning beloved of pilots.

A brand new, rakish, up-to-date machine it is, of highly polished,
beautifully finished wood, fabric as tight as a drum, polished metal,
and every part so perfectly "streamlined" to minimize Drift, which is
the resistance of the air to the passage of the machine, that to the
veriest tyro the remark of the Pilot is obviously justified.

"Clean looking 'bus, looks almost alive and impatient to be off. Ought
to have a turn for speed with those lines."

"Yes," replies the Flight-Commander, "it's the latest of its type and
looks a beauty. Give it a good test. A special report is required on
this machine."

The A.M.'s[8] have now placed the Aeroplane in position facing the
gentle air that is just beginning to make itself evident; the engine
Fitter, having made sure of a sufficiency of oil and petrol in the
tanks, is standing by the Propeller; the Rigger, satisfied with a job
well done, is critically "vetting" the machine by eye, four A.M.'s are
at their posts, ready to hold the Aeroplane from jumping the blocks
which have been placed in front of the wheels; and the Flight-Sergeant
is awaiting the Pilot's orders.

As the Pilot approaches the Aeroplane the Rigger springs to attention
and reports, "All correct, sir," but the Fitter does not this morning
report the condition of the Engine, for well he knows that this Pilot
always personally looks after the preliminary engine test. The latter,
in leathern kit, warm flying boots and goggled, climbs into his seat,
and now, even more than before, has the Aeroplane an almost living
appearance, as if straining to be off and away. First he moves the
Controls to see that everything is clear, for sometimes when the
Aeroplane is on the ground the control lever or "joy-stick" is lashed
fast to prevent the wind from blowing the controlling surfaces about and
possibly damaging them.

The air of this early dawn is distinctly chilly, and the A.M.'s are
beginning to stamp their cold feet upon the dewy grass, but very careful
and circumspect is the Pilot, as he mutters to himself, "Don't worry and
flurry, or you'll die in a hurry."

At last he fumbles for his safety belt, but with a start remembers the
Pilot Air Speed Indicator, and, adjusting it to zero, smiles as he hears
the Pilot-head's gruff voice, "Well, I should think so, twenty miles an
hour I was registering. That's likely to cause a green pilot to stall
the Aeroplane. Pancake, they call it." And the Pilot, who is an old hand
and has learned a lot of things in the air that mere earth-dwellers know
nothing about, distinctly heard the Pilot Tube, whose mouth is open to
the air to receive its pressure, stammer. "Oh Lor! I've got an earwig
already--hope to goodness the Rigger blows me out when I come down--and
this morning air simply fills me with moisture; I'll never keep the
Liquid steady in the Gauge. I'm not sure of my rubber connections
either."

"Oh, shut up!" cry all the Wires in unison, "haven't we got our troubles
too? We're in the most horrible state of tension. It's simply murdering
our Factor of Safety, and how we can possibly stand it when we get the
Lift only the Designer knows."

"That's all right," squeak all the little Wire loops, "we're that
accommodating, we're sure to elongate a bit and so relieve your
tension." For the whole Aeroplane is braced together with innumerable
wires, many of which are at their ends bent over in the form of loops
in order to connect with the metal fittings on the spars and
elsewhere--cheap and easy way of making connection.

"Elongate, you little devils, would you?" fairly shout the Angles of
Incidence, Dihedral and Stagger, amid a chorus of groans from all parts
of the Aeroplane. "What's going to happen to us then? How are we going
to keep our adjustments upon which good flying depends?"

"Butt us and screw us,"[9] wail the Wires. "Butt us and screw us, and
death to the Loops. That's what we sang to the Designer, but he only
looked sad and scowled at the Directors."

"And who on earth are they?" asked the Loops, trembling for their
troublesome little lives.

"Oh earth indeed," sniffed Efficiency, who had not spoken before, having
been rendered rather shy by being badly compromised in the Drawing
Office. "I'd like to get some of them up between Heaven and Earth,
I would. I'd give 'em something to think of besides their Debits and
Credits--but all the same the Designer will get his way in the end. I'm
his Best Girl, you know, and if we could only get rid of the Directors,
the little Tin god, and the Man-who-takes-the-credit, we should be quite
happy." Then she abruptly subsides, feeling that perhaps the less said
the better until she has made a reputation in the Air. The matter of
that Compromise still rankled, and indeed it does seem hardly fit that
a bold bad Tin god should flirt with Efficiency. You see there was
a little Tin god, and he said "Boom, Boom BOOM! Nonsense! It MUST be
done," and things like that in a very loud voice, and the Designer
tore his hair and was furious, but the Directors, who were thinking of
nothing but Orders and Dividends, had the whip-hand of HIM, and so there
you are, and so poor beautiful Miss Efficiency was compromised.

All this time the Pilot is carefully buckling his belt and making
himself perfectly easy and comfortable, as all good pilots do. As
he straightens himself up from a careful inspection of the Deviation
Curve[10] of the Compass and takes command of the Controls, the
Throttle and the Ignition, the voices grow fainter and fainter until
there is nothing but a trembling of the Lift and Drift wires to indicate
to his understanding eye their state of tension in expectancy of the
Great Test.

"Petrol on?" shouts the Fitter to the Pilot.

"Petrol on," replies the Pilot.

"Ignition off?"

"Ignition off."

Round goes the Propeller, the Engine sucking in the Petrol Vapour with
satisfied gulps. And then--

"Contact?" from the Fitter.

"Contact," says the Pilot.

Now one swing of the Propeller by the Fitter, and the Engine is awake
and working. Slowly at first though, and in a weak voice demanding, "Not
too much Throttle, please. I'm very cold and mustn't run fast until my
Oil has thinned and is circulating freely. Three minutes slowly, as you
love me, Pilot."

Faster and faster turn the Engine and Propeller, and the Aeroplane,
trembling in all its parts, strains to jump the blocks and be off.
Carefully the Pilot listens to what the Engine Revolution Indicator
says. At last, "Steady at 1,500 revs. and I'll pick up the rest in the
Air." Then does he throttle down the Engine, carefully putting the lever
back to the last notch to make sure that in such position the Throttle
is still sufficiently open for the Engine to continue working, as
otherwise it might lead to him "losing" his Engine in the air when
throttling down the power for descent. Then, giving the official signal,
he sees the blocks removed from the wheels, and the Flight-Sergeant
saluting he knows that all is clear to ascend. One more signal, and all
the A.M.'s run clear of the Aeroplane.

Then gently, gently mind you, with none of the "crashing on" bad
Pilots think so fine, he opens the Throttle and, the Propeller Thrust
overcoming its enemy the Drift, the Aeroplane moves forward.

"Ah!" says the Wind-screen, "that's Discipline, that is. Through
my little window I see most things, and don't I just know that poor
discipline always results in poor work in the air, and don't you forget
it."

"Discipline is it?" complains the Under-carriage, as its wheels roll
swiftly over the rather rough ground. "I'm bump getting it; and bump,
bump, all I want, bang, bump, rattle, too!" But, as the Lift increases
with the Speed, the complaints of the Under-carriage are stilled, and
then, the friendly Lift becoming greater than the Weight, the Aeroplane
swiftly and easily takes to the air.

Below is left the Earth with all its bumps and troubles. Up into the
clean clear Air moves with incredible speed and steadiness this triumph
of the Designer, the result of how much mental effort, imagination,
trials and errors, failures and successes, and many a life lost in high
endeavour.

Now is the mighty voice of the Engine heard as he turns the Propeller
nine hundred times a minute. Now does the Thrust fight the Drift for all
it's worth, and the Air Speed Indicator gasps with delight, "One hundred
miles an hour!"

And now does the burden of work fall upon the Lift and Drift Wires,
and they scream to the Turnbuckles whose business it is to hold them in
tension, "This is the limit! the Limit! THE LIMIT! Release us, if only
a quarter turn." But the Turnbuckles are locked too fast to turn their
eyes or utter a word. Only the Locking Wires thus: "Ha! ha! the Rigger
knew his job. He knew the trick, and there's no release here." For
an expert rigger will always use the locking wire in such a way as to
oppose the slightest tendency of the turnbuckle to unscrew. The other
kind of rigger will often use the wire in such a way as to allow the
turnbuckle, to the "eyes" of which the wires are attached, to unscrew a
quarter of a turn or more, with the result that the correct adjustment
of the wires may be lost; and upon their fine adjustment much depends.

And the Struts and the Spars groan in compression and pray to keep
straight, for once "out of truth" there is, in addition to possible
collapse, the certainty that in bending they will throw many wires out
of adjustment.

And the Fabric's quite mixed in its mind, and ejaculates, "Now, who
would have thought I got more Lift from the top of the Surface than its
bottom?" And then truculently to the Distance Pieces, which run from
rib to rib, "Just keep the Ribs from rolling, will you? or you'll see me
strip. I'm an Irishman, I am, and if my coat comes off---- Yes, Irish, I
said. I used to come from Egypt, but I've got naturalized since the War
began."

Then the Air Speed Indicator catches the eye of the Pilot. "Good
enough," he says as he gently deflects the Elevator and points the nose
of the Aeroplane upwards in search of the elusive Best Climbing Angle.

"Ha! ha!" shouts the Drift, growing stronger with the increased Angle
of Incidence. "Ha! ha!" he laughs to the Thrust. "Now I've got you. Now
who's Master?"

And the Propeller shrieks hysterically, "Oh! look at me. I'm a
helicopter. That's not fair. Where's Efficiency?" And she can only sadly
reply, "Yes, indeed, but you see we're a Compromise."

And the Drift has hopes of reaching the Maximum Angle of Incidence
and vanquishing the Thrust and the Lift. And he grows very bold as he
strangles the Thrust; but the situation is saved by the Propeller,
who is now bravely helicopting skywards, somewhat to the chagrin of
Efficiency.

"Much ado about nothing," quotes the Aeroplane learnedly. "Compromise
or not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'll
confirm it."

And so indeed it was. The vacuum box of the Altimeter was steadily
expanding under the decreased pressure of the rarefied air, and by means
of its little levers and its wonderful chain no larger than a hair it
was moving the needle round the gauge and indicating the ascent at the
rate of a thousand feet a minute.

And lo! the Aeroplane has almost reached the clouds! But what's this?
A sudden gust, and down sinks one wing and up goes the other. "Oh, my
Horizontal Equivalent!" despairingly call the Planes: "it's eloping with
the Lift, and what in the name of Gravity will happen? Surely there was
enough scandal in the Factory without this, too!" For the lift varies
with the horizontal equivalent of the planes, so that if the aeroplane
tilts sideways beyond a certain angle, the lift becomes less than the
weight of the machine, which must then fall. A fall in such a position
is known as a "side-slip."

But the ever-watchful Pilot instantly depresses one aileron, elevating
the other, with just a touch of the rudder to keep on the course, and
the Planes welcome back their precious Lift as the Aeroplane flicks back
to its normal position.

"Bit bumpy here under these clouds," is all the Pilot says as he heads
for a gap between them, and the next minute the Aeroplane shoots up into
a new world of space.

"My eye!" ejaculates the Wind-screen, "talk about a view!" And indeed
mere words will always fail to express the wonder of it. Six thousand
feet up now, and look! The sun is rising quicker than ever mortal on
earth witnessed its ascent. Far below is Mother Earth, wrapt in mists
and deep blue shadows, and far above are those light, filmy, ethereal
clouds now faintly tinged with pink And all about great mountains of
cloud, lazily floating in space. The sun rises and they take on all
colours, blending one with the other, from dazzling white to crimson
and deep violet-blue. Lakes and rivers here and there in the enormous
expanse of country below refract the level rays of the sun and, like so
many immense diamonds, send dazzling shafts of light far upwards. The
tops of the hills now laugh to the light of the sun, but the valleys are
still mysterious dark blue caverns, clowned with white filmy lace-like
streaks of vapour. And withal the increasing sense with altitude of
vast, clean, silent solitudes of space.

Lives there the man who can adequately describe this Wonder? "Never,"
says the Pilot, who has seen it many times, but to whom it is ever new
and more wonderful.

Up, up, up, and still up, unfalteringly speeds the Pilot and his mount.
Sweet the drone of the Engine and steady the Thrust as the Propeller
exultingly battles with the Drift.

And look! What is that bright silver streak all along the horizon? It
puzzled the Pilot when first he saw it, but now he knows it for the Sea,
full fifty miles away!

And on his right is the brightness of the Morn and the smiling Earth
unveiling itself to the ardent rays of the Sun; and on his left, so
high is he, there is yet black Night, hiding innumerable Cities, Towns,
Villages and all those places where soon teeming multitudes of men shall
awake, and by their unceasing toil and the spirit within them produce
marvels of which the Aeroplane is but the harbinger.

And the Pilot's soul is refreshed, and his vision, now exalted, sees
the Earth a very garden, even as it appears at that height, with discord
banished and a happy time come, when the Designer shall have at last
captured Efficiency, and the Man-who-takes-the-credit is he who has
earned it, and when kisses are the only things that go by favour.

Now the Pilot anxiously scans the Barograph, which is an instrument much
the same as the Altimeter; but in this case the expansion of the vacuum
box causes a pen to trace a line upon a roll of paper. This paper is
made by clockwork to pass over the point of the pen, and so a curved
line is made which accurately registers the speed of the ascent in feet
per minute. No longer is the ascent at the rate of a thousand feet a
minute, and the Propeller complains to the Engine, "I'm losing my Revs.
and the Thrust. Buck up with the Power, for the Lift is decreasing,
though the Weight remains much the same."

Quoth the Engine: "I strangle for Air. A certain proportion, and that
of right density, I must have to one part of Petrol, in order to give me
full power and compression, and here at an altitude of ten thousand feet
the Air is only two-thirds as dense as at sea-level. Oh, where is he who
will invent a contrivance to keep me supplied with Air of right density
and quality? It should not be impossible within certain limits."

"We fully agree," said the dying Power and Thrust. "Only maintain Us and
you shall be surprised at the result. For our enemy Drift decreases in
respect of distance with the increase of altitude and rarity of air,
and there is no limit to the speed through space if only our strength
remains. And with oxygen for Pilot and Passengers and a steeper
pitch[11] for the Propeller we may then circle the Earth in a day!"

Ah, Reader, smile not unbelievingly, as you smiled but a few years past.
There may be greater wonders yet. Consider that as the speed increases,
so does the momentum or stored-up force in the mass of the aeroplane
become terrific. And, bearing that in mind, remember that with
altitude gravity decreases. There may yet be literally other worlds to
conquer.[12]

Now at fifteen thousand feet the conditions are chilly and rare, and the
Pilot, with thoughts of breakfast far below, exclaims, "High enough!
I had better get on with the Test." And then, as he depresses the
Elevator, the Aeroplane with relief assumes its normal horizontal
position. Then, almost closing the Throttle, the Thrust dies away. Now,
the nose of the Aeroplane should sink of its own volition, and the craft
glide downward at flying speed, which is in this case a hundred miles
an hour. That is what should happen if the Designer has carefully
calculated the weight of every part and arranged for the centre of
gravity to be just the right distance in front of the centre of lift.
Thus is the Aeroplane "nose-heavy" as a glider, and just so to a degree
ensuring a speed of glide equal to its flying speed. And the Air Speed
Indicator is steady at one hundred miles an hour, and "That's all
right!" exclaims the Pilot. "And very useful, too, in a fog or a cloud,"
he reflects, for then he can safely leave the angle of the glide to
itself, and give all his attention, and he will need it all, to keeping
the Aeroplane horizontal from wing-tip to wing-tip, and to keeping
it straight on its course. The latter he will manage with the rudder,
controlled by his feet, and the Compass will tell him whether a straight
course is kept. The former he will control by the Ailerons, or
little wings hinged to the tips of the planes, and the bubble in the
Inclinometer in front of him must be kept in the middle.

A Pilot, being only human, may be able to do two things at once, but
three is a tall order, so was this Pilot relieved to find the Design not
at fault and his craft a "natural glider." To correct this nose-heavy
tendency when the Engine is running, and descent not required, the
centre of Thrust is arranged to be a little below the centre of Drift or
Resistance, and thus acts as a counter-balance.

But what is this stream of bad language from the Exhaust Pipe,
accompanied by gouts of smoke and vapour? The Engine, now revolving at
no more than one-tenth its normal speed, has upset the proportion of
petrol to air, and combustion is taking place intermittently or in the
Exhaust Pipe, where it has no business to be.

"Crash, Bang, Rattle----!----!----!" and worse than that, yells the
Exhaust, and the Aeroplane, who is a gentleman and not a box kite,[13]
remonstrates with the severity of a Senior Officer. "See the Medical
Officer, you young Hun. Go and see a doctor. Vocal diarrhoea, that's
your complaint, and a very nasty one too. Bad form, bad for discipline,
and a nuisance in the Mess. What's your Regiment? Special Reserve, you
say? Humph! Sounds like Secondhand Bicycle Trade to me!"

Now the Pilot decides to change the straight gliding descent to a spiral
one, and, obedient to the Rudder, the Aeroplane turns to the left. But
the Momentum (two tons at 100 miles per hour is no small affair) heavily
resents this change of direction, and tries its level best to prevent
it and to pull the machine sideways and outwards from its spiral
course--that is, to make it "side-skid" outwards. But the Pilot deflects
the Ailerons and "banks" up the planes to the correct angle, and, the
Aeroplane skidding sideways and outwards, the lowest surfaces of
the planes press up against the air until the pressure equals the
centrifugal force of the Momentum, and the Aeroplane spirals steadily
downwards.

Down, down, down, and the air grows denser, and the Pilot gulps largely,
filling his lungs with the heavier air to counteract the increasing
pressure from without. Down through a gap in the clouds, and the
Aerodrome springs into view, appearing no larger than a saucer, and the
Pilot, having by now got the "feel" of the Controls, proceeds to put
the Aeroplane through its paces. First at its Maximum Angle, staggering
along tail-down and just maintaining horizontal flight; then a dive at
far over flying speed, finishing with a perfect loop; then sharp turns
with attendant vertical "banks" and then a wonderful switchback
flight, speeding down at a hundred and fifty miles an hour with short,
exhilarating ascents at the rate of two thousand feet a minute!

All the parts are now working well together. Such wires as were before
in undue tension have secured relief by slightly elongating their loops,
and each one is now doing its bit, and all are sharing the burden of
work together.

The Struts and the Spars, which felt so awkward at first, have bedded
themselves in their sockets, and are taking the compression stresses
uncomplainingly.

The Control Cables of twisted wire, a bit tight before, have slightly
lengthened by perhaps the eighth of an inch, and, the Controls instantly
responding to the delicate touch of the Pilot, the Aeroplane, at the
will of its Master, darts this way and that way, dives, loops, spirals,
and at last, in one long, magnificent glide, lands gently in front of
its shed.

"Well, what result?" calls the Flight-Commander to the Pilot.

"A hundred miles an hour and a thousand feet a minute," he briefly
replies.

"And a very good result too," says the Aeroplane, complacently, as he is
carefully wheeled into his shed.


That is the way Aeroplanes speak to those who love them and understand
them. Lots of Pilots know all about it, and can spin you wonderful
yarns, much better than this one, if you catch them in a confidential
mood--on leave, for instance, and after a good dinner.



PART IV. 'CROSS COUNTRY

The Aeroplane had been designed and built, and tested in the air, and
now stood on the Aerodrome ready for its first 'cross-country flight.

It had run the gauntlet of pseudo-designers, crank inventors, press
"experts," and politicians; of manufacturers keen on cheap work and
large profits; of poor pilots who had funked it, and good pilots who had
expected too much of it. Thousands of pounds had been wasted on it,
many had gone bankrupt over it, and others it had provided with safe fat
jobs.

Somehow, and despite every conceivable obstacle, it had managed to
muddle through, and now it was ready for its work. It was not perfect,
for there were fifty different ways in which it might be improved, some
of them shamefully obvious. But it was fairly sound mechanically, had a
little inherent stability, was easily controlled, could climb a thousand
feet a minute, and its speed was a hundred miles an hour. In short,
quite a creditable machine, though of course the right man had not got
the credit.

It is rough, unsettled weather with a thirty mile an hour wind on the
ground, and that means fifty more or less aloft. Lots of clouds at
different altitudes to bother the Pilot, and the air none to clear for
the observation of landmarks.

As the Pilot and Observer approach the Aeroplane the former is clearly
not in the best of tempers. "It's rotten luck," he is saying, "a blank
shame that I should have to take this blessed 'bus and join X Reserve
Squadron, stationed a hundred and fifty miles from anywhere; and just
as I have licked my Flight into shape. Now some slack blighter will, I
suppose, command it and get the credit of all my work!"

"Shut up, you grouser," said the Observer. "Do you think you're the only
one with troubles? Haven't I been through it too? Oh! I know all about
it! You're from the Special Reserve and your C.O. doesn't like your
style of beauty, and you won't lick his boots, and you were a bit of a
technical knut in civil life, but now you've jolly well got to know less
than those senior to you. Well! It's a jolly good experience for most of
us. Perhaps conceit won't be at quite such a premium after this war. And
what's the use of grousing? That never helped anyone. So buck up, old
chap. Your day will come yet. Here's our machine, and I must say it
looks a beauty!"

And, as the Pilot approaches the Aeroplane, his face brightens and he
soon forgets his troubles as he critically inspects the craft which is
to transport him and the Observer over the hills and far away. Turning
to the Flight-Sergeant he inquires, "Tank full of petrol and oil?"

"Yes, sir," he replies, "and everything else all correct. Propeller,
engine, and body covers on board, sir; tool kit checked over and in the
locker; engine and Aeroplane logbooks written up, signed, and under your
seat; engine revs. up to mark, and all the control cables in perfect
condition and tension."

"Very good," said the Pilot; and then turning to the Observer, "Before
we start you had better have a look at the course I have mapped out.

"A is where we stand and we have to reach B, a hundred and fifty miles
due North. I judge that, at the altitude we shall fly, there will be
an East wind, for although it is not quite East on the ground it is
probably about twenty degrees different aloft, the wind usually moving
round clockways to about that extent. I think that it is blowing at the
rate of about fifty miles an hour, and I therefore take a line on the
map to C, fifty miles due West of A. The Aeroplane's speed is a hundred
miles an hour, and so I take a line of one hundred miles from C to D.
Our compass course will then be in the direction A--E, which is always a
line parallel to C--D. That is, to be exact, it will be fourteen degrees
off the C--D course, as, in this part of the globe, there is that much
difference between the North and South lines on the map and the magnetic
North to which the compass needle points. If the compass has an error,
as it may have of a few degrees, that, too, must be taken into account,
and the deviation or error curve on the dashboard will indicate it.

"The Aeroplane will then always be pointing in a direction parallel to
A--E, but, owing to the side wind, it will be actually travelling over
the course A--B, though in a rather sideways attitude to that course.

"The distance we shall travel over the A--B course in one hour is A--D.
That is nearly eighty-seven miles, so we ought to accomplish our journey
of a hundred and fifty miles in about one and three-quarter hours.

"I hope that's quite clear to you. It's a very simple way of calculating
the compass course, and I always do it like that."

"Yes, that's plain enough. You have drafted what engineers call 'a
parallelogram of forces'; but suppose you have miscalculated the
velocity of the wind, or that it should change in velocity or
direction?"

"Well, that of course will more or less alter matters," replies the
Pilot. "But there are any number of good landmarks such as lakes,
rivers, towns, and railway lines. They will help to keep us on the right
course, and the compass will, at any rate, prevent us from going far
astray when between them."

"Well, we'd better be off, old chap. Hop aboard." This from the Observer
as he climbs into the front seat from which he will command a good view
over the lower plane; and the Pilot takes his place in the rear seat,
and, after making himself perfectly comfortable, fixing his safety belt,
and moving the control levers to make sure that they are working freely,
he gives the signal to the Engine Fitter to turn the propeller and so
start the engine.

Round buzzes the Propeller, and the Pilot, giving the official signal,
the Aeroplane is released and rolls swiftly over the ground in the teeth
of the gusty wind.

In less than fifty yards it takes to the air and begins to climb rapidly
upwards, but how different are the conditions to the calm morning of
yesterday! If the air were visible it would be seen to be acting in the
most extraordinary manner; crazily swirling, lifting and dropping, gusts
viciously colliding--a mad phantasmagoria of forces!

Wickedly it seizes and shakes the Aeroplane; then tries to turn it over
sideways; then instantly changes its mind and in a second drops it into
a hole a hundred feet deep, and if it were not for his safety belt the
Pilot might find his seat sinking away from beneath him.

Gusts strike the front of the craft like so many slaps in the face; and
others, with the motion of mountainous waves, sometimes lift it hundreds
of feet in a few seconds, hoping to see it plunge over the summit in a
death-dive--and so it goes on, but the Pilot, perfectly at one with
his mount and instantly alert to its slightest motion, is skilfully and
naturally making perhaps fifty movements a minute of hand and feet;
the former lightly grasping the "joy-stick" which controls the Elevator
hinged to the tail, and also the Ailerons or little wings hinged to the
wing-tips; and the latter moving the Rudder control-bar.

A strain on the Pilot? Not a bit of it, for this is his Work which he
loves and excels in; and given a cool head, alert eye, and a sensitive
touch for the controls, what sport can compare with these ever-changing
battles of the air?

The Aeroplane has all this time been climbing in great wide circles,
and is now some three thousand feet above the Aerodrome which from such
height looks absurdly small. The buildings below now seem quite squat;
the hills appear to have sunk away into the ground, and the whole
country below, cut up into diminutive fields, has the appearance of
having been lately tidied and thoroughly spring-cleaned! A doll's
country it looks, with tiny horses and cows ornamenting the fields
and little model motor-cars and carts stuck on the roads, the latter
stretching away across the country like ribbons accidentally dropped.

At three thousand feet altitude the Pilot is satisfied that he is now
sufficiently high to secure, in the event of engine failure, a
long enough glide to earth to enable him to choose and reach a good
landing-place; and, being furthermore content with the steady running of
the engine, he decides to climb no more but to follow the course he has
mapped out. Consulting the compass, he places the Aeroplane on the A--E
course and, using the Elevator, he gives his craft its minimum angle of
incidence at which it will just maintain horizontal flight and secure
its maximum speed.

Swiftly he speeds away, and few thoughts he has now for the changing
panorama of country, cloud, and colour. Ever present in his mind are the
three great 'cross-country queries. "Am I on my right course? Can I see
a good landing-ground within gliding distance?" And "How is the Engine
running?"

Keenly both he and the Observer compare their maps with the country
below. The roads, khaki-coloured ribbons, are easily seen but are not
of much use, for there are so many of them and they all look alike from
such an altitude.

Now where can that lake be which the map shows so plainly? He feels that
surely he should see it by now, and has an uncomfortable feeling that
he is flying too far West. What pilot is there indeed who has not many
times experienced such unpleasant sensation? Few things in the air
can create greater anxiety. Wisely, however, he sticks to his compass
course, and the next minute he is rewarded by the sight of the lake,
though indeed he now sees that the direction of his travel will not take
him over it, as should be the case if he were flying over the shortest
route to his destination. He must have slightly miscalculated the
velocity or direction of the side-wind.

"About ten degrees off," he mutters, and, using the Rudder, corrects his
course accordingly.

Now he feels happier and that he is well on his way. The gusts, too,
have ceased to trouble him as, at this altitude, they are not nearly so
bad as they were near the ground the broken surface of which does much
to produce them; and sometimes for miles he makes but a movement or two
of the controls.

The clouds just above race by with dizzy and uniform speed; the country
below slowly unrolls, and the steady drone of the Engine is almost
hypnotic in effect. "Sleep, sleep, sleep," it insidiously suggests.
"Listen to me and watch the clouds; there's nothing else to do. Dream,
dream, dream of speeding through space for ever, and ever, and ever; and
rest, rest, rest to the sound of my rhythmical hum. Droning on and on,
nothing whatever matters. All things now are merged into speed through
space and a sleepy monotonous d-d-r-r-o-o-n-n-e - - - - -." But the
Pilot pulls himself together with a start and peers far ahead in search
of the next landmark. This time it is a little country town, red-roofed
his map tells him, and roughly of cruciform shape; and, sure enough,
there in the right direction are the broken outlines of a few red roofs
peeping out from between the trees.

Another minute and he can see this little town, a fairy town it appears,
nestling down between the hills with its red roofs and picturesque
shape, a glowing and lovely contrast with the dark green of the
surrounding moors.

So extraordinarily clean and tidy it looks from such a height, and
laid out in such orderly fashion with perfectly defined squares, parks,
avenues, and public buildings, it indeed appears hardly real, but rather
as if it has this very day materialized from some delightful children's
book!

Every city and town you must know has its distinct individuality to the
Pilot's eye. Some are not fairy places at all, but great dark ugly blots
upon the fair countryside, and with tall shafts belching forth murky
columns of smoke to defile clean space. Others, melancholy-looking
masses of grey, slate-roofed houses, are always sad and dispirited;
never welcoming the glad sunshine, but ever calling for leaden skies
and a weeping Heaven. Others again, little coquettes with village green,
white palings everywhere, bright gravel roads, and an irrepressible air
of brightness and gaiety.

Then there are the rivers, silvery streaks peacefully winding far, far
away to the distant horizon; they and the lakes the finest landmarks
the Pilot can have. And the forests. How can I describe them? The trees
cannot be seen separately, but merge altogether into enormous irregular
dark green masses sprawling over the country, and sometimes with great
ungainly arms half encircling some town or village; and the wind passing
over the foliage at times gives the forest an almost living appearance,
as of some great dragon of olden times rousing itself from slumber to
devour the peaceful villages which its arms encircle.

And the Pilot and Observer fly on and on, seeing these things and many
others which baffle my poor skill to describe--things, dear Reader, that
you shall see, and poets sing of, and great artists paint in the days
to come when the Designer has captured Efficiency. Then, and the time
is near, shall you see this beautiful world as you have never seen it
before, the garden it is, the peace it breathes, and the wonder of it.

The Pilot, flying on, is now anxiously looking for the railway line
which midway on his journey should point the course. Ah! There it is
at last, but suddenly (and the map at fault) it plunges into the
earth! Well the writer remembers when that happened to him on a long
'cross-country flight in the early days of aviation. Anxiously he
wondered "Are tunnels always straight?" and with what relief, keeping on
a straight course, he picked up the line again some three miles farther
on!

Now at last the Pilot sees the sea, just a streak on the north-eastern
horizon, and he knows that his flight is two-thirds over. Indeed, he
should have seen it before, but the air is none too clear, and he is not
yet able to discern the river which soon should cross his path. As he
swiftly speeds on the air becomes denser and denser with what he fears
must be the beginning of a sea-fog, perhaps drifting inland along the
course of the river. Now does he feel real anxiety, for it is the DUTY
of a Pilot to fear fog, his deadliest enemy. Fog not only hides the
landmarks by which he keeps his course, but makes the control of the
Aeroplane a matter of the greatest difficulty. He may not realize
it, but, in keeping his machine on an even keel, he is unconsciously
balancing it against the horizon, and with the horizon gone he is
lost indeed. Not only that, but it also prevents him from choosing his
landing-place, and the chances are that, landing in a fog, he will smash
into a tree, hedge, or building, with disastrous results. The best and
boldest pilot 'wares a fog, and so this one, finding the conditions
becoming worse and yet worse, and being forced to descend lower and
lower in order to keep the earth within view, wisely decides to choose a
landing-place while there is yet time to do so.

Throttling down the power of the engine he spirals downwards, keenly
observing the country below. There are plenty of green fields to lure
him, and his great object is to avoid one in which the grass is long,
for that would bring his machine to a stop so suddenly as to turn it
over; or one of rough surface likely to break the under-carriage. Now is
perfect eyesight and a cool head indispensable. He sees and decides upon
a field and, knowing his job, he sticks to that field with no change
of mind to confuse him. It is none too large, and gliding just over the
trees and head on to the wind he skilfully "stalls" his machine; that
is, the speed having decreased sufficiently to avoid such a manoeuvre
resulting in ascent, he, by means of the Elevator, gives the Aeroplane
as large an angle of incidence as possible, and the undersides of the
planes meeting the air at such a large angle act as an air-brake, and
the Aeroplane, skimming over the ground, lessens its speed and finally
stops just at the farther end of the field.

Then, after driving the Aeroplane up to and under the lee of the hedge,
he stops the engine, and quickly lashing the joy-stick fast in order
to prevent the wind from blowing the controlling surfaces about and
possibly damaging them, he hurriedly alights. Now running to the tail he
lifts it up on to his shoulder, for the wind has become rough indeed and
there is danger of the Aeroplane becoming unmanageable. By this action
he decreases the angle at which the planes are inclined to the wind and
so minimizes the latter's effect upon them. Then to the Observer, "Hurry
up, old fellow, and try to find some rope, wire, or anything with which
to picket the machine. The wind is rising and I shan't be able to hold
the 'bus steady for long. Don't forget the wire-cutters. They're in the
tool kit." And the Observer rushes off in frantic haste, before long
triumphantly returning with a long length of wire from a neighbouring
fence. Blocking up the tail with some debris at hand, they soon succeed,
with the aid of the wire, in stoutly picketing the Aeroplane to the
roots of the high hedge in front of it; done with much care, too,
so that the wire shall not fray the fabric or set up dangerous
bending-stresses in the woodwork. Their work is not done yet, for the
Observer remarking, "I don't like the look of this thick weather and
rather fear a heavy rain-storm," the Pilot replies, "Well, it's a
fearful bore, but the first rule of our game is never to take an
unnecessary risk, so out with the engine and body covers."

Working with a will they soon have the engine and the open part of the
body which contains the seats, controls, and instruments snugly housed
with their waterproof covers, and the Aeroplane is ready to weather the
possible storm.

Says the Observer, "I'm remarkably peckish, and methinks I spy the
towers of one of England's stately homes showing themselves just beyond
that wood, less than a quarter of a mile away. What ho! for a raid. What
do you say?"

"All right, you cut along and I'll stop here, for the Aeroplane must not
be left alone. Get back as quickly as possible."

And the Observer trots off, leaving the Pilot filling his pipe and
anxiously scrutinizing the weather conditions. Very thick it is now, but
the day is yet young, and he has hopes of the fog lifting sufficiently
to enable the flight to be resumed. A little impatiently he awaits the
return of his comrade, but with never a doubt of the result, for the
hospitality of the country house is proverbial among pilots! What old
hand among them is there who cannot instance many a forced landing made
pleasant by such hospitality? Never too late or too early to help with
food, petrol, oil, tools, and assistants. Many a grateful thought has
the writer for such kind help given in the days before the war (how long
ago they seem!), when aeroplanes were still more imperfect than they are
now, and involuntary descents often a part of 'cross-country flying.

Ah! those early days! How fresh and inspiring they were! As one started
off on one's first 'cross-country flight, on a machine the first of its
design, and with everything yet to learn, and the wonders of the air yet
to explore; then the joy of accomplishment, the dreams of Efficiency,
the hard work and long hours better than leisure; and what a field of
endeavour--the realms of space to conquer! And the battle still goes on
with ever-increasing success. Who is bold enough to say what its limits
shall be?

So ruminates this Pilot-Designer, as he puffs at his pipe, until his
reverie is abruptly disturbed by the return of the Observer.

"Wake up, you AIRMAN," the latter shouts. "Here's the very thing the
doctor ordered! A basket of first-class grub and something to keep the
fog out, too."

"Well, that's splendid, but don't call me newspaper names or you'll
spoil my appetite!"

Then, with hunger such as only flying can produce, they appreciatively
discuss their lunch, and with many a grateful thought for the
donors--and they talk shop. They can't help it, and even golf is a poor
second to flight talk. Says the Pilot, who must have his grievance,
"Just observe where I managed to stop the machine. Not twenty feet from
this hedge! A little more and we should have been through it and into
Kingdom Come! I stalled as well as one could, but the tail touched
the ground and so I could not give the Aeroplane any larger angle of
incidence. Could I have given it a larger angle, then the planes would
have become a much more effective air-brake, and we should have come to
rest in a much shorter distance. It's all the fault of the tail. There's
hardly a type of Aeroplane in existence in which the tail could not be
raised several feet, and that would make all the difference. High tails
mean a large angle of incidence when the machine touches ground and,
with enough angle, I'll guarantee to safely land the fastest machine in
a five-acre field. You can, I am sure, imagine what a difference that
would make where forced landings are concerned!" Then rapidly sketching
in his notebook, he shows the Observer the following illustration:

"That's very pretty," said the Observer, "but how about Mechanical
Difficulties, and Efficiency in respect of Flight? And, anyway, why
hasn't such an obvious thing been done already?"

"As regards the first part of your question I assure you that there's
nothing in it, and I'll prove it to you as follows----"

"Oh! That's all right, old chap. I'll take your word for it," hurriedly
replies the Observer, whose soul isn't tuned to a technical key.

"As regards the latter part of your inquiry," went on the Pilot, a
little nettled at having such a poor listener, "it's very simple.
Aeroplanes have 'just growed' like Topsy, and they consequently contain
this and many another relic of early day design when Aeroplanes were
more or less thrown together and anything was good enough that could get
off the ground."

"By Jove," interrupts the Observer, "I do believe the fog is lifting.
Hadn't we better get the engine and body covers off, just in case it's
really so?"

"I believe you're right. I am sure those hills over there could not
be seen a few minutes ago, and look--there's sunshine over there. We'd
better hurry up."

Ten minutes' hard work and the covers are off, neatly folded and stowed
aboard; the picketing wires are cast adrift, and the Pilot is once more
in his seat. The Aeroplane has been turned to face the other end of the
field, and, the Observer swinging round the propeller, the engine is
awake again and slowly ticking over. Quickly the Observer climbs into
his seat in front of the Pilot, and, the latter slightly opening the
throttle, the Aeroplane leisurely rolls over the ground towards the
other end of the field, from which the ascent will be made.

Arriving there the Pilot turns the Aeroplane in order to face the wind
and thus secure a quick "get-off." Then he opens the throttle fully and
the mighty voice of the Engine roars out "Now see me clear that hedge!"
and the Aeroplane races forward at its minimum angle of incidence. Tail
up, and with ever-increasing speed, it rushes towards the hedge under
the lee of which it has lately been at rest; and then, just as the
Observer involuntarily pulls back an imaginary "joy-stick," the Pilot
moves the real one and places the machine at its best climbing angle.
Like a living thing it responds, and instantly leaves the ground,
clearing the hedge like a--well, like an Aeroplane with an excellent
margin of lift. Upwards it climbs with even and powerful lift, and the
familiar scenes below again gladden the eyes of the Pilot. Smaller and
more and more squat grow the houses and hills; more and more doll-like
appear the fields which are clearly outlined by the hedges; and soon the
country below is easily identified with the map. Now they can see the
river before them and a bay of the sea which must be crossed or skirted.
The fog still lingers along the course of the river and between the
hills, but is fast rolling away in grey, ghost-like masses. Out to sea
it obscures the horizon, making it difficult to be sure where water ends
and fog begins, and creating a strange, rather weird effect by which
ships at a certain distance appear to be floating in space.

Now the Aeroplane is almost over the river, and the next instant it
suddenly drops into a "hole in the air." With great suddenness it
happens, and for some two hundred feet it drops nose-down and tilted
over sideways; but the Pilot is prepared and has put his craft on an
even keel in less time than it takes to tell you about it; for well he
knows that he must expect such conditions when passing over a shore
or, indeed, any well-defined change in the composition of the earth's
surface. Especially is this so on a hot and sunny day, for then the warm
surface of the earth creates columns of ascending air, the speed of the
ascent depending upon the composition of the surface. Sandy soil, for
instance, such as borders this river produces a quickly ascending column
of air, whereas water and forests have not such a marked effect. Thus,
when our Aeroplane passed over the shore of the river, it suddenly lost
the lift due to the ascending air produced by the warm sandy soil, and
it consequently dropped just as if it had fallen into a hole.

Now the Aeroplane is over the bay and, the sea being calm, the Pilot
looks down, down through the water, and clearly sees the bottom,
hundreds of feet below the surface. Down through the reflection of the
blue sky and clouds, and one might think that is all, but it isn't. Only
those who fly know the beauties of the sea as viewed from above;
its dappled pearly tints; its soft dark blue shadows; the beautiful
contrasts of unusual shades of colour which are always differing and
shifting with the changing sunshine and the ever moving position of the
aerial observer. Ah! for some better pen than mine to describe these
things! One with glowing words and a magic rhythm to express the wonders
of the air and the beauty of the garden beneath--the immensity of the
sea--the sense of space and of one's littleness there--the realization
of the Power moving the multitudes below--the exaltation of spirit
altitude produces--the joy of speed. A new world of sensation!

Now the bay is almost crossed and the Aerodrome at B can be
distinguished.

On the Aerodrome is a little crowd waiting and watching for the arrival
of the Aeroplane, for it is of a new and improved type and its first
'cross-country performance is of keen interest to these men; men who
really know something about flight.

There is the Squadron Commander who has done some real flying in
his time; several well-seasoned Flight-Commanders; a dozen or more
Flight-Lieutenants; a knowledgeable Flight-Sergeant; a number of Air
Mechanics, and, a little on one side and almost unnoticed, the Designer.

"I hope they are all right," said someone, "and that they haven't had
difficulties with the fog. It rolled up very quickly, you know."

"Never fear," remarked a Flight-Commander. "I know the Pilot well and
he's a good 'un; far too good to carry on into a fog."

"They say the machine is really something out of the ordinary," said
another, "and that, for once, the Designer has been allowed full play;
that he hasn't been forced to unduly standardize ribs, spars, struts,
etc., and has more or less had his own way. I wonder who he is. It seems
strange we hear so little of him."

"Ah! my boy. You do a bit more flying and you'll discover that things
are not always as they appear from a distance!"

"There she is, sir!" cries the Flight-Sergeant. "Just a speck over the
silvery corner of that cloud."

A tiny speck it looks, some six miles distant and three thousand feet
high; but, racing along, it rapidly appears larger and soon its outlines
can be traced and the sunlight be seen playing upon the whirling
propeller.

Now the distant drone of the engine can be heard, but not for long, for
suddenly it ceases and, the nose of the Aeroplane sinking, the craft
commences gliding downwards.

"Surely too far away," says a subaltern. "It will be a wonderful machine
if, from that distance and height, it can glide into the Aerodrome."
And more than one express the opinion that it cannot be done; but
the Designer smiles to himself, yet with a little anxiety, for his
reputation is at stake, and Efficiency, the main reward he desires, is
perhaps, or perhaps not, at last within his grasp!

Swiftly the machine glides downwards towards them, and it can now be
seen how surprisingly little it is affected by the rough weather and
gusts; so much so that a little chorus of approval is heard.

"Jolly good gliding angle," says someone; and another, "Beautifully
quick controls, what?" and from yet another, "By Jove! The Pilot must be
sure of the machine. Look, he's stopped the engine entirely."

Then the Aeroplane with noiseless engine glides over the boundary of the
Aerodrome, and, with just a soft soughing sound from the air it cleaves,
lands gently not fifty yards from the onlookers.

"Glad to see you," says the Squadron Commander to the Pilot. "How do you
like the machine?" And the Pilot replies:

"I never want a better one, sir. It almost flies itself!"

And the Designer turns his face homewards and towards his beloved
drawing-office; well satisfied, but still dreaming dreams of the future
and... looking far ahead whom should he see but Efficiency at last
coming towards him! And to him she is all things. In her hair is the
morning sunshine; her eyes hold the blue of the sky, and on her cheeks
is the pearly tint of the clouds as seen from above. The passion of
speed, the lure of space, the sense of power, and the wonder of the
future... all these things she holds for him.

"Ah!" he cries. "You'll never leave me now, when at last there is no one
between us?"

And Efficiency, smiling and blushing, but practical as ever, says:

"And you will never throw those Compromises in my face?"

"My dear, I love you for them! Haven't they been my life ever since I
began striving for you ten long years ago?"

And so they walked off very happily, arm-in-arm together; and if this
hasn't bored you and you'd like some more of the same sort of thing, I'd
just love to tell you some day of the wonderful things they accomplish
together, and of what they dream the future holds in store.

And that's the end of the Prologue.



CHAPTER I. FLIGHT

Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum.
It therefore obeys Newton's laws[14] and resists movement. It is that
resistance or reaction which makes flight possible.

Flight is secured by driving through the air a surface[15] inclined
upwards and towards the direction of motion.

S = Side view of surface.

M = Direction of motion.

CHORD.--The Chord is, for practical purposes, taken to be a straight
line from the leading edge of the surface to its trailing edge.

N = A line through the surface starting from its trailing edge. The
position of this line, which I call the Neutral Lift Line, is found by
means of wind-tunnel research, and it varies with differences in
the camber (curvature) of surfaces. In order to secure flight, the
inclination of the surface must be such that the neutral lift line makes
an angle with and ABOVE the line of motion. If it is coincident with M,
there is no lift. If it makes an angle with M and BELOW it, then there
is a pressure tending to force the surface down.

I = Angle of Incidence. This angle is generally defined as the angle the
chord makes with the direction of motion, but that is a bad definition,
as it leads to misconception. The angle of incidence is best described
as the angle the neutral lift line makes with the direction of motion
relative to the air. You will, however, find that in nearly all rigging
specifications the angle of incidence is taken to mean the angle the
chord makes with a line parallel to the propeller thrust. This is
necessary from the point of view of the practical mechanic who has to
rig the aeroplane, for he could not find the neutral lift line, whereas
he can easily find the chord. Again, he would certainly be in doubt as
to "the direction of motion relative to the air," whereas he can easily
find a line parallel to the propeller thrust. It is a pity, however,
that these practical considerations have resulted in a bad definition
of the angle of incidence becoming prevalent, a consequence of which has
been the widespread fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be secured with a
negative angle of chord, but never with a negative inclination of the
surface. All this is only applicable to cambered surfaces. In the case
of flat surfaces the neutral lift line coincides with the chord and the
definition I have criticised adversely is then applicable. Flat lifting
surfaces are, however, never used.

The surface acts upon the air in the following manner:


As the bottom of the surface meets the air, it compresses it and
accelerates it DOWNWARDS. As a result of this definite action there is,
of course, an equal and opposite reaction UPWARDS.

The top surface, in moving forward, tends to leave the air behind
it, thus creating a semi-vacuum or rarefied area over the top of the
surface. Consequently the pressure of air on the top of the surface
is decreased, thus assisting the reaction below to lift the surface
UPWARDS.

The reaction increases approximately as the square of the velocity. It
is the result of (1) the mass of air engaged, and (2) the velocity and
consequent force with which the surface engages the air. If the reaction
was produced by only one of those factors it would increase in direct
proportion to the velocity, but, since it is the product of both
factors, it increases as V(2S).

Approximately three-fifths of the reaction is due to the decrease of
density (and consequent decrease of downward pressure) on the top of the
surface; and only some two-fifths is due to the upward reaction secured
by the action of the bottom surface upon the air. A practical point in
respect of this is that, in the event of the fabric covering the surface
getting into bad condition, it is more likely to strip off the top than
off the bottom.

The direction of the reaction is approximately at right-angles to the
chord of the surface, as illustrated above; and it is, in considering
flight, convenient to divide it into two component parts or values,
thus:

1. The vertical component of the reaction, i.e., Lift, which is opposed
to Gravity, i.e., the weight of the aeroplane.

2. The horizontal component, i.e., Drift (sometimes called Resistance),
to which is opposed the thrust of the propeller.

The direction of the reaction is, of course, the resultant of the forces
Lift and Drift.

The Lift is the useful part of the reaction, for it lifts the weight of
the aeroplane.

The Drift is the villain of the piece, and must be overcome by the
Thrust in order to secure the necessary velocity to produce the
requisite Lift for flight.

DRIFT.--The drift of the whole aeroplane (we have considered only the
lifting surface heretofore) may be conveniently divided into three
parts, as follows:

Active Drift, which is the drift produced by the lifting surfaces.

Passive Drift, which is the drift produced by all the rest of the
aeroplane--the struts, wires, fuselage, under-carriage, etc., all of
which is known as "detrimental surface."

Skin Friction, which is the drift produced by the friction of the air
with roughnesses of surface. The latter is practically negligible
having regard to the smooth surface of the modern aeroplane, and its
comparatively slow velocity compared with, for instance, the velocity of
a propeller blade.

LIFT-DRIFT RATIO.--The proportion of lift to drift is known as the
lift-drift ratio, and is of paramount importance, for it expresses the
efficiency of the aeroplane (as distinct from engine and propeller). A
knowledge of the factors governing the lift-drift ratio is, as will be
seen later, an absolute necessity to anyone responsible for the rigging
of an aeroplane, and the maintenance of it in an efficient and safe
condition.

Those factors are as follows:

1. Velocity.--The greater the velocity the greater the proportion of
drift to lift, and consequently the less the efficiency. Considering
the lifting surfaces alone, both the lift and the (active) drift, being
component parts of the reaction, increase as the square of the velocity,
and the efficiency remains the same at all speeds. But, considering the
whole aeroplane, we must remember the passive drift. It also increases
as the square of the velocity (with no attendant lift), and, adding
itself to the active drift, results in increasing the proportion of
total drift (active + passive) to lift.

But for the increase in passive drift the efficiency of the aeroplane
would not fall with increasing velocity, and it would be possible, by
doubling the thrust, to approximately double the speed or lift--a happy
state of affairs which can never be, but which we may, in a measure,
approach by doing everything possible to diminish the passive drift.

Every effort is then made to decrease it by "stream-lining," i.e., by
giving all "detrimental" parts of the aeroplane a form by which they
will pass through the air with the least possible drift. Even the wires
bracing the aeroplane together are, in many cases, stream-lined, and
with a markedly good effect upon the lift-drift ratio. In the case of a
certain well-known type of aeroplane the replacing of the ordinary wires
by stream-lined wires added over five miles an hour to the flight speed.

Head-resistance is a term often applied to passive drift, but it is apt
to convey a wrong impression, as the drift is not nearly so much the
result of the head or forward part of struts, wires, etc., as it is of
the rarefied area behind.

Above is illustrated the flow of air round two objects moving in the
direction of the arrow M.

In the case of A, you will note that the rarefied area DD is of very
considerable extent; whereas in the case of B, the air flows round it
in such a way as to meet very closely to the rear of the object, thus
DECREASING DD.

The greater the rarefied area DD. then, the less the density, and,
consequently, the less the pressure of air upon the rear of the object.
The less such pressure, then, the better is head-resistance D able to
get its work in, and the more thrust will be required to overcome it.

The "fineness" of the stream-line shape, i.e., the proportion of length
to width, is determined by the velocity--the greater the velocity, the
greater the fineness. The best degree of fineness for any given velocity
is found by means of wind-tunnel research.

The practical application of all this is, from a rigging point of view,
the importance of adjusting all stream-line parts to be dead-on in the
line of flight, but more of that later on.

2. Angle of Incidence.--The most efficient angle of incidence varies
with the thrust at the disposal of the designer, the weight to be
carried, and the climb-velocity ratio desired.

The best angles of incidence for these varying factors are found by
means of wind-tunnel research and practical trial and error. Generally
speaking, the greater the velocity the smaller should be the angle of
incidence, in order to preserve a clean, stream-line shape of rarefied
area and freedom from eddies. Should the angle be too great for the
velocity, then the rarefied area becomes of irregular shape with
attendant turbulent eddies. Such eddies possess no lift value, and since
it has taken power to produce them, they represent drift and adversely
affect the lift-drift ratio.

From a rigging point of view, one must presume that every standard
aeroplane has its lifting surface set at the most efficient angle, and
the practical application of all this is in taking the greatest possible
care to rig the surface at the correct angle and to maintain it at such
angle. Any deviation will adversely affect the lift-drift ratio, i.e.,
the efficiency.

3. Camber.--(Refer to the second illustration in this chapter.) The
lifting surfaces are cambered, i.e., curved, in order to decrease the
horizontal component of the reaction, i.e., the drift.

The bottom camber: If the bottom of the surface was flat, every particle
of air meeting it would do so with a shock, and such shock would produce
a very considerable horizontal reaction or drift. By curving it such
shock is diminished, and the curve should be such as to produce a
uniform (not necessarily constant) acceleration and compression of the
air from the leading edge to the trailing edge. Any unevenness in the
acceleration and compression of the air produces drift.

The top camber: If this was flat it would produce a rarefied area of
irregular shape. I have already explained the bad effect this has
upon the lift-drift ratio. The top surface is then curved to produce a
rarefied area the shape of which shall be as stream-line and free from
attendant eddies as possible.

The camber varies with the angle of incidence, the velocity, and the
thickness of the surface. Generally speaking, the greater the velocity,
the less the camber and angle of incidence. With infinite velocity the
surface would be set at no angle of incidence (the neutral lift line
coincident with the direction of motion relative to the air), and would
be, top and bottom, of pure streamline form--i.e., of infinite fineness.
This is, of course, carrying theory to absurdity as the surface would
then cease to exist.

The best cambers for varying velocities, angles of incidence, and
thicknesses of surface, are found by means of wind-tunnel research.
The practical application of all this is in taking the greatest care to
prevent the surface from becoming distorted and thus spoiling the camber
and consequently the lift-drift ratio.

4. Aspect Ratio.--This is the proportion of span to chord. Thus, if the
span is, for instance, 50 feet and the chord 5 feet, the surface would
be said to have an aspect ratio of 10 to 1.

For A GIVEN VELOCITY and A GIVEN AREA of surface, the greater the
aspect ratio, the greater the reaction. It is obvious, I think, that the
greater the span, the greater the mass of air engaged, and, as already
explained, the reaction is partly the result of the mass of air engaged.

Not only that, but, PROVIDED the chord is not decreased to an extent
making it impossible to secure the best camber owing to the thickness
of the surface, the greater the aspect ratio, the better the lift-drift
ratio. The reason of this is rather obscure. It is sometimes advanced
that it is owing to the "spill" of air from under the wing-tips. With
a high aspect ratio the chord is less than would otherwise be the case.
Less chord results in smaller wing-tips and consequently less "spill."
This, however, appears to be a rather inadequate reason for the high
aspect ratio producing the high lift-drift ratio. Other reasons are also
advanced, but they are of such a contentious nature I do not think it
well to go into them here. They are of interest to designers, but this
is written for the practical pilot and rigger.

5. Stagger.--This is the advancement of the top surface relative to the
bottom surface, and is not, of course, applicable to a single surface,
i.e., a monoplane. In the case of a biplane having no stagger, there
will be "interference" and consequent loss of Efficiency unless the
gap between the top and bottom surfaces is equal to not less than 1 1/2
times the chord. If less than that, the air engaged by the bottom of the
top surface will have a tendency to be drawn into the rarefied area over
the top of the bottom surface, with the result that the surfaces will
not secure as good a reaction as would otherwise be the case.

It is not practicable to have a gap of much more than a distance equal
to the chord, owing to the drift produced by the great length of struts
and wires such a large gap would necessitate. By staggering the top
surface forward, however, it is removed from the action of the lower
surface and engages undisturbed air, with the result that the efficiency
can in this way be increased by about 5 per cent. Theoretically the top
plane should be staggered forward for a distance equal to about 30 per
cent. of the chord, the exact distance depending upon the velocity
and angle of incidence; but this is not always possible to arrange
in designing an aeroplane, owing to difficulties of balance, desired
position, and view of pilot, observer, etc.

6. Horizontal Equivalent.--The vertical component of the reaction, i.e.,
lift, varies as the horizontal equivalent (H.E.) of the surface, but
the drift remains the same. Then it follows that if H.E. grows less, the
ratio of lift to drift must do the same.

A, B, and C are front views of three surfaces.

A has its full H.E., and therefore, from the point of view from which
we are at the moment considering efficiency, it has its best lift-drift
ratio.

B and C both possess the same surface as A, but one is inclined upwards
from its centre and the other is straight but tilted. For these reasons
their H.E.'s are, as illustrated, less than in the case of A. That means
less vertical lift, and, the drift remaining the same (for there is
the same amount of surface as in A to produce it), the lift-drift ratio
falls.

THE MARGIN OF POWER is the power available above that necessary to
maintain horizontal flight.

THE MARGIN OF LIFT is the height an aeroplane can gain in a given time
and starting from a given altitude. As an example, thus: 1,000 feet the
first minute, and starting from an altitude of 500 feet above sea-level.

The margin of lift decreases with altitude, owing to the decrease in
the density of the air, which adversely affects the engine. Provided
the engine maintained its impulse with altitude, then, if we ignore the
problem of the propeller, which I will go into later on, the margin of
lift would not disappear. Moreover, greater velocity for a given power
would be secured at a greater altitude, owing to the decreased density
of air to be overcome. After reading that, you may like to light your
pipe and indulge in dreams of the wonderful possibilities which may
become realities if some brilliant genius shows us some day how to
secure a constant power with increasing altitude. I am afraid, however,
that will always remain impossible; but it is probable that some very
interesting steps may be taken in that direction.

THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift
ratio is highest. In modern aeroplanes it is that angle of incidence
possessed by the surface when the axis of the propeller is horizontal.

THE BEST CLIMBING ANGLE is approximately half-way between the maximum
and the optimum angles.

All present-day aeroplanes are a compromise between Climb and horizontal
Velocity. We will compare the essentials for two aeroplanes, one
designed for maximum climb, and the other for maximum velocity.


ESSENTIALS FOR MAXIMUM CLIMB:

1. Low velocity, in order to secure the best lift-drift ratio.

2. Having a low velocity, a large surface will be necessary in order to
engage the necessary mass of air to secure the requisite lift.

3. Since (1) such a climbing machine will move along an upward sloping
path, and (2) will climb with its propeller thrust horizontal, then a
large angle relative to the direction of the thrust will be necessary in
order to secure the requisite angle relative to the direction of motion.

The propeller thrust should be always horizontal, because the most
efficient flying-machine (having regard to climb OR velocity) has, so
far, been found to be an arrangement of an inclined surface driven by
a HORIZONTAL thrust--the surface lifting the weight, and the thrust
overcoming the drift. This is, in practice, a far more efficient
arrangement than the helicopter, i.e., the air-screw revolving about
a vertical axis and producing a thrust opposed to gravity. If, when
climbing, the propeller thrust is at such an angle as to tend to haul
the aeroplane upwards, then it is, in a measure, acting as a helicopter,
and that means inefficiency. The reason of a helicopter being
inefficient in practice is due to the fact that, owing to mechanical
difficulties, it is impossible to construct within a reasonable weight
an air-screw of the requisite dimensions. That being so, it would be
necessary, in order to absorb the power of the engine, to revolve the
comparatively small-surfaced air screw at an immensely greater velocity
than that of the aeroplane's surface. As already explained, the
lift-drift ratio falls with velocity on account of the increase in
passive drift. This applies to a blade of a propeller or air-screw,
which is nothing but a revolving surface set at angle of incidence, and
which it is impossible to construct without a good deal of detrimental
surface near the central boss.

4. The velocity being low, then it follows that for that reason also the
angle of incidence should be comparatively large.

5. Camber.--Since such an aeroplane would be of low velocity, and
therefore possess a large angle of incidence, a large camber would be
necessary.

Let us now consider the essentials for an aeroplane of maximum velocity
for its power, and possessing merely enough lift to get off the ground,
but no margin of lift.

1. Comparatively HIGH VELOCITY.

2. A comparatively SMALL SURFACE, because, being of greater velocity
than the maximum climber, a greater mass of air will be engaged for
a given surface and time, and therefore a smaller surface will be
sufficient to secure the requisit lift.

3. A small angle relative to the propeller thrust, since the latter
coincides with the direction of motion.

4. A comparatively small angle of incidence by reason of the high
velocity.

5. A comparatively small camber follows as a result of the small angle
of incidence.


SUMMARY.

   Essentials for Maximum            Essentials for Maximum
           Climb.                            Velocity

   1. Low velocity.                  High velocity.
   2. Large surface.                 Small surface.
   3. Large angle relative to        Small angle relative to
      propeller thrust.                 propeller thrust.
   4. Large angle relative to        Small angle relative to direction
      direction of motion.              of motion.
   5. Large camber.                  Small camber.


It is mechanically impossible to construct an aeroplane of reasonable
weight of which it would be possible to very the above opposing
essentials. Therefore, all aeroplanes are designed as a compromise
between Climb and Velocity.

As a rule aeroplanes are designed to have at low altitude a slight
margin of lift when the propeller thrust is horizontal.


ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNED
AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHT
MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL

MINIMUM ANGLE.

This gives the greatest velocity during horizontal flight at a low
altitude. Greater velocity would be secured if the surface, angle, and
camber were smaller and designed to just maintain horizontal flight
with a horizontal thrust. Also, in such case, the propeller would not
be thrusting downwards, but along a horizontal line which is obviously
a more efficient arrangement if we regard the aeroplane merely from one
point of view, i.e., either with reference to velocity OR climb.

OPTIMUM ANGLE (Thrust horizontal)

The velocity is less than at the smaller minimum angle, and, as
aeroplanes are designed to-day, the area and angle of incidence of the
surface is such as to secure a slight ascent at a low altitude. The
camber of the surface is designed for this angle of incidence and
velocity. The lift-drift ratio is best at this angle.

BEST CLIMBING ANGLE

The velocity is now still less by reason of the increased angle
producing increase of drift. Less velocity at A GIVEN ANGLE produces
less lift, but the increased angle more or less offsets the loss of
lift due to the decreased velocity, and in addition, the thrust is now
hauling the aeroplane upwards.

MAXIMUM ANGLE

The greater angle has now produced so much drift as to lessen the
velocity to a point where the combined lifts from the surface and from
the thrust are only just able to maintain horizontal flight. Any greater
angle will result in a still lower lift-drift ratio. The lift will then
become less than the weight and the aeroplane will consequently fall.
Such a fall is known as "stalling" or "pancaking."

NOTE.--The golden rule for beginners: Never exceed the Best Climbing
Angle. Always maintain the flying speed of the aeroplane.


By this means, when the altitude is reached where the margin of
lift disappears (on account of loss of engine power), and which is,
consequently, the altitude where it is just possible to maintain
horizontal flight, the aeroplane is flying with its thrust horizontal
and with maximum efficiency (as distinct from engine and propeller
efficiency).

The margin of lift at low altitude, and when the thrust is horizontal,
should then be such that the higher altitude at which the margin of lift
is lost is that altitude at which most of the aeroplane's horizontal
flight work is done. That ensures maximum velocity when most required.

Unfortunately, where aeroplanes designed for fighting are concerned, the
altitude where most of the work is done is that at which both maximum
velocity and maximum margin of lift for power are required.

Perhaps some day a brilliant inventor will design an aeroplane of
reasonable weight and drift of which it will be possible for the pilot
to vary at will the above-mentioned opposing essentials. Then we shall
get maximum velocity, or maximum margin of lift, for power as required.
Until then the design of the aeroplane must remain a compromise between
Velocity and Climb.



CHAPTER II. STABILITY AND CONTROL

STABILITY is a condition whereby an object disturbed has a natural
tendency to return to its first and normal position. Example: a weight
suspended by a cord.

INSTABILITY is a condition whereby an object disturbed has a natural
tendency to move as far as possible away from its first position, with
no tendency to return. Example: a stick balanced vertically upon your
finger.

NEUTRAL INSTABILITY is a condition whereby an object disturbed has no
tendency to move farther than displaced by the force of the disturbance,
and no tendency to return to its first position.

In order that an aeroplane may be reasonably controllable, it is
necessary for it to possess some degree of stability longitudinally,
laterally, and directionally.

LONGITUDINAL STABILITY in an aeroplane is its stability about an axis
transverse to the direction of normal horizontal flight, and without
which it would pitch and toss.

LATERAL STABILITY is its stability about its longitudinal axis, and
without which it would roll sideways.

DIRECTIONAL STABILITY is its stability about its vertical axis, and
without which it would have no tendency to keep its course.

For such directional stability to exist there must be, in effect,[16]
more "keel-surface" behind the vertical axis than there is in front of
it. By keel-surface I mean every-thing to be seen when looking at an
aeroplane from the side of it--the sides of the body, undercarriage,
struts, wires, etc. The same thing applies to a weathercock. You know
what would happen if there was insufficient keel-surface behind the
vertical axis upon which it is pivoted. It would turn off its proper
course, which is opposite to the direction of the wind. It is very much
the same in the case of an aeroplane.

The above illustration represents an aeroplane (directionally stable)
flying along the course B. A gust striking it as indicated acts upon the
greater proportion of keel-surface behind the turning axis and throws it
into the new course. It does not, however, travel along the new course,
owing to its momentum in the direction B. It travels, as long as such
momentum lasts, in a direction which is the resultant of the two forces
Thrust and Momentum. But the centre line of the aeroplane is pointing in
the direction of the new course. Therefore its attitude, relative to
the direction of motion, is more or less sideways, and it consequently
receives an air pressure in the direction C. Such pressure, acting upon
the keel-surface, presses the tail back towards its first position in
which the aeroplane is upon its course B.

What I have described is continually going on during flight, but in
a well-designed aeroplane such stabilizing movements are, most of the
time, so slight as to be imperceptible to the pilot.

If an aeroplane was not stabilized in this way, it would not only be
continually trying to leave its course, but it would also possess a
dangerous tendency to "nose away" from the direction of the side gusts.
In such case the gust shown in the above illustration would turn the
aeroplane round the opposite way a very considerable distance; and the
right wing, being on the outside of the turn, would travel with greater
velocity than the left wing. Increased velocity means increased lift;
and so, the right wing lifting, the aeroplane would turn over sideways
very quickly.

LONGITUDINAL STABILITY.--Flat surfaces are longitudinally stable owing
to the fact that with decreasing angles of incidence the centre line of
pressure (C.P.) moves forward.

The C.P. is a line taken across the surface, transverse to the direction
of motion, and about which all the air forces may be said to balance, or
through which they may be said to act.


Imagine A to be a flat surface, attitude vertical, travelling through
the air in the direction of motion M. Its C.P. is then obviously along
the exact centre line of the surface as illustrated.

In B, C, and D the surfaces are shown with angles of incidence
decreasing to nothing, and you will note that the C.P. moves forward
with the decreasing angle.

Now, should some gust or eddy tend to make the surface decrease the
angle, i.e., dive, then the C.P. moves forward and pushes the front of
the surface up. Should the surface tend to assume too large an angle,
then the reverse happens--the C.P. moves back and pushes the rear of the
surface up.

Flat surfaces are, then, theoretically stable longitudinally. They are
not, however, used, on account of their poor lift-drift ratio.

As already explained, cambered surfaces are used, and these are
longitudinally unstable at those angles of incidence producing a
reasonable lift-drift ratio, i.e., at angles below: about 12 degrees.

A is a cambered surface, attitude approximately vertical, moving through
the air in the direction M. Obviously the C. P. coincides with the
transverse centre line of the surface.

With decreasing angles, down to angles of about 30 degrees, the C.P.
moves forward as in the case of flat surfaces (see B), but angles above
30 degrees do not interest us, since they produce a very low ratio of
lift to drift.

Below angles of about 30 degrees (see C) the dipping front part of the
surface assumes a negative angle of incidence resulting in the DOWNWARD
air pressure D, and the more the angle of incidence is decreased, the
greater such negative angle and its resultant pressure D. Since the
C.P. is the resultant of all the air forces, its position is naturally
affected by D, which causes it to move backwards. Now, should some gust
or eddy tend to make the surface decrease its angle of incidence, i.e.,
dive, then the C.P. moves backwards, and, pushing up the rear of the
surface, causes it to dive the more. Should the surface tend to assume
too large an angle, then the reverse happens; the pressure D decreases,
with the result that C.P. moves forward and pushes up the front of the
surface, thus increasing the angle still further, the final result being
a "tail-slide."

It is therefore necessary to find a means of stabilizing the naturally
unstable cambered surface. This is usually secured by means of a
stabilizing surface fixed some distance in the rear of the main surface,
and it is a necessary condition that the neutral lift lines of the two
surfaces, when projected to meet each other, make a dihedral angle. In
other words, the rear stabilizing surface must have a lesser angle of
incidence than the main surface--certainly not more than one-third of
that of the main surface. This is known as the longitudinal dihedral.

I may add that the tail-plane is sometimes mounted upon the aeroplane at
the same angle as the main surface, but, in such cases, it attacks air
which has received a downward deflection from the main surface, thus:

The angle at which the tail surface attacks the air (the angle of
incidence) is therefore less than the angle of incidence of the main
surface.

I will now, by means of the following illustration, try to explain how
the longitudinal dihedral secures stability:

First, imagine the aeroplane travelling in the direction of motion,
which coincides with the direction of thrust T. The weight is, of
course, balanced about a C.P., the resultant of the C.P. of the main
surface and the C.P. of the stabilizing surface. For the sake of
illustration, the stabilizing surface has been given an angle of
incidence, and therefore has a lift and C.P. In practice the stabilizer
is often set at no angle of incidence. In such case the proposition
remains the same, but it is, perhaps, a little easier to illustrate it
as above.

Now, we will suppose that a gust or eddy throws the machine into the
lower position. It no longer travels in the direction of T, since the
momentum in the old direction pulls it off that course. M is now the
resultant of the Thrust and the Momentum, and you will note that this
results in a decrease in the angle our old friend the neutral lift line
makes with M, i.e., a decrease in the angle of incidence and therefore a
decrease in lift.

We will suppose that this decrease is 2 degrees. Such decrease applies
to both main surface and stabilizer, since both are fixed rigidly to the
aeroplane.

The main surface, which had 12 degrees angle, has now only 10 degrees,
i.e., a loss of ONE-SIXTH.

The stabilizer, which had 4 degrees angle, has now only 2 degrees, i.e.,
a loss of ONE-HALF.

The latter has therefore lost a greater PROPORTION of its angle of
incidence, and consequently its lift, than has the main surface. It must
then fall relative to the main surface. The tail falling, the aeroplane
then assumes its first position, though at a slightly less altitude.

Should a gust throw the nose of the aeroplane up, then the reverse
happens. Both main surface and stabilizer increase their angles of
incidence in the same amount, but the angle, and therefore the lift, of
the stabilizer increases in greater proportion than does the lift of the
main surface, with the result that it lifts the tail. The aeroplane then
assumes its first position, though at a slightly greater altitude.

Do not fall into the widespread error that the angle of incidence varies
as the angle of the aeroplane to the horizontal. It varies with such
angle, but not as anything approaching it. Remember that the stabilizing
effect of the longitudinal dihedral lasts only as long as there is
momentum in the direction of the first course.

These stabilizing movements are taking place all the time, even though
imperceptible to the pilot.

Aeroplanes have, in the past, been built with a stabilizing surface in
front of the main surface instead of at the rear of it. In such
design the main surface (which is then the tail surface as well as the
principal lifting surface) must be set at a less angle than the forward
stabilizing surface, in order to secure a longitudinal dihedral. The
defect of such design lies in the fact that the main surface must have
a certain angle to lift the weight--say 5 degrees. Then, in order to
secure a sufficiency of longitudinal stability, it is necessary to
set the forward stabilizer at about 15 degrees. Such a large angle of
incidence results in a very poor lift-drift ratio (and consequently
great loss of efficiency), except at very low velocities compared with
the speed of modern aeroplanes. At the time such aeroplanes were built
velocities were comparatively low, and this defect was; for that reason,
not sufficiently appreciated. In the end it killed the "canard" or
"tail-first" design.

Aeroplanes of the Dunne and similar types possess no stabilizing surface
distinct from the main surface, but they have a longitudinal dihedral
which renders them stable.

The main surface towards the wing-tips is given a decreasing angle
of incidence and corresponding camber. The wing-tips then act as
longitudinal stabilizers.

This design of aeroplane, while very interesting, has not proved very
practicable, owing to the following disadvantages: (1) The plan design
is not, from a mechanical point of view, so sound as that of the
ordinary aeroplane surface, which is, in plan, a parallelogram. It is,
then, necessary to make the strength of construction greater than would
otherwise be the case. That means extra weight. (2) The plan of the
surface area is such that the aspect ratio is not so high as if the
surface was arranged with its leading edges at right angles to the
direction of motion. The lower the aspect ratio, then, the less the
lift. This design, then, produces less lift for weight of surface than
would the same surface if arranged as a parallelogram. (3) In order to
secure the longitudinal dihedral, the angle of incidence has to be very
much decreased towards the wing-tips. Then, in order that the lift-drift
ratio may be preserved, there must be a corresponding decrease in the
camber. That calls for surface ribs of varying cambers, and results in
an expensive and lengthy job for the builder. (4) In order to secure
directional stability, the surface is, in the centre, arranged to dip
down in the form of a V, pointing towards the direction of motion.
Should the aeroplane turn off its course, then its momentum in the
direction of its first course causes it to move in a direction the
resultant of the thrust and the momentum. It then moves in a more or
less sideways attitude, which results in an air pressure upon one
side of the V, and which tends to turn the aeroplane back to its first
course. This arrangement of the surface results in a bad drift. Vertical
surfaces at the wing-tips may also be set at an angle producing the same
stabilizing effect, but they also increase the drift.

The gyroscopic action of a rotary engine will affect the longitudinal
stability when an aeroplane is turned to right or left. In the case of
a Gnome engine, such gyroscopic action will tend to depress the nose of
the aeroplane when it is turned to the left, and to elevate it when
it is turned to the right. In modern aeroplanes this tendency is not
sufficiently important to bother about. In the old days of crudely
designed and under-powered aeroplanes this gyroscopic action was very
marked, and led the majority of pilots to dislike turning an aeroplane
to the right, since, in doing so, there was some danger of "stalling."


LATERAL STABILITY is far more difficult for the designer to secure
than is longitudinal or directional stability. Some degree of lateral
stability may be secured by means of the "lateral dihedral," i.e., the
upward inclination of the surface towards its wing-tips thus:

Imagine the top V, illustrated opposite, to be the front view of a
surface flying towards you. The horizontal equivalent (H.E.) of the left
wing is the same as that of the right wing. Therefore, the lift of one
wing is equal to the lift of the other, and the weight, being situated
always in the centre, is balanced.

If some movement of the air causes the surface to tilt sideways, as in
the lower illustration, then you will note that the H.E. of the left
wing increases, and the H.E. of the right wing decreases. The left wing
then, having the greatest lift, rises; and the surface assumes its first
and normal position.

Unfortunately however, the righting effect is not proportional to the
difference between the right and left H.E.'s.



In the case of A, the resultant direction of the reaction of both wings
is opposed to the direction of gravity or weight. The two forces R R
and gravity are then evenly balanced, and the surface is in a state of
equilibrium.

In the case of B, you will note that the R R is not directly opposed
to gravity. This results in the appearance of M, and so the resultant
direction of motion of the aeroplane is no longer directly forward, but
is along a line the resultant of the thrust and M. In other words,
it is, while flying forward, at the same time moving sideways in the
direction M.

In moving sideways, the keel-surface receives, of course, a pressure
from the air equal and opposite to M. Since such surface is greatest in
effect towards the tail, then the latter must be pushed sideways. That
causes the aeroplane to turn; and, the highest wing being on the
outside of the turn, it has a greater velocity than the lower wing. That
produces greater lift, and tends to tilt the aeroplane over still more.
Such tilting tendency is, however, opposed by the difference in the
H.E.'s of the two wings.

It then follows that, for the lateral dihedral angle to be effective,
such angle must be large enough to produce, when the aeroplane tilts,
a difference in the H.E.'s of the two wings, which difference must be
sufficient to not only oppose the tilting tendency due to the aeroplane
turning, but sufficient to also force the aeroplane back to its original
position of equilibrium.

It is now, I hope, clear to the reader that the lateral dihedral is
not quite so effective as would appear at first sight. Some designers,
indeed, prefer not to use it, since its effect is not very great, and
since it must be paid for in loss of H.E. and consequently loss of lift,
thus decreasing the lift-drift ratio, i.e., the efficiency. Also, it is
sometimes advanced that the lateral dihedral increases the "spill" of
air from the wing-tips and that this adversely affects the lift-drift
ratio.

The disposition of the keel-surface affects the lateral stability. It
should be, in effect, equally divided by the longitudinal turning axis
of the aeroplane. If there is an excess of keel-surface above or below
such axis, then a side gust striking it will tend to turn the aeroplane
over sideways.

The position of the centre of gravity affects lateral stability. If
too low, it produces a pendulum effect and causes the aeroplane to roll
sideways.

If too high, it acts as a stick balanced vertically would act. If
disturbed, it tends to travel to a position as far as possible from its
original position. It would then tend, when moved, to turn the aeroplane
over sideways and into an upside-down position.

From the point of view of lateral stability, the best position for the
centre of gravity is one a little below the centre of drift.

Propeller torque affects lateral stability. An aeroplane tends to turn
over sideways in the opposite direction to which the propeller revolves.

This tendency is offset by increasing the angle of incidence (and
consequently the lift) of the side tending to fall; and it is always
advisable, if practical considerations allow it, to also decrease the
angle upon the other side. In that way it is not necessary to depart so
far from the normal angle of incidence at which the lift-drift ratio is
highest.

Wash-in is the term applied to the increased angle.

Wash-out is the term applied to the decreased angle.

Both lateral and directional stability may be improved by washing out
the angle of incidence on both sides of the surface, thus:

The decreased angle decreases the drift and therefore the effect of
gusts upon the wing-tips which is just where they have the most effect
upon the aeroplane, owing to the distance from the turning axis.

The wash-out also renders the ailerons (lateral controlling services)
more effective, as, in order to operate them, it is not then necessary
to give them such a large angle of incidence as would otherwise be
required.



The less the angle of incidence of the ailerons, the better their
lift-drift ratio, i.e., their efficiency. You will note that, while the
aileron attached to the surface with washed-out angle is operated to the
same extent as the aileron illustrated above it, its angle of incidence
is considerably less. Its efficiency is therefore greater.

The advantages of the wash-in must, of course be paid for in some loss
of lift, as the lift decreases with the decreased angle.

In order to secure all the above described advantages, a combination is
sometimes effected, thus:

BANKING.--An aeroplane turned off its course to right or left does not
at once proceed along its new course. Its momentum in the direction of
its first course causes it to travel along a line the resultant of such
momentum and the thrust. In other words, it more or less skids sideways
and away from the centre of the turn. Its lifting surfaces do not then
meet the air in their correct attitude, and the lift may fall to such
an extent as to become less than the weight, in which case the aeroplane
must fall. This bad effect is minimized by "banking," i.e., tilting the
aeroplane sideways. The bottom of the lifting surface is in that way
opposed to the air through which it is moving in the direction of the
momentum and receives an opposite air pressure. The rarefied area over
the top of the surface is rendered still more rare, and this, of course,
assists the air pressure in opposing the momentum.

The velocity of the "skid," or sideways movement, is then only such
as is necessary to secure an air pressure equal and opposite to the
centrifugal force of the turn.

The sharper the turn, the greater the effect of the centrifugal force,
and therefore the steeper should be the "bank." Experentia docet.

The position of the centre of gravity affects banking. A low C.G. will
tend to swing outward from the centre of the turn, and will cause the
aeroplane to bank--perhaps too much, in which case the pilot must remedy
matters by operating the ailerons.

A high C.G. also tends to swing outward from the centre of the turn. It
will tend to make the aeroplane bank the wrong way, and such effect must
be remedied by means of the ailerons.

The pleasantest machine from a banking point of view is one in which
the C.G. is a little below the centre of drift. It tends to bank the
aeroplane the right way for the turn, and the pilot can, if necessary,
perfect the bank by means of the ailerons.

The disposition of the keel-surface affects banking. It should be,
in effect, evenly divided by the longitudinal axis. An excess of
keel-surface above the longitudinal axis will, when banking, receive an
air pressure causing the aeroplane to bank, perhaps too much. An excess
of keel-surface below the axis has the reverse effect.


SIDE-SLIPPING.--This usually occurs as a result of over-banking. It is
always the result of the aeroplane tilting sideways and thus decreasing
the horizontal equivalent, and therefore the lift, of the surface. An
excessive "bank," or sideways tilt, results in the H.E., and therefore
the lift, becoming less than the weight, when, of course, the aeroplane
must fall, i.e., side-slip.


When making a very sharp turn it is necessary to bank very steeply
indeed. If, at the same time, the longitudinal axis of the aeroplane
remains approximately horizontal, then there must be a fall, and the
direction of motion will be the resultant of the thrust and the fall as
illustrated above in sketch A. The lifting surfaces and the controlling
surfaces are not then meeting the air in the correct attitude, with the
result that, in addition to falling, the aeroplane will probably become
quite unmanageable.

The Pilot, however, prevents such a state of affairs from happening by
"nosing-down," i.e., by operating the rudder to turn the nose of the
aeroplane downward and towards the direction of motion as illustrated
in sketch B. This results in the higher wing, which is on the outside
of the turn, travelling with greater velocity, and therefore securing a
greater reaction than the lower wing, thus tending to tilt the aeroplane
over still more. The aeroplane is now almost upside-down, but its
attitude relative to the direction of motion is correct and the
controlling surfaces are all of them working efficiently. The recovery
of a normal attitude relative to the Earth is then made as illustrated
in sketch C.

The Pilot must then learn to know just the angle of bank at which the
margin of lift is lost, and, if a sharp turn necessitates banking beyond
that angle, he must "nose-down."

In this matter of banking and nosing-down, and, indeed, regarding
stability and control generally, the golden rule for all but very
experienced pilots should be: Keep the aeroplane in such an attitude
that the air pressure is always directly in the pilot's face. The
aeroplane is then always engaging the air as designed to do so, and
both lifting and controlling surfaces are acting efficiently. The only
exception to this rule is a vertical dive, and I think that is obviously
not an attitude for any but very experienced pilots to hanker after.

SPINNING.--This is the worst of all predicaments the pilot can find
himself in. Fortunately it rarely happens.

It is due to the combination of (1) a very steep spiral descent of small
radius, and (2) insufficiency of keel-surface behind the vertical axis,
or the jamming of the rudder end or elevator into a position by which
the aeroplane is forced into an increasingly steep and small spiral.

Owing to the small radius of such a spiral, the mass of the aeroplane
may gain a rotary momentum greater, in effect, than the air pressure of
the keel-surface or controlling surfaces opposed to it; and, when once
such a condition occurs, it is difficult to see what can be done by the
pilot to remedy it. The sensible pilot will not go beyond reasonable
limits of steepness and radius when executing spiral descents.

GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should
be, designed to assume their gliding angle when the power and thrust is
cut off. This relieves the pilot of work, worry, and danger should he
find himself in a fog or cloud. The Pilot, although he may not realize
it, maintains the correct attitude of the aeroplane by observing its
position relative to the horizon. Flying into a fog or cloud the horizon
is lost to view, and he must then rely upon his instruments--(1) the
compass for direction; (2) an inclinometer (arched spirit-level) mounted
transversely to the longitudinal axis, for lateral stability; and (3) an
inclinometer mounted parallel to the longitudinal axis, or the airspeed
indicator, which will indicate a nose-down position by increase in air
speed, and a tail-down position by decrease in air speed.

The pilot is then under the necessity of watching three instruments
and manipulating his three controls to keep the instruments indicating
longitudinal, lateral, and directional stability. That is a feat beyond
the capacity of the ordinary man. If, however, by the simple movement
of throttling down the power and thrust, he can be relieved of looking
after the longitudinal stability, he then has only two instruments to
watch. That is no small job in itself, but it is, at any rate, fairly
practicable.

Aeroplanes are, then, designed, or should be, so that the centre of
gravity is slightly forward of centre of lift. The aeroplane is then, as
a glider, nose-heavy--and the distance the C.G. is placed in advance
of the C.L. should be such as to ensure a gliding angle producing a
velocity the same as the normal flying speed (for which the strength of
construction has been designed).

In order that this nose-heavy tendency should not exist when the thrust
is working and descent not required, the centre of thrust is placed a
little below the centre of drift or resistance, and thus tends to pull
up the nose of the aeroplane.

The distance the centre of thrust is placed below the centre of drift
should be such as to produce a force equal and opposite to that due to
the C.G. being forward of the C.L.

LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired, it is best to
throttle the engine down at point A. The C.G. being forward of the C.P.,
then causes the aeroplane to nose-down, and assists the pilot in making
a reasonably small loop along the course C and in securing a quick
recovery. If the engine is not throttled down, then the aeroplane may
be expected to follow the course D, which results in a longer nose dive
than in the case of the course C.

A steady, gentle movement of the elevator is necessary. A jerky movement
may change the direction of motion so suddenly as to produce dangerous
air stresses upon the surfaces, in which case there is a possibility of
collapse.

If an upside-down flight is desired, the engine may, or may not, be
throttled down at point A. If not throttled down, then the elevator must
be operated to secure a course approximately in the direction B. If it
is throttled down, then the course must be one of a steeper angle than
B, or there will be danger of stalling.


Diagram p. 88.--This is not set at quite the correct angle. Path B
should slope slightly downwards from Position A.



CHAPTER III. RIGGING

In order to rig an aeroplane intelligently, and to maintain it in an
efficient and safe condition, it is necessary to possess a knowledge
of the stresses it is called upon to endure, and the strains likely to
appear.


STRESS is the load or burden a body is called upon to bear. It is
usually expressed by the result found by dividing the load by the number
of superficial square inches contained in the cross-sectional area of
the body.

Thus, if, for instance, the object illustrated above contains 4 square
inches of cross-sectional area, and the total load it is called upon to
endure is 10 tons, the stress would be expressed as 2 1/2 tons.


STRAIN is the deformation produced by stress.


THE FACTOR OF SAFETY is usually expressed by the result found by
dividing the stress at which it is known the body will collapse, by
the maximum stress it will be called upon to endure. For instance, if a
control wire be called upon to endure a maximum stress of 2 cwts., and
the known stress at which it will collapse is 10 cwts., the factor of
safety is then 5.

[cwts. = centerweights = 100 pound units as in cent & century.
Interestingly enough, this word only exists today in abbreviation form,
probably of centreweights, but the dictionary entries, even from a
hundred years ago do not list this as a word, but do list c. or C. as
the previous popular abbreviation as in Roman Numerals] The word listed
is "hundredweight. Michael S. Hart, 1997]


COMPRESSION.--The simple stress of compression tends to produce a
crushing strain. Example: the interplane and fuselage struts.


TENSION.--The simple stress of tension tends to produce the strain of
elongation. Example: all the wires.


BENDING.--The compound stress of bending is a combination of compression
and tension.

The above sketch illustrates a straight piece of wood of which the top,
centre, and bottom lines are of equal length. We will now imagine it
bent to form a circle, thus:

The centre line is still the same length as before being bent; but the
top line, being farther from the centre of the circle, is now longer
than the centre line. That can be due only to the strain of elongation
produced by the stress of tension. The wood between the centre line and
the top line is then in tension; and the farther from the centre, the
greater the strain, and consequently the greater the tension.

The bottom line, being nearest to the centre of the circle, is now
shorter than the centre line. That can be due only to the strain of
crushing produced by the stress of compression. The wood between the
centre and bottom lines is then in compression; and the nearer the
centre of the circle, the greater the strain, and consequently the
greater the compression.

It then follows that there is neither tension nor compression, i.e., no
stress, at the centre line, and that the wood immediately surrounding it
is under considerably less stress than the wood farther away. This being
so, the wood in the centre may be hollowed out without unduly weakening
struts and spars. In this way 25 to 33 per cent. is saved in the weight
of wood in an aeroplane.

The strength of wood is in its fibres, which should, as far as possible,
run without break from one end of a strut or spar to the other end. A
point to remember is that the outside fibres, being farthest removed
from the centre line, are doing by far the greatest work.


SHEAR STRESS IS such that, when material collapses under it, one part
slides over the other. Example: all the locking pins.

Some of the bolts are also in shear or "sideways" stress, owing to lugs
under their heads and from which wires are taken. Such a wire, exerting
a sideways pull upon a bolt, tries to break it in such a way as to make
one piece of the bolt slide over the other piece.

TORSION.--This is a twisting stress compounded of compression, tension,
and shear stresses. Example: the propeller shaft.


NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress
of compression far better than any other stress. For instance: a
walking-stick of less than 1 lb. in weight will, if kept perfectly
straight, probably stand up to a compression stress of a ton or more
before crushing; whereas, if the same stick is put under a bending
stress, it will probably collapse to a stress of not more than about
50 lb. That is a very great difference, and, since weight is of the
greatest importance, the design of an aeroplane is always such as to,
as far as possible, keep the various wooden parts of its construction in
direct compression. Weight being of such vital importance, and designers
all trying to outdo each other in saving weight, it follows that the
factor of safety is rather low in an aeroplane. The parts in direct
compression will, however, take the stresses safely provided the
following conditions are carefully observed.


CONDITIONS TO BE OBSERVED:


1. All the spars and struts must be perfectly straight.

The above sketch illustrates a section through an interplane strut. If
the strut is to be kept straight, i.e., prevented from bending, then
the stress of compression must be equally disposed about the centre of
strength. If it is not straight, then there will be more compression
on one side of the centre of strength than on the other side. That is
a step towards getting compression on one side and tension on the other
side, in which case it may be forced to take a bending stress for which
it is not designed. Even if it does not collapse it will, in effect,
become shorter, and thus throw out of adjustment the gap and all the
wires attached to the top and bottom of the strut, with the result that
the flight efficiency of the aeroplane will be spoiled.

The only exception to the above condition is what is known as the
Arch. For instance, in the case of the Maurice Farman, the spars of the
centre-section plane, which have to take the weight of the nacelle, are
arched upwards. If this was not done, it is possible that rough
landings might result in the weight causing the spars to become slightly
distorted downwards. That would produce a dangerous bending stress,
but, as long as the wood is arched, or, at any rate, kept from bending
downwards, it will remain in direct compression and no danger can
result.


2. Struts and spars must be symmetrical. By that I mean that the
cross-sectional dimensions must be correct, as otherwise there will be
bulging places on the outside, with the result that the stress will not
be evenly disposed about the centre of strength, and a bending stress
may be produced.


3. Struts, spars, etc., must be undamaged. Remember that, from what I
have already explained about bending stresses, the outside fibres of
the wood are doing by far the most work. If these get bruised or scored,
then the strut or spar suffers in strength much more than one might
think at first sight; and, if it ever gets a tendency to bend, it is
likely to collapse at that point.


4. The wood must have a good, clear grain with no cross-grain, knots,
or shakes. Such blemishes produce weak places and, if a tendency to bend
appears, then it may collapse at such a point.

5. The struts, spars, etc., must be properly bedded into their sockets
or fittings. To begin with, they must be of good pushing or gentle
tapping fit. They must never be driven in with a heavy hammer. Then
again, a strut must bed well down all over its cross-sectional area
as illustrated above; otherwise the stress of compression will not be
evenly disposed about the centre of strength, and that may produce a
bending stress. The bottom of the strut or spar should be covered
with some sort of paint, bedded into the socket or fitting, and then
withdrawn to see if the paint has stuck all over the bed.


6. The atmosphere is sometimes much damper than at other times, and this
causes wood to expand and contract appreciably. This would not matter
but for the fact that it does not expand and contract uniformly, but
becomes unsymmetrical, i.e., distorted. I have already explained
the danger of that in condition 2. This should be minimized by WELL
VARNISHING THE WOOD to keep the moisture out of it.


FUNCTION OF INTERPLANE STRUTS.--These struts have to keep the lifting
surfaces or "planes" apart, but this is only part of their work. They
must keep the planes apart, so that the latter are in their correct
attitude. That is only so when the spars of the bottom plane are
parallel with those of the top plane. Also, the chord of the top plane
must be parallel with the chord of the bottom plane. If that is not so,
then one plane will not have the same angle of incidence as the other
one. At first sight one might think that all that is necessary is to cut
all the struts to be the same length, but that is not the case.

Sometimes, as illustrated above, the rear spar is not so thick as the
main spar, and it is then necessary to make up for that difference by
making the rear struts correspondingly longer. If that is not done, then
the top and bottom chords will not be parallel, and the top and bottom
planes will have different angles of incidence. Also, the sockets or
fittings, or even the spars upon which they are placed, sometimes
vary in thickness owing to faulty manufacture. This must be offset by
altering the length of the struts. The best way to proceed is to measure
the distance between the top and bottom spars by the side of each strut,
and if that distance, or "gap" as it is called, is not as stated in the
aeroplane's specifications, then make it correct by changing the length
of the strut. This applies to both front and rear interplane struts.
When measuring the gap, always be careful to measure from the centre
of the spar, as it may be set at an angle, and the rear of it may be
considerably lower than its front.


BORING HOLES IN WOOD.--It should be a strict rule that no spar be used
which has an unnecessary hole in it. Before boring a hole, its position
should be confirmed by whoever is in charge of the workshop. A bolt-hole
should be of a size to enable the bolt to be pushed in, or, at any rate,
not more than gently tapped in. Bolts should not be hammered in, as that
may split the spar. On the other hand, a bolt should not be slack in its
hole, as, in such a case, it may work sideways and split the spar, not
to speak of throwing out of adjustment the wires leading from the lug or
socket under the bolt-head.


WASHERS.--Under the bolt-head, and also under the nut, a washer must be
placed--a very large washer compared with the size which would be used
in all-metal construction. This is to disperse the stress over a large
area; otherwise the washer may be pulled into the wood and weaken it,
besides possibly throwing out of adjustment the wires attached to the
bolt or the fitting it is holding to the spar.


LOCKING.--Now as regards locking the bolts. If split pins are used,
be sure to see that they are used in such a way that the nut cannot
possibly unscrew at all. The split pin should be passed through the bolt
as near as possible to the nut. It should not be passed through both nut
and bolt.

If it is locked by burring over the edge of the bolt, do not use a heavy
hammer and try to spread the whole head of the bolt. That might damage
the woodwork inside the fabric-covered surface. Use a small, light
hammer, and gently tap round the edge of the bolt until it is burred
over.


TURNBUCKLES.--A turnbuckle is composed of a central barrel into each end
of which is screwed an eye-bolt. Wires are taken from the eyes of the
eye-bolt, and so, by turning the barrel, they can be adjusted to their
proper tension. Eye-bolts must be a good fit in the barrel; that is to
say, not slack and not very tight. Theoretically it is not necessary
to screw the eye-bolt into the barrel for a distance greater than the
diameter of the bolt, but, in practice, it is better to screw it in
for a considerably greater distance than that if a reasonable degree of
safety is to be secured.

Now about turning the barrel to secure the right adjustment. The barrel
looks solid, but, as a matter of fact, it is hollow and much more frail
than it appears. For that reason it should not be turned by seizing it
with pliers, as that may distort it and spoil the bore within it. The
best method is to pass a piece of wire through the hole in its centre,
and to use that as a lever. When the correct adjustment has been
secured, the turnbuckle must be locked to prevent it from unscrewing. It
is quite possible to lock it in such a way as to allow it to unscrew a
quarter or a half turn, and that would throw the wires out of the very
fine adjustment necessary. The proper way is to use the locking wire so
that its direction is such as to oppose the tendency of the barrel to
unscrew, thus:


WIRES.--The following points should be carefully observed where wire is
concerned:

1. Quality.--It must not be too hard or too soft. An easy practical way
of learning to know the approximate quality of wire is as follows:

Take three pieces, all of the same gauge, and each about a foot in
length. One piece should be too soft, another too hard, and the third
piece of the right quality. Fix them in a vice, about an inch apart and
in a vertical position, and with the light from a window shining upon
them. Burnish them if necessary, and you will see a band of light
reflected from each wire.

Now bend the wires over as far as possible and away from the light.
Where the soft wire is concerned, it will squash out at the bend, and
this will be indicated by the band of light, which will broaden at that
point. In the case of the wire which is too hard, the band of light will
broaden very little at the turn, but, if you look carefully, you will
see some little roughnesses of surface. In the case of the wire of the
right quality, the band of light may broaden a very little at the turn,
but there will be no roughnesses of surface.

By making this experiment two or three times one can soon learn to know
really bad wire from good, and also learn to know the strength of hand
necessary to bend the right quality.

2. It must not be damaged. That is to say, it must be unkinked,
rustless, and unscored.

3. Now as regards keeping wire in good condition. Where outside wires
are concerned, they should be kept WELL GREASED OR OILED, especially
where bent over at the ends. Internal bracing wires cannot be reached
for the purpose of regreasing them, as they are inside fabric-covered
surfaces. They should be prevented from rusting by being painted with
an anti-rust mixture. Great care should be taken to see that the wire
is perfectly clean and dry before being painted. A greasy finger-mark is
sufficient to stop the paint from sticking to the wire. In such a case
there will be a little space between the paint and the wire. Air may
enter there and cause the wire to rust.

4. Tension of Wires.--The tension to which the wires are adjusted is
of the greatest importance. All the wires should be of the same tension
when the aeroplane is supported in such a way as to throw no stress upon
them. If some wires are in greater tension than others, the aeroplane
will quickly become distorted and lose its efficiency.

In order to secure the same tension of all wires, the aeroplane, when
being rigged, should be supported by packing underneath the lower
surfaces as well as by packing underneath the fuselage or nacelle. In
this way the anti-lift wires are relieved of the weight, and there is no
stress upon any of the wires.

As a general rule the wires of an aeroplane are tensioned too much. The
tension should be sufficient to keep the framework rigid. Anything
more than that lowers the factor of safety, throws various parts of the
framework into undue compression, pulls the fittings into the wood, and
will, in the end, distort the whole framework of the aeroplane.

Only experience will teach the rigger what tension to employ. Much may
be done by learning the construction of the various types of aeroplanes,
the work the various parts do, and in cultivating a touch for tensioning
wires by constantly handling them.

5. Wires with no Opposition Wires.--In some few cases wires will be
found which have no opposition wires pulling in the opposite direction.
For instance, an auxiliary lift wire may run from the bottom of a strut
to a spar in the top plane at a point between struts. In such a case
great care should be taken not to tighten the wire beyond barely taking
up the slack.

Such a wire must be a little slack, or, as illustrated above, it will
distort the framework. That, in the example given, will spoil the camber
(curvature) of the surface, and result in changing both the lift and
the drift at that part of the surface. Such a condition will cause the
aeroplane to lose its directional stability and also to fly one wing
down.

I cannot impress this matter of tension upon the reader too strongly.
It is of the utmost importance. When this, and also accuracy in securing
the various adjustments, has been learned, one is on the way to becoming
a good rigger.

6. Wire Loops.--Wire is often bent over at its end in the form of a
loop, in order to connect with a turnbuckle or fitting. These loops,
even when made as perfectly as possible, have a tendency to elongate,
thus spoiling the adjustment of the wires Great care should be taken
to minimize this as far as possible. The rules to be observed are as
follows:

(a) The size of the loop should be as small as possible within reason.
By that I mean it should not be so small as to create the possibility of
the wire breaking.


(b) The shape of the loop should be symmetrical.


(c) It should have well-defined shoulders in order to prevent the
ferrule from slipping up. At the same time, a shoulder should not have
an angular place.


(d) When the loop is finished it should be undamaged, and it should not
be, as is often the case, badly scored.


7. Stranded Wire Cable.--No splice should be served with twine until it
has been inspected by whoever is in charge of the workshop. The serving
may cover bad work.

Should a strand become broken, then the cable should be replaced at once
by another one.

Control cables have a way of wearing out and fraying wherever they pass
round pulleys. Every time an aeroplane comes down from flight the rigger
should carefully examine the cables, especially where they pass round
pulleys. If he finds a strand broken, he should replace the cable.

The ailerons' balance cable on the top of the top plane is often
forgotten, since it is necessary to fetch a high pair of steps in
order to examine it. Don't slack this, or some gusty day the pilot may
unexpectedly find himself minus the aileron control.

CONTROLLING SURFACES.--The greatest care should be exercised in rigging
the aileron, rudder, and elevator properly, for the pilot entirely
depends upon them in managing the aeroplane.

The ailerons and elevator should be rigged so that, when the aeroplane
is in flight, they are in a fair true line with the surface in front and
to which they are hinged.

If the surface to which they are hinged is not a lifting surface, then
they should be rigged to be in a fair true line with it as illustrated
above.

If the controlling surface is, as illustrated, hinged to the back of a
lifting surface, then it should be rigged a little below the position it
would occupy if in a fair true line with the surface in front. This is
because, in such a case, it is set at an angle of incidence. This angle
will, during flight, cause it to lift a little above the position in
which it has been rigged. It is able to lift owing to a certain amount
of slack in the control wire holding it--and one cannot adjust the
control wire to have no slack, because that would cause it to bind
against the pulleys and make the operation of it too hard for the pilot.
It is therefore necessary to rig it a little below the position it would
occupy if it was rigged in a fair true line with the surface in front.
Remember that this only applies when it is hinged to a lifting surface.
The greater the angle of incidence (and therefore the lift) of the
surface in front, then the more the controlling surface will have to be
rigged down.

As a general rule it is safe to rig it down so that its trailing

edge is 1/2 to 3/4 inch below the position it would occupy if in a fair
line with the surface in front; or about 1/2 inch down for every 18
inches of chord of the controlling surface.

When making these adjustments the pilot's control levers should be in
their neutral positions. It is not sufficient to lash them. They should
be rigidly blocked into position with wood packing.

The surfaces must not be distorted in any way. If they are held true by
bracing wires, then such wires must be carefully adjusted. If they are
distorted and there are no bracing wires with which to true them, then
some of the internal framework will probably have to be replaced.

The controlling surfaces should never be adjusted with a view to
altering the stability of the aeroplane. Nothing can be accomplished in
that way. The only result will be to spoil the control of the aeroplane.


FABRIC-COVERED SURFACES.--First of all make sure that there is no
distortion of spars or ribs, and that they are perfectly sound. Then
adjust the internal bracing wires so that the ribs are parallel to the
direction of flight. The ribs usually cause the fabric to make a ridge
where they occur, and, if such ridge is not parallel to the direction of
flight, it will produce excessive drift. As a rule the ribs are at right
angles to both main and rear spars.

The tension of the internal bracing wires should be just sufficient to
give rigidity to the framework. They should not be tensioned above that
unless the wires are, at their ends, bent to form loops. In that case a
little extra tension may be given to offset the probable elongation of
the loops.

The turnbuckles must now be generously greased, and served round with
adhesive tape. The wires must be rendered perfectly dry and clean,
and then painted with an anti-rust mixture. The woodwork must be well
varnished.

If it is necessary to bore holes in the spars for the purpose of
receiving, for instance, socket bolts, then their places should be
marked before being bored and their positions confirmed by whoever is in
charge of the workshop. All is now ready for the sail-maker to cover the
surface with fabric.


ADJUSTMENT OF CONTROL CABLES.--The adjustment of the control cables is
quite an art, and upon it will depend to a large degree the quick and
easy control of the aeroplane by the pilot.

The method is as follows:

After having rigged the controlling surfaces, and as far as possible
secured the correct adjustment of the control cables, then remove the
packing which has kept the control levers rigid. Then, sitting in the
pilot's seat, move the control levers SMARTLY. Tension the control
cables so that when the levers are smartly moved there is no perceptible
snatch or lag. Be careful not to tension the cables more than necessary
to take out the snatch. If tensioned too much they will (1) bind round
the pulleys and result in hard work for the pilot; (2) throw dangerous
stresses upon the controlling surfaces, which are of rather flimsy
construction; and (3) cause the cables to fray round the pulleys quicker
than would otherwise be the case.

Now, after having tensioned the cables sufficiently to take out the
snatch, place the levers in their neutral positions, and move them to
and fro about 1/8 inch either side of such positions. If the adjustment
is correct, it should be possible to see the controlling surfaces move.
If they do not move, then the control cables are too slack.


FLYING POSITION.--Before rigging an aeroplane or making any adjustments
it is necessary to place it in what is known as its "flying position." I
may add that it would be better termed its "rigging position."

In the case of an aeroplane fitted with a stationary engine this is
secured by packing up the machine so that the engine foundations are
perfectly horizontal both longitudinally and laterally. This position
is found by placing a straight-edge and a spirit-level across the engine
foundations (both longitudinally and laterally), and great care should
be taken to see that the bubble is exactly in the centre of the level.
The slightest error will assume magnitude towards the extremities of the
aeroplane. Great care should be taken to block up the aeroplane rigidly.
In case it gets accidentally disturbed while the work is going on, it
is well to constantly verify the flying position by running the
straight-edge and spirit-level over the engine foundations. The
straight-edge should be carefully tested before being used, as, being
generally made of wood, it will not remain true long. Place it lightly
in a vice, and in such a position that a spirit-level on top shows
the bubble exactly in the centre. Now slowly move the level along the
straight-edge, and the bubble should remain exactly in the centre. If
it does not do so, then the straight-edge is not true and must be
corrected. THIS SHOULD NEVER BE OMITTED.

In the case of aeroplanes fitted with engines of the rotary type, the
"flying position" is some special attitude laid down in the aeroplane's
specifications, and great care should be taken to secure accuracy.


ANGLE OF INCIDENCE.--One method of finding the angle of incidence is as
follows:

First place the aeroplane in its flying position. The corner of the
straight-edge must be placed underneath and against the CENTRE of the
rear spar, and held in a horizontal position parallel to the ribs. This
is secured by using a spirit-level. The set measurement will then be
from the top of the straight-edge to the centre of the bottom surface
of the main spar, or it may be from the top of the straight-edge to the
lowest part of the leading edge. Care should be taken to measure from
the centre of the spar and to see that the bubble is exactly in the
centre of the level. Remember that all this will be useless if the
aeroplane has not been placed accurately in its flying position.

This method of finding the angle of incidence must be used under every
part of the lower surface where struts occur. It should not be used
between the struts, because, in such places, the spars may have taken a
slight permanent set up or down; not, perhaps, sufficiently bad to make
any material difference to the flying of the machine, but quite bad
enough to throw out the angle of incidence, which cannot be corrected at
such a place.

If the angle is wrong, it should then be corrected as follows:

If it is too great, then the rear spar must be warped up until it is
right, and this is done by slackening ALL the wires going to the top of
the strut, and then tightening ALL the wires going to the bottom of the
strut.

If the angle is too small, then slacken ALL the wires going to the
bottom of the strut, and tighten ALL the wires going to the top of the
strut, until the correct adjustment is secured.

Never attempt to adjust the angle by warping the main spar.

The set measurement, which is of course stated in the aeroplane's
specifications, should be accurate to 1/16 inch.


LATERAL DIHEDRAL ANGLE.--One method of securing this is as follows,
and this method will, at the same time, secure the correct angle of
incidence:

The strings, drawn very tight, must be taken over both the main and rear
spars of the top surface. They must run between points on the spars just
inside the outer struts. The set measurement (which should be accurate
to 1/16 inch or less) is then from the strings down to four points on
the main and rear spars of the centre-section surface. These points
should be just inside the four centre-section struts; that is to say,
as far as possible away from the centre of the centre-section. Do
not attempt to take the set measurement near the centre of the
centre-section.

The strings should be as tight as possible, and, if it can be arranged,
the best way to accomplish that is as shown in the above illustration,
i.e., by weighting the strings down to the spars by means of weights
and tying each end of the strings to a strut. This will give a tight and
motionless string.

However carefully the above adjustment is made, there is sure to be some
slight error. This is of no great importance, provided it is divided
equally between the left- and right-hand wings. In order to make sure of
this, certain check measurements should be taken as follows:

Each bay must be diagonally measured, and such measurements must be the
same to within 1/16 inch on each side of the aeroplane. As a rule such
diagonal measurements are taken from the bottom socket of one strut to
the top socket of another strut, but this is bad practice, because of
possible inaccuracies due to faulty manufacture.

The points between which the diagonal measurements are taken should be
at fixed distances from the butts of the spars, such distances being the
same on each side of the aeroplane, thus:


It would be better to use the centre line of the aeroplane rather than
the butts of the spars. It is not practicable to do so, however, as the
centre line probably runs through the petrol tanks, etc.


THE DIHEDRAL BOARD.--Another method of securing the dihedral angle, and
also the angle of incidence, is by means of the dihedral board. It is
a light handy thing to use, but leads to many errors, and should not be
used unless necessary. The reasons are as follows:

The dihedral board is probably not true. If it must be used, then it
should be very carefully tested for truth before-hand. Another reason
against its use is that it has to be placed on the spars in a position
between the struts, and that is just where the spars may have a little
permanent set up or down, or some inaccuracy of surface which will, of
course, throw out the accuracy of the adjustment. The method of using it
is as follows:

The board is cut to the same angle as that specified for the upward
inclination of the surface towards its wing-tips. It is placed on the
spar as indicated above, and it is provided with two short legs to raise
it above the flanges of the ribs (which cross over the spars), as they
may vary in depth. A spirit-level is then placed on the board, and the
wires must be adjusted to give the surface such an inclination as to
result in the bubble being in the centre of the level. This operation
must be performed in respect of each bay both front and rear. The bays
must then be diagonally measured as already explained.


YET ANOTHER METHOD of finding the dihedral angle, and at the same time
the angle of incidence, is as follows:

A horizontal line is taken from underneath the butt of each spar, and
the set measurement is either the angle it makes with the spar, or
a fixed measurement from the line to the spar taken at a specified
distance from the butt. This operation must be performed in respect of
both main and rear spars, and all the bays must be measured diagonally
afterwards.



Whichever method is used, be sure that after the job is done the spars
are perfectly straight.


STAGGER.--The stagger is the distance the top surface is in advance of
the bottom surface when the aeroplane is in flying position. The set
measurement is obtained as follows:

Plumb-lines must be dropped over the leading edge of the top surface
wherever struts occur, and also near the fuselage. The set measurement
is taken from the front of the lower leading edge to the plumb-lines. It
makes a difference whether the measurement is taken along a horizontal
line (which can be found by using a straight-edge and a spirit-level)
or along a projection of the chord. The line along which the measurement
should be taken is laid down in the aeroplane's specifications.

If a mistake is made and the measurement taken along the wrong line, it
may result in a difference of perhaps 1/4 will, in flight, be nose-heavy
or tail-heavy.

After the adjustments of the angles of incidence, dihedral, and stagger
have been secured, it is as well to confirm all of them, as, in making
the last adjustment, the first one may have been spoiled.


OVER-ALL ADJUSTMENTS.--The following over-all check measurements should
now be taken.

The straight lines AC and BC should be equal to within 1/8 inch. The
point C is the centre of the propeller, or, in the case of a "pusher"
aeroplane, the centre of the nacelle. The points A and B are marked on
the main spar, and must in each case be the same distance from the butt
of the spar. The rigger should not attempt to make A and B merely the
sockets of the outer struts, as they may not have been placed quite
accurately by the manufacturer. The lines AC and BC must be taken
from both top and bottom spars--two measurements on each side of the
aeroplane.

The two measurements FD and FE should be equal to within 1/8 inch. F is
the centre of the fuselage or rudder-post. D and E are points marked on
both top and bottom rear spars, and each must be the same fixed
distance from the butt of the spar. Two measurements on each side of the
aeroplane.

If these over-all measurements are not correct, then it is probably due
to some of the drift or anti-drift wires being too tight or too slack.
It may possibly be due to the fuselage being out of truth, but of course
the rigger should have made quite sure that the fuselage was true before
rigging the rest of the machine. Again, it may be due to the internal
bracing wires within the lifting surfaces not being accurately adjusted,
but of course this should have been seen to before covering the surfaces
with fabric.


FUSELAGE.--The method of truing the fuselage is laid down in the
aeroplane's specifications. After it has been adjusted according to the
specified directions, it should then be arranged on trestles in such
a way as to make about three-quarters of it towards the tail stick out
unsupported. In this way it will assume a condition as near as
possible to flying conditions, and when it is in this position the set
measurements should be confirmed. If this is not done it may be out of
truth, but perhaps appear all right when supported by trestles at both
ends, as, in such case, its weight may keep it true as long as it is
resting upon the trestles.


THE TAIL-PLANE (EMPENNAGE).--The exact angle of incidence of the
tail-plane is laid down in the aeroplane's specifications. It is
necessary to make sure that the spars are horizontal when the aeroplane
is in flying position and the tail unsupported as explained above under
the heading of Fuselage. If the spars are tapered, then make sure that
their centre lines are horizontal.


UNDERCARRIAGE.--The undercarriage must be very carefully aligned as laid
down in the specifications.

1. The aeroplane must be placed in its flying position and sufficiently
high to ensure the wheels being off the ground when rigged. When in this
position the axle must be horizontal and the bracing wires adjusted to
secure the various set measurements stated in the specifications.

2. Make sure that the struts bed well down into their sockets.

3. Make sure that the shock absorbers are of equal tension. In the case
of rubber shock absorbers, both the number of turns and the lengths must
be equal.


HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL.


DIRECTIONAL STABILITY will be badly affected if there is more drift
(i.e., resistance) on one side of the aeroplane than there is on the
other side. The aeroplane will tend to turn towards the side having the
most drift. This may be caused as follows:

1. The angle of incidence of the main surface or the tail surface may
be wrong. The greater the angle of incidence, the greater the drift. The
less the angle, the less the drift.

2. If the alignment of the fuselage, fin in front of the rudder, the
struts or stream-line wires, or, in the case of the Maurice Farman, the
front outriggers, are not absolutely correct--that is to say, if they
are turned a little to the left or to the right instead of being in line
with the direction of flight--then they will act as a rudder and cause
the aeroplane to turn off its course.

3. If any part of the surface is distorted, it will cause the aeroplane
to turn off its course. The surface is cambered, i.e., curved, to pass
through the air with the least possible drift. If, owing perhaps to the
leading edge, spars, or trailing edge becoming bent, the curvature is
spoiled, that will result in changing the amount of drift on one side of
the aeroplane, which will then have a tendency to turn off its course.


LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible reason
for such a condition is a difference in the lifts of right and left
wings. That may be caused as follows:

1. The angle of incidence may be wrong. If it is too great, it will
produce more lift than on the other side of the aeroplane; and if too
small, it will produce less lift than on the other side--the result
being that, in either case, the aeroplane will try to fly one wing down.

2. Distorted Surfaces.--If some part of the surface is distorted, then
its camber is spoiled, and the lift will not be the same on both sides
of the aeroplane, and that, of course, will cause it to fly one wing
down.


LONGITUDINAL INSTABILITY may be due to the following reasons:

1. The stagger may be wrong. The top surface may have drifted back a
little owing to some of the wires, probably the incidence wires, having
elongated their loops or having pulled the fittings into the wood. If
the top surface is not staggered forward to the correct degree, then
consequently the whole of its lift is too far back, and it will then
have a tendency to lift up the tail of the machine too much. The
aeroplane would then be said to be "nose-heavy."

A 1/4-inch area in the stagger will make a very considerable difference
to the longitudinal stability.

2. If the angle of incidence of the main surface is not right, it will
have a bad effect, especially in the case of an aeroplane with a lifting
tail-plane.

If the angle is too great, it will produce an excess of lift, and that
may lift up the nose of the aeroplane and result in a tendency to fly
"tail-down." If the angle is too small, it will produce a decreased
lift, and the aeroplane may have a tendency to fly "nose-down."

3. The fuselage may have become warped upward or downward, thus giving
the tail-plane an incorrect angle of incidence. If it has too much
angle, it will lift too much, and the aeroplane will be "nose-heavy." If
it has too little angle, then it will not lift enough, and the aeroplane
will be "tail-heavy."

4. (The least likely reason.) The tail-plane may be mounted upon
the fuselage at a wrong angle of incidence, in which case it must
be corrected. If nose-heavy, it should be given a smaller angle of
incidence. If tail-heavy, it should be given a larger angle; but
care should be taken not to give it too great an angle, because the
longitudinal stability entirely depends upon the tail-plane being set at
a much smaller angle of incidence than is the main surface, and if
that difference is decreased too much, the aeroplane will become
uncontrollable longitudinally. Sometimes the tail-plane is mounted on
the aeroplane at the same angle as the main surface, but it actually
engages the air at a lesser angle, owing to the air being deflected
downwards by the main surface. There is then, in effect, a longitudinal
dihedral as explained and illustrated in Chapter I.


CLIMBS BADLY.--Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, or (2) too small an
angle of incidence.


FLIGHT SPEED POOR.--Such a condition is, apart from engine or propeller
trouble, probably due to (1) distorted surfaces, (2) too great an
angle of incidence, or (3) dirt or mud, and consequently excessive
skin-friction.


INEFFICIENT CONTROL is probably due to (1) wrong setting of control
surfaces, (2) distortion of control surfaces, or (3) control cables
being badly tensioned.


WILL NOT TAXI STRAIGHT.--If the aeroplane is uncontrollable on the
ground, it is probably due to (1) alignment of undercarriage being
wrong, or (2) unequal tension of shock absorbers.



CHAPTER IV. THE PROPELLER, OR "AIR-SCREW"

The sole object of the propeller is to translate the power of the engine
into thrust.

The propeller screws through the air, and its blades, being set at an
angle inclined to the direction of motion, secure a reaction, as in the
case of the aeroplane's lifting surface.

This reaction may be conveniently divided into two component parts or
values, namely, Thrust and Drift.

The Thrust is opposed to the Drift of the aeroplane, and must be equal
and opposite to it at flying speed. If it falls off in power, then the
flying speed must decrease to a velocity, at which the aeroplane drift
equals the decreased thrust.

The Drift of the propeller may be conveniently divided into the
following component values:


Active Drift, produced by the useful thrusting part of the propeller.


Passive Drift, produced by all the rest of the propeller, i.e., by its
detrimental surface.


Skin Friction, produced by the friction of the air with roughnesses of
surface.


Eddies attending the movement of the air caused by the action of the
propeller.


Cavitation (very marked at excessive speed of revolution). A tendency of
the propeller to produce a cavity or semi-vacuum in which it revolves,
the thrust decreasing with increase of speed and cavitation.


THRUST-DRIFT RATIO.--The proportion of thrust to drift is of paramount
importance, for it expresses the efficiency of the propeller. It is
affected by the following factors: Speed of Revolution.--The greater the
speed, the greater the proportion of drift to thrust. This is due to
the increase with speed of the passive drift, which carries with it no
increase in thrust. For this reason propellers are often geared down to
revolve at a lower speed than that of the engine.


Angle of Incidence.--The same reasons as in the case of the aeroplane
surface.

Surface Area.--Ditto.

Aspect Ratio.--Ditto.

Camber.--Ditto.

In addition to the above factors there are, when it comes to actually
designing a propeller, mechanical difficulties to consider. For
instance, the blades must be of a certain strength and consequent
thickness. That, in itself, limits the aspect ratio, for it will
necessitate a chord long enough in proportion to the thickness to make
a good camber possible. Again, the diameter of the propeller must be
limited, having regard to the fact that greater diameters than those
used to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage to keep the
propeller off the ground, and such undercarriage would not only produce
excessive drift, but would also tend to make the aeroplane stand on
its nose when alighting. The latter difficulty cannot be overcome by
mounting the propeller higher, as the centre of its thrust must be
approximately coincident with the centre of aeroplane drift.


MAINTENANCE OF EFFICIENCY.


The following conditions must be observed:


1. PITCH ANGLE.--The angle, at any given point on the propeller, at
which the blade is set is known as the pitch angle, and it must be
correct to half a degree if reasonable efficiency is to be maintained.

This angle secures the "pitch," which is the distance the propeller
advances during one revolution, supposing the air to be solid. The air,
as a matter of fact, gives back to the thrust of the blades just as the
pebbles slip back as one ascends a shingle beach. Such "give-back" is
known as Slip. If a propeller has a pitch of, say, 10 feet, but actually
advances, say, only 8 feet owing to slip, then it will be said to
possess 20 per cent. slip.

Thus, the pitch must equal the flying speed of the aeroplane plus
the slip of the propeller. For example, let us find the pitch of a
propeller, given the following conditions:

          Flying speed.............. 70 miles per hour.
          Propeller revolutions..... 1,200 per minute.
          Slip...................... 15 per cent.

First find the distance in feet the aeroplane will travel forward in one
minute. That is--

   369,600 feet (70 miles)
   ------------------------ = 6,160 feet per minute.
      60     " (minutes)


Now divide the feet per minute by the propeller revolutions per minute,
add 15 per cent. for the slip, and the result will be the propeller
pitch:

     6,160
     ----- + 15 per cent. = 5 feet 1 3/5 inches.
     1,200


In order to secure a constant pitch from root to tip of blade, the pitch
angle decreases towards the tip. This is necessary, since the end of the
blade travels faster than its root, and yet must advance forward at the
same speed as the rest of the propeller. For example, two men ascending
a hill. One prefers to walk fast and the other slowly, but they wish to
arrive at the top of the hill simultaneously. Then the fast walker
must travel a farther distance than the slow one, and his angle of path
(pitch angle) must be smaller than the angle of path taken by the slow
walker. Their pitch angles are different, but their pitch (in this case
altitude reached in a given time) is the same.

In order to test the pitch angle, the propeller must be mounted upon
a shaft at right angles to a beam the face of which must be perfectly
level, thus:

First select a point on the blade at some distance (say about 2 feet)
from the centre of the propeller. At that point find, by means of a
protractor, the angle a projection of the chord makes with the face of
the beam. That angle is the pitch angle of the blade at that point.

Now lay out the angle on paper, thus:

The line above and parallel to the circumference line must be placed
in a position making the distance between the two lines equal to the
specified pitch, which is, or should be, marked upon the boss of the
propeller.

Now find the circumference of the propeller where the pitch angle is
being tested. For example, if that place is 2 feet radius from the
centre, then the circumference will be 2 feet X 2 = 4 feet diameter,
which, if multiplied by 3.1416 = 15.56 feet circumference.

Now mark off the circumference distance, which is represented above by
A-B, and reduce it in scale for convenience.

The distance a vertical line makes between B and the chord dine is
the pitch at the point where the angle is being tested, and it should
coincide with the specified pitch. You will note, from the above
illustration, that the actual pitch line should meet the junction of the
chord line and top line.

The propeller should be tested at several points, about a foot apart, on
each blade; and the diagram, provided the propeller is not faulty, will
then look like this:

At each point tested the actual pitch coincides with the specified
pitch: a satisfactory condition.

A faulty propeller will produce a diagram something like this:


At every point tested the pitch angle is wrong, for nowhere does the
actual pitch coincide with the specified pitch. Angles A, C, and D, are
too large, and B is too small. The angle should be correct to half a
degree if reasonable efficiency is to be maintained.

A fault in the pitch angle may be due to (1) faulty manufacture,
(2) distortion, or (3) the shaft hole through the boss being out of
position.


2. STRAIGHTNESS.--To test for straightness the propeller must be mounted
upon a shaft. Now bring the tip of one blade round to graze some fixed
object. Mark the point it grazes. Now bring the other tip round, and it
should come within 1/8 inch of the mark. If it does not do so, it is due
to (1) faulty manufacture, (2) distortion, or (3) to the hole through
the boss being out of position.


3. LENGTH.--The blades should be of equal length to inch.


4. BALANCE.--The usual method of testing a propeller for balance is as
follows: Mount it upon a shaft, which must be on ball-bearings. Place
the propeller in a horizontal position, and it should remain in that
position. If a weight of a trifle over an ounce placed in a bolt-hole on
one side of the boss fails to disturb the balance, then the propeller is
usually regarded as unfit for use.

The above method is rather futile, as it does not test for the balance
of centrifugal force, which comes into play as soon as the propeller
revolves. It can be tested as follows:


The propeller must be in a horizontal position, and then weighed at
fixed points, such as A, B, C, D, E, and F, and the weights noted. The
points A, B, and C must, of course, be at the same fixed distances from
the centre of the propeller as the points D, E, and F. Now reverse the
propeller and weigh at each point again. Note the results. The first
series of weights should correspond to the second series, thus:

      Weight A should equal weight F.
         "  B   "     "     "  E.
         "  C   "     "     "  D.


There is no standard practice as to the degree of error permissible, but
if there are any appreciable differences the propeller is unfit for use.


5. SURFACE AREA.--The surface area of the blades should be equal. Test
with callipers thus:

The points between which the distances are taken must, of course, be at
the same distance from the centre in the case of each blade.

There is no standard practice as to the degree of error permissible.
If, however, there is an error of over 1/8 inch, the propeller is really
unfit for use.


6. CAMBER.--The camber (curvature) of the blades should be (1) equal,
(2) decrease evenly towards the tips of the blades, and (3) the greatest
depth of the curve should, at any point of the blade, be approximately
at the same percentage of the chord from the leading edge as at other
points.

It is difficult to test the top camber without a set of templates, but
a fairly accurate idea of the concave camber can be secured by slowly
passing a straight-edge along the blade, thus:

The camber can now be easily seen, and as the straight-edge is passed
along the blade, the observer should look for any irregularities of the
curvature, which should gradually and evenly decrease towards the tip of
the blade.


7. THE JOINTS.--The usual method for testing the glued joints is by
revolving the propeller at greater speed than it will be called upon to
make during flight, and then carefully examining the joints to see if
they have opened. It is not likely, however, that the reader will have
the opportunity of making this test. He should, however, examine all the
joints very carefully, trying by hand to see if they are quite sound.
Suspect a propeller of which the joints appear to hold any thickness of
glue. Sometimes the joints in the boss open a little, but this is not
dangerous unless they extend to the blades, as the bolts will hold the
laminations together.


8. CONDITION OF SURFACE.--The surface should be very smooth, especially
towards the tips of the blades. Some propeller tips have a speed of
over 30,000 feet a minute, and any roughness will produce a bad drift or
resistance and lower the efficiency.


9. MOUNTING.--Great care should be taken to see that the propeller
is mounted quite straight on its shaft. Test in the same way as for
straightness. If it is not straight, it is possibly due to some of the
propeller bolts being too slack or to others having been pulled up too
tightly.


FLUTTER.--Propeller "flutter," or vibration, may be due to faulty pitch
angle, balance, camber, or surface area. It causes a condition sometimes
mistaken for engine trouble, and one which may easily lead to the
collapse of the propeller.


CARE OF PROPELLERS.--The care of propellers is of the greatest
importance, as they become distorted very easily.


1. Do not store them in a very damp or a very dry place.


2. Do not store them where the sun will shine upon them.


3. Never leave them long in a horizontal position or leaning up against
a wall.


4. They should be hung on horizontal pegs, and the position of the
propellers should be vertical.


If the points I have impressed upon you in these notes are not attended
to, you may be sure of the following results:


1. Lack of efficiency, resulting in less aeroplane speed and climb than
would otherwise be the case.


2. Propeller "flutter" and possible collapse.


3. A bad stress upon the propeller shaft and its bearings.


TRACTOR.--A propeller mounted in front of the main surface.


PUSHER.--A propeller mounted behind the main surface.


FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when
the pitch is comparatively large.

For a given pitch, and having regard to "interference," they are not so
efficient as two-bladed propellers.

The smaller the pitch, the less the "gap," i.e., the distance, measured
in the direction of the thrust, between the spiral courses of the
blades.

If the gap is too small, then the following blade will engage air
which the preceding blade has put into motion, with the result that the
following blade will not secure as good a reaction as would otherwise be
the case. It is very much the same as in the case of the aeroplane gap.

For a given pitch, the gap of a four-bladed propeller is only half
that of a two-bladed one. Therefore the four-bladed propeller is only
suitable for large pitch, as such pitch produces spirals with a large
gap, thus offsetting the decrease in gap caused by the numerous blades.

The greater the speed of rotation, the less the pitch for a given
aeroplane speed. Then, in order to secure a large pitch and consequently
a good gap, the four-bladed propeller is usually geared to rotate at a
lower speed than would be the case if directly attached to the engine
crank-shaft.



CHAPTER V. MAINTENANCE

CLEANLINESS.--The fabric must be kept clean and free from oil, as that
will rot it. To take out dirt or oily patches, try acetone. If that will
not remedy matters, then try petrol, but use it sparingly, as otherwise
it will take off an unnecessary amount of dope. If that will not remove
the dirt, then hot water and soap will do so, but, in that case, be
sure to use soap having no alkali in it, as otherwise it may injure the
fabric. Use the water sparingly, or it may get inside the planes and
rust the internal bracing wires, or cause some of the wooden framework
to swell.

The wheels of the undercarriage have a way of throwing up mud on to
the lower surface. This should, if possible, be taken off while wet. It
should never be scraped off when dry, as that may injure the fabric. If
dry, then it should be moistened before being removed.

Measures should be taken to prevent dirt from collecting upon any
part of the aeroplane, as, otherwise, excessive skin-friction will be
produced with resultant loss of flight speed. The wires, being greasy,
collect dirt very easily.


CONTROL CABLES.--After every flight the rigger should pass his hand over
the control cables and carefully examine them near pulleys. Removal of
grease may be necessary to make a close inspection possible. If only one
strand is broken the wire should be replaced. Do not forget the aileron
balance wire on the top surface.

Once a day try the tension of the control cables by smartly moving the
control levers about as explained elsewhere.


WIRES.--All the wires should be kept well greased or oiled, and in the
correct tension. When examining the wires, it is necessary to place the
aeroplane on level ground, as otherwise it may be twisted, thus throwing
some wires into undue tension and slackening others. The best way, if
there is time, is to pack the machine up into its "flying position."

If you see a slack wire, do not jump to the conclusion that it must
be tensioned. Perhaps its opposition wire is too tight, in which case
slacken it, and possibly you will find that will tighten the slack wire.

Carefully examine all wires and their connections near the propeller,
and be sure that they are snaked round with safety wire, so that the
latter may keep them out of the way of the propeller if they come
adrift.

The wires inside the fuselage should be cleaned and regreased about once
a fortnight.


STRUTS AND SOCKETS.--These should be carefully examined to see if any
splitting has occurred.


DISTORTION.--Carefully examine all surfaces, including the controlling
surfaces, to see whether any distortion has occurred. If distortion can
be corrected by the adjustment of wires, well and good; but if not, then
some of the internal framework probably requires replacement.


ADJUSTMENTS.--Verify the angles of incidence; dihedral, and stagger, and
the rigging position of the controlling-surfaces, as often as possible.


UNDERCARRIAGE.--Constantly examine the alignment and fittings of the
undercarriage, and the condition of tyres and shock absorbers. The
latter, when made of rubber, wear quickest underneath. Inspect axles and
skids to see if there are any signs of them becoming bent. The wheels
should be taken off occasionally and greased.


LOCKING ARRANGEMENTS.--Constantly inspect the locking arrangements of
turnbuckles, bolts, etc. Pay particular attention to the control cable
connections, and to all moving parts in respect of the controls.


LUBRICATION.--Keep all moving parts, such as pulleys, control levers,
and hinges of controlling surfaces, well greased.


SPECIAL INSPECTION.--Apart from constantly examining the aeroplane with
reference to the above points I have made, I think that, in the case of
an aeroplane in constant use it is an excellent thing to make a special
inspection of every part, say once a week. This will take from two to
three hours, according to the type of aeroplane. In order to carry it
out methodically, the rigger should have a list of every part down to
the smallest split-pin. He can then check the parts as he examines them,
and nothing will be passed over. This, I know from experience, greatly
increases the confidence of the pilot, and tends to produce good work in
the air.


WINDY WEATHER.--The aeroplane, when on the ground, should face the
wind; and it is advisable to lash the control lever fast, so that the
controlling surfaces may not be blown about and possibly damaged.


"VETTING" BY EYE.--This should be practiced at every opportunity, and,
if persevered in, it is possible to become quite expert in diagnosing by
eye faults in flight efficiency, stability and control.

The aeroplane should be standing upon level ground, or, better than
that, packed up into its "flying position."

Now stand in front of it and line up the leading edge with the main
spar, rear spar, and trailing edge. Their shadows can usually be seen
through the fabric. Allowance must, of course, be made for wash-in and
wash-out; otherwise, the parts I have specified should be parallel with
each other.

Now line up the centre part of the main-plane with the tail-plane. The
latter should be horizontal.

Next, sight each interplane front strut with its rear strut. They should
be parallel.

Then, standing on one side of the aeroplane, sight all the front struts.
The one nearest to you should cover all the others. This applies to the
rear struts also.

Look for distortion of leading edges, main and rear spars, trailing
edges, tail-plane and controlling surfaces.

This sort of thing, if practiced constantly, will not only develop an
expert eye for diagnosis of faults, but will also greatly assist in
impressing upon the memory the characteristics and possible troubles of
the various types of aeroplanes.


MISHANDLING OF THE GROUND.--This is the cause of a lot of unnecessary
damage. The golden rule to observe is: PRODUCE NO BENDING STRESSES.

Nearly all the wood in an aeroplane is designed to take merely the
stress of direct compression, and it cannot be bent safely. Therefore,
in packing an aeroplane up from the ground, or in pulling or pushing it
about, be careful to stress it in such a way as to produce, as far
as possible, only direct compression stresses. For instance, if it is
necessary to support the lifting surface, then the packing should be
arranged to come directly under the struts so that they may take the
stress in the form of compression for which they are designed. Such
supports should be covered with soft packing in order to prevent the
fabric from becoming damaged.

When pulling an aeroplane along, if possible, pull from the top of the
undercarriage struts. If necessary to pull from elsewhere, then do so by
grasping the interplane struts as low down as possible.

Never lay fabric-covered parts upon a concrete floor. Any slight
movement will cause the fabric to scrape over the floor with resultant
damage.

Struts, spars, etc., should never be left about the floor, as in such
position they are likely to become scored. I have already explained the
importance of protecting the outside fibres of the wood. Remember
also that wood becomes distorted easily. This particularly applies to
interplane struts. If there are no proper racks to stand them in, then
the best plan is to lean them up against the wall in as near a vertical
position as possible.


TIME.--Learn to know the time necessary to complete any of the various
rigging jobs. This is really important. Ignorance of this will lead
to bitter disappointments in civil life; and, where Service flying
is concerned, it will, to say the least of it, earn unpopularity with
senior officers, and fail to develop respect and good work where men are
concerned.


THE AEROPLANE SHED.--This should be kept as clean and orderly as
possible. A clean, smart shed produces briskness, energy, and pride of
work. A dirty, disorderly shed nearly always produces slackness and poor
quality of work, lost tools and mislaid material.



GLOSSARY

Aeronautics--The science of aerial navigation.

Aerofoil--A rigid structure, of large superficial area relative to its
thickness, designed to obtain, when driven through the air at an
angle inclined to the direction of motion, a reaction from the air
approximately at right angles to its surface. Always cambered when
intended to secure a reaction in one direction only. As the term
"aerofoil" is hardly ever used in practical aeronautics, I have,
throughout this book, used the term SURFACE, which, while academically
incorrect, since it does not indicate thickness, is a term usually
used to describe the cambered lifting surfaces, i.e., the "planes" or
"wings," and the stabilizers and the controlling aerofoils.

Aerodrome--The name usually applied to a ground used for the practice
of aviation. It really means "flying machine," but is never used in that
sense nowadays.

Aeroplane--A power-driven aerofoil with stabilizing and controlling
surfaces.

Acceleration--The rate of change of velocity.

Angle of Incidence--The angle at which the "neutral lift line" of a
surface attacks the air.

Angle of Incidence, Rigger's--The angle the chord of a surface makes
with a line parallel to the axis of the propeller.

Angle of Incidence, Maximum--The greatest angle of incidence at which,
for a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

Angle of Incidence, Minimum--The smallest angle of incidence at which,
for a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

Angle of Incidence, Best Climbing--That angle of incidence at which an
aeroplane ascends quickest. An angle approximately halfway between the
maximum and optimum angles.

Angle of Incidence, Optimum--The angle of incidence at which the
lift-drift ratio is the highest.


Angle, Gliding--The angle between the horizontal and the path along
which an aeroplane at normal flying speed, but not under engine power,
descends in still air.

Angle, Dihedral--The angle between two planes.

Angle, Lateral Dihedral--The lifting surface of an aeroplane is said to
be at a lateral dihedral angle when it is inclined upward towards its
wing-tips.

Angle, Longitudinal Dihedral--The main surface and tail surface are said
to be at a longitudinal dihedral angle when the projections of their
neutral lift lines meet and produce an angle above them.

Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting "chords"
for "neutral life lines."

Angle, Pitch--The angle at any given point of a propeller, at which
the blade is inclined to the direction of motion when the propeller is
revolving but the aeroplane stationary.

Altimeter--An instrument used for measuring height.

Air-Speed Indicator--An instrument used for measuring air pressures or
velocities. It consequently indicates whether the surface is securing
the requisite reaction for flight. Usually calibrated in miles per hour,
in which case it indicates the correct number of miles per hour at only
one altitude. This is owing to the density of the air decreasing with
increase of altitude and necessitating a greater speed through space
to secure the same air pressure as would be secured by less speed at a
lower altitude. It would be more correct to calibrate it in units of air
pressure.

Air Pocket--A local movement or condition of the air causing an
aeroplane to drop or lose its correct attitude.

Aspect-Ratio--The proportion of span to chord of a surface.

Air-Screw (Propeller)--A surface so shaped that its rotation about an
axis produces a force (thrust) in the direction of its axis.

Aileron--A controlling surface, usually situated at the wing-tip, the
operation of which turns an aeroplane about its longitudinal axis;
causes an aeroplane to tilt sideways.


Aviation--The art of driving an aeroplane.

Aviator--The driver of an aeroplane.

Barograph--A recording barometer, the charts of which can be calibrated
for showing air density or height.

Barometer--An instrument used for indicating the density of air.

Bank, to--To turn an aeroplane about its longitudinal axis (to tilt
sideways) when turning to left or right.

Biplane--An aeroplane of which the main lifting surface consists of a
surface or pair of wings mounted above another surface or pair of wings.

Bay--The space enclosed by two struts and whatever they are fixed to.

Boom--A term usually applied to the long spars joining the tail of a
"pusher" aeroplane to its main lifting surface.

Bracing--A system of struts and tie wires to transfer a force from one
point to another.

Canard--Literally "duck." The name which was given to a type of
aeroplane of which the longitudinal stabilizing surface (empennage)
was mounted in front of the main lifting surface. Sometimes termed
"tail-first" aeroplanes, but such term is erroneous, as in such a design
the main lifting surface acts as, and is, the empennage.

Cabre--To fly or glide at an excessive angle of incidence; tail down.

Camber--Curvature.

Chord--Usually taken to be a straight line between the trailing and
leading edges of a surface.

Cell--The whole of the lower surface, that part of the upper surface
directly over it, together with the struts and wires holding them
together.

Centre (Line) of Pressure--A line running from wing-tip to wing-tip, and
through which all the air forces acting upon the surface may be said to
act, or about which they may be said to balance.

Centre (Line) of Pressure, Resultant--A line transverse to the
longitudinal axis, and the position of which is the resultant of the
centres of pressure of two or more surfaces.

Centre of Gravity--The centre of weight.

Cabane--A combination of two pylons, situated over the fuselage, and
from which anti-lift wires are suspended.

Cloche--Literally "bell." Is applied to the bell-shaped construction
which forms the lower part of the pilot's control lever in a Bleriot
monoplane, and to which the control cables are attached.

Centrifugal Force--Every body which moves in a curved path is urged
outwards from the centre of the curve by a force termed "centrifugal."

Control Lever--A lever by means of which the controlling surfaces
are operated. It usually operates the ailerons and elevator. The
"joy-stick".

Cavitation, Propeller--The tendency to produce a cavity in the air.

Distance Piece--A long, thin piece of wood (sometimes tape) passing
through and attached to all the ribs in order to prevent them from
rolling over sideways.

Displacement--Change of position.

Drift (of an aeroplane as distinct from the propeller)--The horizontal
component of the reaction produced by the action of driving through the
air a surface inclined upwards and towards its direction of motion PLUS
the horizontal component of the reaction produced by the "detrimental"
surface PLUS resistance due to "skin-friction." Sometimes termed
"head-resistance."

Drift, Active--Drift produced by the lifting surface.

Drift, Passive--Drift produced by the detrimental surface.

Drift (of a propeller)--Analogous to the drift of an aeroplane. It is
convenient to include "cavitation" within this term.

Drift, to--To be carried by a current of air; to make leeway.

Dive, to--To descend so steeply as to produce a speed greater than the
normal flying speed.

Dope, to--To paint a fabric with a special fluid for the purpose of
tightening and protecting it.

Density--Mass of unit volume, for instance, pounds per cubic foot.

Efficiency--Output            Input

Efficiency (of an aeroplane as distinct from engine and propeller)--

       Lift and Velocity
    Thrust (= aeroplane drift)

Efficiency, Engine--Brake horse-power

       Indicated horse-power

Efficiency, Propeller--

                              Thrust horse-power
                       Horse-power received from engine
                               (= propeller drift)

NOTE.--The above terms can, of course, be expressed in foot-pounds. It
is then only necessary to divide the upper term by the lower one to find
the measure of efficiency.

Elevator--A controlling surface, usually hinged to the rear of the
tail-plane, the operation of which turns an aeroplane about an axis
which is transverse to the direction of normal horizontal flight.

Empennage--See "Tail-plane."

Energy--Stored work. For instance, a given weight of coal or petroleum
stores a given quantity of energy which may be expressed in foot-pounds.

Extension--That part of the upper surface extending beyond the span of
the lower surface.

Edge, Leading--The front edge of a surface relative to its normal
direction of motion.

Edge, Trailing--The rear edge of a surface relative to its normal
direction of motion.

Factor of Safety--Usually taken to mean the result found by dividing the
stress at which a body will collapse by the maximum stress it will be
called upon to bear.

Fineness (of stream-line)--The proportion of length to maximum width.

Flying Position--A special position in which an aeroplane must be placed
when rigging it or making adjustments. It varies with different types of
aeroplanes. Would be more correctly described as "rigging position."

Fuselage--That part of an aeroplane containing the pilot, and to which
is fixed the tail-plane.

Fin--Additional keel-surface, usually mounted at the rear of an
aeroplane.

Flange (of a rib)--That horizontal part of a rib which prevents it from
bending sideways.

Flight--The sustenance of a body heavier than air by means of its action
upon the air.

Foot-pound--A measure of work representing the weight of 1 lb. raised 1
foot.

Fairing--Usually made of thin sheet aluminum, wood, or a light
construction of wood and fabric; and bent round detrimental surface in
order to give it a "fair" or "stream-like" shape.

Gravity--Is the force of the Earth's attraction upon a body. It
decreases with increase of distance from the Earth. See "Weight."

Gravity, Specific--Density of substance                   Density of water.
Thus, if the density of water is 10 lb. per unit volume, the same unit
volume of petrol, if weighing 7 lb., would be said to have a specific
gravity of 7/10, i.e., 0.7.

Gap (of an aeroplane)--The distance between the upper and lower surfaces
of a biplane. In a triplane or multiplane, the distance between a
surface and the one first above it.

Gap, Propeller--The distance, measured in the direction of the thrust,
between the spiral courses of the blades.

Girder--A structure designed to resist bending, and to combine lightness
and strength.

Gyroscope--A heavy circular wheel revolving at high speed, the effect of
which is a tendency to maintain its plane of rotation against disturbing
forces.

Hangar--An aeroplane shed.

Head-Resistance--Drift. The resistance of the air to the passage of a
body.

Helicopter--An air-screw revolving about a vertical axis, the direction
of its thrust being opposed to gravity.

Horizontal Equivalent--The plan view of a body whatever its attitude may
be.

Impulse--A force causing a body to gain or lose momentum.

Inclinometer--A curved form of spirit-level used for indicating the
attitude of a body relative to the horizontal.

Instability--An inherent tendency of a body, which, if the body is
disturbed, causes it to move into a position as far as possible away
from its first position.

Instability, Neutral--An inherent tendency of a body to remain in the
position given it by the force of a disturbance, with no tendency to
move farther or to return to its first position.

Inertia--The inherent resistance to displacement of a body as distinct
from resistance the result of an external force.

Joy-Stick--See "Control Lever."

Keel-Surface--Everything to be seen when viewing an aeroplane from the
side of it.

King-Post--A bracing strut; in an aeroplane, usually passing through a
surface and attached to the main spar, and from the end or ends of which
wires are taken to spar, surface, or other part of the construction in
order to prevent distortion. When used in connection with a controlling
surface, it usually performs the additional function of a lever, control
cables connecting its ends with the pilot's control lever.

Lift--The vertical component of the reaction produced by the action
of driving through the air a surface inclined upwards and towards its
direction of motion.

Lift, Margin of--The height an aeroplane can gain in a given time and
starting from a given altitude.

Lift-Drift Ratio--The proportion of lift to drift.

Loading--The weight carried by an aerofoil. Usually expressed in pounds
per square foot of superficial area.

Longeron--The term usually applied to any long spar running length-ways
of a fuselage.

Mass--The mass of a body is a measure of the quantity of material in it.

Momentum--The product of the mass and velocity of a body is known as
"momentum."

Monoplane--An aeroplane of which the main lifting surface consists of
one surface or one pair of wings.

Multiplane--An aeroplane of which the main lifting surface consists of
numerous surfaces or pairs of wings mounted one above the other.

Montant--Fuselage strut.

Nacelle--That part of an aeroplane containing the engine and pilot and
passenger, and to which the tail plane is not fixed.

Neutral Lift Line--A line taken through a surface in a forward direction
relative to its direction of motion, and starting from its trailing
edge. If the attitude of the surface is such as to make the said line
coincident with the direction of motion, it results in no lift, the
reaction then consisting solely of drift. The position of the neutral
lift line, i.e., the angle it makes with the chord, varies with
differences of camber, and it is found by means of wind-tunnel research.

Newton's Laws of Motion--1. If a body be at rest, it will remain at
rest; or, if in motion, it will move uniformly in a straight line until
acted upon by some force.

2. The rate of change of the quantity of motion (momentum) is
proportional to the force which causes it, and takes place in the
direction of the straight line in which the force acts. If a body be
acted upon by several forces, it will obey each as though the others did
not exist, and this whether the body be at rest or in motion.

3. To every action there is opposed an equal and opposite reaction.

Ornithopter (or Orthopter)--A flapping wing design of aircraft intended
to imitate the flight of a bird.

Outrigger--This term is usually applied to the framework connecting the
main surface with an elevator placed in advance of it. Sometimes applied
to the "tail-boom" framework connecting the tail-plane with the main
lifting surface.

Pancake, to--To "stall "

Plane--This term is often applied to a lifting surface. Such application
is not quite correct, since "plane" indicates a flat surface, and the
lifting surfaces are always cambered.

Propeller--See "Air-Screw."

Propeller, Tractor--An air-screw mounted in front of the main lifting
surface.

Propeller, Pusher--An air-screw mounted behind the main lifting surface.

Pusher--An aeroplane of which the propeller is mounted behind the main
lifting surface.

Pylon--Any V-shaped construction from the point of which wires are
taken.

Power--Rate of working.

Power, Horse--One horse-power represents a force sufficient to raise
33,000 lbs. 1 foot in a minute.

Power, Indicated Horse--The I.H.P. of an engine is a measure of the rate
at which work is done by the pressure upon the piston or pistons, as
distinct from the rate at which the engine does work. The latter is
usually termed "brake horse-power," since it may be measured by an
absorption brake.

Power, Margin of--The available quantity of power above that necessary
to maintain horizontal flight at the optimum angle.

Pitot Tube--A form of air-speed indicator consisting of a tube with open
end facing the wind, which, combined with a static pressure or suction
tube, is used in conjunction with a gauge for measuring air pressures or
velocities. (No. 1 in diagram.)

Pitch, Propeller--The distance a propeller advances during one
revolution supposing the air to be solid.

Pitch, to--To plunge nose-down.

Reaction--A force, equal and opposite to the force of the action
producing it.

Rudder--A controlling surface, usually hinged to the tail, the operation
of which turns an aeroplane about an axis which is vertical in normal
horizontal flight; causes an aeroplane to turn to left or right of the
pilot.

Roll, to--To turn about the longitudinal axis.

Rib, Ordinary--A light curved wooden part mounted in a fore and aft
direction within a surface. The ordinary ribs give the surface its
camber, carry the fabric, and transfer the lift from the fabric to the
spars.

Rib, Compression--Acts as an ordinary rib, besides bearing the stress of
compression produced by the tension of the internal bracing wires.

Rib, False--A subsidiary rib, usually used to improve the camber of the
front part of the surface.

Right and Left Hand--Always used relative to the position of the pilot.
When observing an aeroplane from the front of it, the right hand side of
it is then on the left hand of the observer.

Remou--A local movement or condition of the air which may cause
displacement of an aeroplane.

Rudder-Bar--A control lever moved by the pilot's feet, and operating the
rudder.

Surface--See "Aerofoil."

Surface, Detrimental--All exterior parts of an aeroplane including
the propeller, but excluding the (aeroplane) lifting and (propeller)
thrusting surfaces.

Surface, Controlling--A surface the operation of which turns an
aeroplane about one of its axes.

Skin-Friction--The friction of the air with roughness of surface. A form
of drift.

Span---The distance from wing-tip to wing-tip.

Stagger--The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.

Stability--The inherent tendency of a body, when disturbed, to return to
its normal position.

Stability, Directional--The stability about an axis which is vertical
during normal horizontal flight, and without which an aeroplane has no
natural tendency to remain upon its course.

Stability, Longitudinal--The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.

Stability, Lateral--The stability of an aeroplane about its longitudinal
axis, and without which it has no tendency to oppose sideways rolling.

Stabilizer--A surface, such as fin or tail-plane, designed to give an
aeroplane inherent stability.

Stall, to--To give or allow an aeroplane an angle of incidence greater
than the "maximum" angle, the result being a fall in the lift-drift
ratio, the lift consequently becoming less than the weight of the
aeroplane, which must then fall, i.e., "stall" or "pancake."

Stress--Burden or load.

Strain--Deformation produced by stress.

Side-Slip, to--To fall as a result of an excessive "bank" or "roll."

Skid, to--To be carried sideways by centrifugal force when turning to
left or right.

Skid, Undercarriage--A spar, mounted in a fore and aft direction, and to
which the wheels of the undercarriage are sometimes attached. Should
a wheel give way the skid is then supposed to act like the runner of a
sleigh and to support the aeroplane.

Skid, Tail--A piece of wood or other material, orientable, and fitted
with shock absorbers, situated under the tail of an aeroplane in order
to support it upon the ground and to absorb the shock of alighting.


Section--Any separate part of the top surface, that part of the bottom
surface immediately underneath it, with their struts and wires.

Spar--Any long piece of wood or other material.

Spar, Main--A spar within a surface and to which all the ribs are
attached, such spar being the one situated nearest to the centre of
pressure. It transfers more than half the lift from the ribs to the
bracing.

Spar, Rear--A spar within a surface, and to which all the ribs are
attached, such spar being situated at the rear of the centre of pressure
and at a greater distance from it than is the main spar. It transfers
less than half of the lift from the ribs to the bracing.

Strut--Any wooden member intended to take merely the stress of direct
compression.

Strut, Interplane--A strut holding the top and bottom surfaces apart.

Strut, Fuselage--A strut holding the fuselage longerons apart. It should
be stated whether top, bottom, or side. If side, then it should be
stated whether right or left hand. Montant.

Strut, Extension--A strut supporting an "extension" when not in flight.
It may also prevent the extension from collapsing upwards during flight.

Strut, Undercarriage--

Strut, Dope--A strut within a surface, so placed as to prevent the
tension of the doped fabric from distorting the framework.

Serving--To bind round with wire, cord, or similar material. Usually
used in connection with wood joints and wire cable splices.

Slip, Propeller--The pitch less the distance the propeller advances
during one revolution.

Stream-Line--A form or shape of detrimental surface designed to produce
minimum drift.

Toss, to--To plunge tail-down.

Torque, Propeller--The tendency of a propeller to turn an aeroplane
about its longitudinal axis in a direction opposite to that in which the
propeller revolves.

Tail-Slide--A fall whereby the tail of an aeroplane leads.

Tractor--An aeroplane of which the propeller is mounted in front of the
main lifting surface.

Triplane--An aeroplane of which the main lifting surface consists of
three surfaces or pairs of wings mounted one above the other.

Tail-Plane--A horizontal stabilizing surface mounted at some distance
behind the main lifting surface. Empennage.

Turnbuckle--A form of wire-tightener, consisting of a barrel into each
end of which is screwed an eyebolt. Wires are attached to the eyebolts
and the required degree of tension is secured by means of rotating the
barrel.

Thrust, Propeller--See "Air-Screw."

Undercarriage--That part of an aeroplane beneath the fuselage or
nacelle, and intended to support the aeroplane when at rest, and to
absorb the shock of alighting.

Velocity--Rate of displacement; speed.

Volplane--A gliding descent.

Weight--Is a measure of the force of the Earth's attraction (gravity)
upon a body. The standard unit of weight in this country is 1 lb., and
is the force of the Earth's attraction on a piece of platinum called
the standard pound, deposited with the Board of Trade in London. At the
centre of the Earth a body will be attracted with equal force in
every direction. It will therefore have no weight, though its mass
is unchanged. Gravity, of which weight is a measure, decreases with
increase of altitude.

Web (of a rib)--That vertical part of a rib which prevents it from
bending upwards.

Warp, to--To distort a surface in order to vary its angle of incidence.
To vary the angle of incidence of a controlling surface.

Wash--The disturbance of air produced by the flight of an aeroplane.

Wash-in--An increasing angle of incidence of a surface towards its
wing-tip.

Wash-out--A decreasing angle of incidence of a surface towards its
wing-tip.

Wing-tip--The right- or left-hand extremity of a surface.

Wire--A wire is, in Aeronautics, always known by the name of its
function.

Wire, Lift or Flying--A wire opposed to the direction of lift, and used
to prevent a surface from collapsing upward during flight.

Wire, Anti-lift or Landing--A wire opposed to the direction of gravity,
and used to sustain a surface when it is at rest.

Wire, Drift--A wire opposed to the direction of drift, and used to
prevent a surface from collapsing backwards during flight.

Wire, Anti-drift--A wire opposed to the tension of a drift wire, and
used to prevent such tension from distorting the framework.

Wire, Incidence--A wire running from the top of an interplane strut
to the bottom of the interplane strut in front of or behind it.
It maintains the "stagger" and assists in maintaining the angle of
incidence. Sometimes termed "stagger wire."

Wire, Bracing--Any wire holding together the framework of any part of
an aeroplane. It is not, however, usually applied to the wires described
above unless the function performed includes a function additional to
those described above. Thus, a lift wire, while strictly speaking a
bracing wire, is not usually described as one unless it performs the
additional function of bracing some well-defined part such as the
undercarriage. It will then be said to be an "undercarriage bracing lift
wire." It might, perhaps, be acting as a drift wire also, in which
case it will then be de-scribed as an "undercarriage bracing lift-drift
wire." It should always be stated whether a bracing wire is (1) top, (2)
bottom, (3) cross, or (4) side. If a "side bracing wire," then it should
be stated whether right- or left-hand.

Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)
within a surface.

Wire, Top Bracing--A bracing wire, approximately horizontal and situated
between the top longerons of fuselate, between top tail booms, or at the
top of similar construction.

Wire, Bottom Bracing--Ditto, substituting "bottom" for "top."

Wire, Side Bracing--A bracing wire crossing diagonally a side bay of
fuselage, tail boom bay, undercarriage side bay or centre-section side
bay. This term is not usually used with reference to incidence wires,
although they cross diagonally the side bays of the cell. It should be
stated whether right- or left-hand.

Wire, Cross Bracing--A bracing wire, the position of which is diagonal
from right to left when viewing it from the front of an aeroplane.

Wire, Control Bracing--A wire preventing distortion of a controlling
surface.

Wire, Control--A wire connecting a controlling surface with the pilot's
control lever, wheel, or rudder-bar.

Wire, Aileron Gap--A wire connecting top and bottom ailerons.

Wire, Aileron Balance--A wire connecting the right- and left-hand top
ailerons. Sometimes termed the "aileron compensating wire."

Wire, Snaking--A wire, usually of soft metal, wound spirally or tied
round another wire, and attached at each end to the framework. Used to
prevent the wire round which it is "snaked" from becoming, in the event
of its displacement, entangled with the propeller.

Wire, Locking--A wire used to prevent a turnbuckle barrel or other
fitting from losing its adjustment.

Wing--Strictly speaking, a wing is one of the surfaces of an
ornithopter. The term is, however, often applied to the lifting surface
of an aeroplane when such surface is divided into two parts, one being
the left-hand "wing," and the other the right-hand "wing."

Wind-Tunnel--A large tube used for experimenting with surfaces and
models, and through which a current of air is made to flow by artificial
means.

Work--Force X displacement.

Wind-Screen--A small transparent screen mounted in front of the pilot to
protect his face from the air pressure.



FOOTNOTES:


[1] Propeller Slip: As the propeller screws through the air, the
latter to a certain extent gives back to the thrust of the propellor
blades, just as the shingle on the beach slips back as you ascend it.
Such "give-back" is known as "slip," and anyone behind the propellor
will feel the slip as a strong draught of air.

[2] Helicopter. An air-screw revolving upon a vertical axis. If driven
with sufficient power, it will lift vertically, but having regard to the
mechanical difficulties of such construction, it is a most inefficient
way of securing lift compared with the arrangement of an inclined
surface driven by a propeller revolving about a horizontal axis.

[3] Pancakes: Pilot's slang for stalling an aeroplane and dropping
like a pancake.

[4] Morane parasol: A type of Morane monoplane in which the lifting
surfaces are raised above the pilot in order to afford him a good view
of the earth.

[5] Skin friction is that part of the drift due to the friction of the
air with roughnesses upon the surface of the aeroplane.

[6] Banking: When an aeroplane is turned to the left or the right
the centrifugal force of its momentum causes it to skid sideways and
outwards away from the centre of the turn. To minimize such action the
pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the
underside of the planes to the air. The aeroplane will not then skid
outwards beyond the slight skid necessary to secure a sufficient
pressure of air to balance the centrifugal force.

[7] An explanation of the way in which the wash-out is combined with a
wash-in to offset propellor torque will be found on p. 82.

[8] A.M.'s: Air Mechanics.

[9] Butt means to thicken at the end. Screw means to machine a
thread on the butt-end of the wire, and in this way the wire can make
connection with the desired place by being screwed into a metal fitting,
thus eliminating the disadvantage of the unsatisfactory loop.

[10] Deviation curve: A curved line indicating any errors in the
compass.

[11] A propeller screws through the air, and the distance it advances
during one revolution, supposing the air to be solid, is known as the
pitch. The pitch, which depends upon the angle of the propeller blades,
must be equal to the speed of the aeroplane, plus the slip, and if, on
account of the rarity of the air the speed of the aeroplane increases,
then the angle and pitch should be correspondingly increased. Propellers
with a pitch capable of being varied by the pilot are the dream of
propeller designers. For explanation of "slip" see Chapter IV. on
propellers.

[12] Getting out of my depth? Invading the realms of fancy? Well,
perhaps so, but at any rate it is possible that extraordinary speed
through space may be secured if means are found to maintain the impulse
of the engine and the thrust-drift efficiency of the propeller at great
altitude.

[13] Box-kite. The first crude form of biplane.

[14] See Newton's laws in the Glossary at the end of the book.

[15] See "Aerofoil" in the Glossary.

[16] "In effect" because, although there may be actually the greatest
proportion of keel-surface In front of the vertical axis, such surface
may be much nearer to the axis than is the keel-surface towards the
tail. The latter may then be actually less than the surface in front,
but, being farther from the axis, it has a greater leverage, and
consequently is greater in effect than the surface in front.





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