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Title: Model aeroplanes - The building of model monoplanes, biplanes, etc. together with a chapter on building a model airship
Author: Camm, F. J.
Language: English
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Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
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in the original text.
Small capitals have been converted to SOLID capitals.
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Typographical and punctuation errors have been silently corrected.
Many of the illustrations are not in numerical order. This is not a
mistake, it is part of the original book.
MODEL
AEROPLANES
The Building of Model Monoplanes,
Biplanes, etc., together with a
Chapter on Building a Model Airship
BY
F. J. CAMM
WITH 190 ILLUSTRATIONS
NEW YORK
FUNK & WAGNALLS COMPANY
EDITOR’S PREFACE
This is a practical handbook on the principles, constructional details
and methods of building model aeroplanes, written by a well-known model
aeroplane designer and builder. It deals with every part of a machine
and describes a number of different types, including monoplanes,
biplanes, collapsible machines, tractor monoplanes, hydro-monoplanes,
aeroplanes driven by compressed air, etc., etc. The concluding chapter
explains how to build a model airship, and, as in the case of all
the others, is based on the results of practical experience. Readers
in need of further information on the subject should address their
inquiries to “Work,” La Belle Sauvage, London, E.C., through whose
columns (but not by post), assistance will be gladly given.
B. E. J.
CONTENTS
CHAPTER PAGE
1. WHY AN AEROPLANE FLIES 1
2. TYPES OF MODEL AEROPLANES 12
3. PRACTICAL CONSTRUCTION: MODEL AEROPLANE FUSELAGES 19
4. PRACTICAL CONSTRUCTION: CARVING AIR-SCREWS 35
5. PRACTICAL CONSTRUCTION: BENDING AIR-SCREWS 42
6. PRACTICAL CONSTRUCTION: PLANES 47
7. SIMPLE TWIN-SCREW MONOPLANE 54
8. SIMPLE TWIN-SCREW BIPLANE 61
9. WINDERS FOR ELASTIC MOTORS 69
10. COLLAPSIBLE MONOPLANE 73
11. TRACTOR MONOPLANE 80
12. HYDRO-MONOPLANE 87
13. COMPRESSED-AIR ENGINE FOR MODEL AEROPLANE 94
14. BIPLANE DRIVEN BY COMPRESSED-AIR ENGINE 104
15. GENERAL NOTES ON MODEL DESIGNING 120
16. GENERAL NOTES 124
17. EASILY-MADE TAILLESS KITES 136
18. BUILDING A MODEL AIRSHIP 141
INDEX 154
MODEL AEROPLANES
CHAPTER I
Why an Aeroplane Flies
Why does an aeroplane fly? The question is worthy of close examination.
There is one common enemy to aeroplanes—the force of gravity. Were
it not for the existence of this force, which, as Newton put it, “is
unseen and unheard and yet dominates the universe,” the problem of the
aeroplane would have been solved years ago.
[Illustration: Fig. 1.—Bristol Monoplane and Biplane]
Most readers have handled the toy kite, and since the principles
governing the flight of a kite are precisely the same as those which
apply to the aeroplane, the latter will be the more readily understood
if the principles are explained through this medium. Full-size
aeroplanes to which certain models approximate are shown in Fig. 1.
[Illustration: Fig. 1A.—Forces Acting on Kite]
If a kite is launched in a wind it speedily attains a certain height
or altitude, at which it remains so long as the wind does not drop.
The wind is overcoming gravity, which constantly endeavours to bring
the kite to earth, and hence, since the kite remains in the air, the
forces acting on the kite are said to be in equilibrium—that is,
balanced. The forces are shown diagrammatically in Fig. 1A, and include
gravity, which is practically constant and remains unaltered under all
conditions, the air pressure which, when sufficiently intense, lifts
the kite against the action of gravity, and the pull of the string.
The air pressure is really a combination of two forces—lift and drift.
The drift or resistance tends to move the kite in the direction of
the wind, and lift to raise the kite in opposition to gravity. Since,
therefore, drift is an undesirable factor, the resistance of the
machine must be made as low as possible, as it absorbs power, as will
clearly be seen. If the velocity of the wind drops, the kite drops
also, increasing its angle with the horizon, thereby causing it to
capture and force down more air until equilibrium is again restored. If
the string of a kite breaks, the balance of the forces is destroyed,
drift and gravity taking command and so bringing the kite to earth.
If it takes a wind of fifteen miles an hour to lift a kite, similarly
it would lift to exactly the same elevation if the holder of the
kite-string commenced to run at a rate of fifteen miles per hour in
calm air.
Now, an aeroplane is merely a kite with a mechanical arrangement (the
engine and propeller) which supplies the motion necessary to fly it,
and eliminates the necessity for a wind. This statement can easily
be followed. In the aforementioned parallel it was seen that it was
immaterial whether the kite-flyer was standing still with the wind
moving at fifteen miles per hour, or whether he was moving at the rate
of fifteen miles per hour in still air. The result in each case is the
same—the kite flies.
It has been stated that if the kite-string fractured the kite would
fall to the ground. If, however, it were possible at the moment of
rupture to attach a weightless engine and air-screw to the kite capable
of exerting a forward push equal to the drift, the kite would still
remain in the air.
Again, if the wind were suddenly to stop, and the engine and air-screw
were capable of moving the kite forward at the same rate at which the
wind was blowing, the kite would fly, and in all important respects
would constitute an aeroplane.
The kite, it will be assumed, requires a minimum speed of fifteen miles
per hour in order to sustain itself. If the wind be blowing at fifteen
miles an hour the operator can remain stationary. If it blows at ten
miles an hour he must run at five miles an hour against the wind. If it
blows at five miles an hour he must run at ten miles an hour against
the wind, or twenty miles per hour with the wind to maintain the kite.
Hence an aeroplane really has two speeds—its speed relative to the
earth and its air speed. The former is the rate of which it would
travel a given distance, and the latter is the sum of the speed
relative to the earth and the velocity of the wind.
It can readily be seen that an aeroplane travelling at ten miles an
hour relative to the earth against a fifteen-mile-an-hour wind has
really an air speed of twenty-five miles an hour. When the aeroplane,
however, is travelling with the wind, the air speed is the speed
relative to the earth minus the velocity of the wind.
It is also convenient to draw a parallel between the ship and the
aeroplane. The weight of a ship must equal the weight of water it
displaces in order to float. Similarly an aeroplane, by its motion
through the air, must deflect a volume of air equal at least to its own
weight. The aeroplane then would just lift itself from the ground; and
the more air it deflects the higher does it ascend.
Now, if a 1-lb. weight be laid on a table, the table presses against
the weight with a force of 1 lb. If the hand is pressed against the
wall, the wall presses back with an equal pressure. If a person fires
a revolver, the force of explosion tends to force the revolver and the
person in the opposite direction to the travel of the bullet. These
are merely illustrations of the law that action and reaction are equal
and opposite. It is in reality due to this law that the aeroplane can
resist gravity.
[Illustration: Fig. 2.—Deflection of Air]
Fig. 2 represents an end view of a kite—or, for that matter, of an
aeroplane. The arrows indicate the direction of motion of the wind.
Upon contact with the kite the air has a downward action, and the
consequent reaction lifts the kite. Hence the motion of an aeroplane
through the air causes a pressure on the latter, and the resultant is
what is termed lift.
So far, then, the reason why an aeroplane lifts has been dealt with.
Further considerations have to be dealt with after the machine has
left the ground. In technical language these could be summarised into
a single sentence—that is, the centres of pressure and gravity must be
made to coincide, and the machine must also be stable in both lateral
and longitudinal directions.
[Illustration: Fig. 3.—Position of Centre of Gravity]
An ordinary paper glider, cut from a stiff sheet of cartridge paper,
will serve admirably to demonstrate this statement, which at first
sight will convey as much to the reader as Choctaw or other remote
language.
Cut the paper to the dimensions given in Fig. 3 and make sure that
it is flat, by pressing between the leaves of a book. Then project
it horizontally into the air. It does not attain gliding motion. It
performs a series of evolutions, too quickly for the eye to perceive;
but what happens is this. After launching, the front edge turns up
and the sheet glides back. Now the back edge turns up and the glider
dives forward. Again the front edge turns up, the glider slides back,
the back edge turns up, it glides forward, and so on until the glider
reaches the ground. Now fix a couple of small brass paper-fasteners in
the front edge (the correct number of fasteners can however only be
found by experiment, but two will usually be sufficient for the size of
glider indicated), and launch the glider again. It will be noticed that
it glides steadily at a small angle to the ground.
The explanation of this phenomenon is simple. When it was launched in
the first place, the centre of gravity of the plane lay along a line
running through the geometrical centre, parallel with the front edge,
and the glider merely rocked or oscillated about this axis. The centre
of pressure of the surface would be approximately in the position
shown in the illustration. When the correct number of paper-fasteners,
however, are fixed, the centre of gravity is moved forward to a
position coincident with the centre of pressure, the result being that
the glider came to earth in steadiness and poise. But, even though it
is now balanced, it will still show a tendency to rock sidewise or
laterally, and if the wings are bowed up to the dihedral angle shown
in Fig. 4, the rock will be eliminated, and the machine is said to be
laterally stable. Either of the dihedral angles may be used, although
=B= is much to be preferred.
What of stability in a longitudinal direction? Just as important this,
but not quite so easy to obtain.
[Illustration: Fig. 4.—Various Forms of Dihedral]
Fig. 5 is a side elevation of two surfaces fixed to a spar, and shows
how stability is obtained longitudinally. The surfaces of the elevator
or tail, according to whether the machine is “canard” or tractor
(canard being the term for propeller-behind or “pusher” machines), is
placed at a positive angle with the horizon. The correct angle can, of
course, only be found by experiment.
Now, from the foregoing certain laws can be deduced. Firstly, in
order to be stable longitudinally, the centre of pressure must be
kept as near to the centre of gravity as possible, and secondly,
the main surface of the aeroplane must be inclined to preserve
lateral stability. With full-size aeroplanes there are, however,
several exceptions to this rule, as the faster a machine travels the
more stable does it become, and hence the dihedral angle is really
unnecessary.
It may be well at this point to describe the action of a plane.
Strictly speaking, the terms “plane” and “aeroplane” are misnomers,
since no full-size machine has surfaces which even approximate to
planes.
[Illustration: Fig. 5.—Disposition of Angles]
[Illustration: Fig. 6.—Air-flow Round Plane]
[Illustration: Fig. 7.—Air-flow Round Cambered Surface]
[Illustration: Fig. 8.—Air-flow Round Streamline Strut]
[Illustration: Fig. 9.—Air-flow Round Square Strut]
The reason why a perfectly flat plane is never used on full-size
aeroplanes will be followed from Fig. 6, which shows the flow of
air over an inclined plane, the term “plane” being used here in its
technical sense.
It will be noticed that a region of “dead air” or partial vacuum is
caused, which seriously affects the lift of the plane. Fig. 7 shows
the flow of air over a cambered aerofoil (or to use the popular
colloquialism “plane”). Less disturbance occurs in this instance,
the air following very approximately the contour of the surface. It
has been proved by test in the Wind Tunnel at the National Physical
Laboratory at Teddington that an efficiently-designed aerofoil section
has a lift two-thirds greater than a true plane. For a similar reason
all struts or aeroplanes are “streamlined,” as shown by Fig. 8. The air
flow, it will be seen, is less disturbed than by a square strut (Fig.
9).
[Illustration: Fig. 10.—Flow of Air over Ends of Plane]
Fig. 10 shows the air flow round a square-ended and taper-ended plane
respectively. It will be noticed that the air has a tendency to leak
over the end of the square plane, which is obviated by the tapered wing.
It may be thought that such details as these are unimportant; but when
it is remembered that an aeroplane, correctly streamlined, will fly
for one-half the power required to fly a machine not so designed, the
enormous saving in power will be manifest.
The actual thrust required to lift a model aeroplane is roughly equal
to a quarter of its total weight. Thus a model weighing 6 oz. will
require 1½-oz. thrust.
CHAPTER II
Types of Model Aeroplanes
With a view to illustrating some of the models described in this book
complete, some drawings are given of the more successful designs which
have come into prominence during the past eight years. Fig. 11 shows
the Ridley Monoplane, which secured several well-merited rewards in
open competition, and is an excellent machine for distance. Birch
should be used for the longerons, preferably of channelled section.
The main plane is of piano wire, covered with proofed silk, and the
elevator is entirely of extremely thin veneer. Bentwood screws are used
fairly short in diameter and of long pitch. The machine is capable
of flying a quarter of a mile. The Fairey type of model aeroplane
typified in Fig. 12 is a most successful type, and has achieved much
in open competition. It has what is known as a floating tail, with no
leading stabilising surface, but a small vertical fin is used. This
is a practice the waiter is not personally in favour of, as through
such a long lever the slightest wind will cause great instability. If
a vane or fin must be used, it should be placed as far to the rear of
the machine as possible, preferably just behind, or in front of the
propellers for pusher machines, and the extreme end of the tail for the
tractor type.
The swept-back wing tips should have a negative angle of about
two degrees, and the tail should be quite flat in relation to the
horizontal.
[Illustration: Fig. 13.—Clarke Type Monoplane]
[Illustration: Fig. 12.—The Fairey Monoplane]
[Illustration: Fig. 11.—The Ridley Monoplane]
It has been stated that this machine is a highly successful one; it
is also exceedingly intricate in adjustment, and requires very calm
weather indeed to secure successful flights. It has also been flown
with great success by Mr. Houlberg, who at one time held the official
duration record of 89 secs with his machine. The long unrelieved
length of spar projecting forward of the main surface detracts much
from its appearance in the air. A machine of this type should not
weigh more than 8 oz., and is capable of a flight of at least a minute
in duration. The simple 1-1-P¹ type drawn in Fig. 13 was formerly
popularised by Mr. T. W. K. Clarke, of Kingston, who used all-wooden
surfaces, a solid spar and bentwood screw built up in two halves. This
method of screw manufacture is unique, since it enables the two blades
to be prepared from jigs to a greater degree of accuracy than when it
is bent from one piece. Moreover, the lapping of the two halves at the
boss imparts strength to the boss where it is most needed. Fig. 14 is
a type of tractor monoplane very successful for duration, capable of
doing a minute at an altitude of forty feet. The rudder of tractor
machines must always be placed _above_ the thrust line and also above
the centre of gravity, so that should a side gust strike the machine,
the latter does not rock laterally in the air, as a couple is set up
between the underhung load and the rudder.
The winner of the Wakefield Gold Challenge Cup is shown in Fig. 15. It
was designed by Mr. E. W Twining, one of the early experimenters, for
duration, and in the winning flight scored a duration of sixty-five
seconds. It is a very pretty and stable flyer, and will rise from the
ground after a run of about five feet.
[Illustration: Fig. 14.—Tractor Monoplane]
[Illustration: Fig. 15.—Twining Monoplane]
[Illustration: Fig. 16.—Bragg-Smith Biplane]
Fig. 16 is of the Bragg-Smith biplane, which first came into prominence
at Wembley in 1909. The original machine was a huge machine some four
feet in span, possessing a propeller of large diameter, large blades,
and large pitch—quite the antithesis to ordinary practice. Latterly,
however, Mr. Smith has developed his machines into twin-screw, and no
doubt is entertained that even better results are obtained with this
arrangement.
[Illustration: Fig. 18.—Tandem Monoplane]
[Illustration: Fig. 17.—Tractor Hydro-Biplane]
[Illustration: Fig. 21.—Fuselage Biplane]
[Illustration: Fig. 22.—Blériot Type Tractor Monoplane]
[Illustration: Fig. 20.—Tractor Biplane]
[Illustration: Fig. 19.—Twining Hand-launched Biplane]
It should be pointed out, in passing, that this machine is the subject
of a patent for stability, it being claimed that greater lateral
stability is obtainable from the curved lower mainplane. A sketch is
also given in Fig. 17 of a tractor hydro-biplane. This should weigh
about 12 ozs. finished. The tandem monoplane shown by Fig. 18 is
another machine which has scored many successes in the early days of
model aeroplaning at the Crystal Palace. Figs. 19, 20, 21, 22 show the
Twining hand-launched biplane, a tractor biplane built by the writer, a
fuselage biplane (canard or screw behind), and a Blériot type tractor
monoplane.
A tractor machine is one having the screw in front, and a “canard” or
“pusher” machine has its screw behind. It is best to designate the
machine by the type formula. Thus, a pusher monoplane with twin screws
would be a 1-1-P²-0 type. If it had a tail it would be 1-1-P²-1. A
twin-screw “pusher” biplane with or without tail would be 1-2-P²-1 and
1-2-P²-0 respectively. A pusher monoplane with only one screw is a
1-1-P¹ type. A tractor monoplane with single screw is P¹-1-1; a tractor
biplane with single screw is P¹-2-1. If a biplane tail is also used it
becomes P¹-2-2. If twin screws are used it would then become P²-2-2,
and so on.
CHAPTER III
Practical Construction: Model Aeroplane Fuselages
In no other portion of a model aeroplane has standardisation become
more marked than in the design and construction of the fuselage or main
frame, both with regard to general details, methods, and materials.
This fact is singular, because in other components contributing in a
greater degree to the success of the model great diversity of opinion
exists. It is difficult to ascribe this lack of uniformity to any
particular reason, unless it is the failure on the part of zealous
amateurs to appreciate the meaning of the term “efficiency.” Very few,
it is thought, endeavour to extract the maximum amount of work for a
minimum expenditure of power from the propellers, surfaces, and so
forth, and the writer, in judging and tabulating some of the model
aeroplane competitions held in different parts of the country, has
found models giving excellent spectacular results which, judged on an
efficiency basis, such as
(distance flown × duration of flight)
-------------------------------------
(weight of rubber)
show a very poor result. The model should be made to fly the longest
possible distance, and to remain in the air the longest possible time
with the smallest possible amount of elastic.
This chapter is devoted to the various types of fuselage for flying
models (as distinct from “scale” models of full-size prototypes) and
methods of constructing them, and the list is as representative of
best practice as it has been possible for the writer, in his extensive
connection with this subject, to make it.
The first shown is the =A= frame (Fig. 23), brought into prominence by
Mr. R. F. Mann. It should have birch longitudinals and spruce cross
members. Quite the best section wood to employ is that shown at =B=,
which forms a convenient seating for the cross members, the latter
being pinned and glued into position. The middle bay of such a frame
requires to be braced, to counteract the torque or distortion caused
by the elastic skein when the latter is in torsion. Diagram =A= shows
the joint at the juncture of the longerons or longitudinals. The hooks
which embrace the elastic skeins are formed from one continuous length
of wire following round the nose of the machine. The bearings may be of
brass, with a lug to follow round the end of the longeron to which it
is bound.
Fig. 24 shows the =T= or cantilever frame, so named because of its
resemblance to that letter. It is usual to make the spar of this
hollow, by channelling out two pieces of wood, and gluing and cramping
them together under pressure. Where the bracing kingpost passes through
the channel should be packed, previous to gluing the two half spars
together, with a piece of hard wood, so that the assembled spar is not
weakened by the piercing necessary for the insertion of the kingpost.
Such a spar should not exceed 4 ft. in length. =C= is a section of the
spar.
[Illustration: Fig. 23.—=A= Frame]
[Illustration: Fig. 26.—=T= Frame]
[Illustration: Fig. 25.—=A= and =T= Frame]
[Illustration: Fig. 24.—=T= Frame]
A much stronger twin-screw fuselage, which is a combination of the =A=
and =T= frames, is shown by Fig. 25. Here, again, a single-channelled
longeron should be used, although the propeller bar and supports should
be solid. The channelled spar can be of silver spruce or birch, and the
bar and supports of mahogany. A section of the spar is given at =D=.
Fig. 26 shows a =T= frame made from a hollow spar, having the ends
splayed out to give the required support to the elastic skein. No
propeller bar is used but there is a tension wire from bearing to
bearing to prevent the splayed section of the spar from spreading when
the rubber is wound up. =E= shows a section of the spar. It should
be pointed out that the greatest width of a spar should be placed
vertically. Never use a square-sectioned spar. Moreover, the bearing
centres should only exceed the propeller diameter by ½ in., since,
apart from consideration of weight, greater rigidity is obtainable from
short spars than from long ones. It is in details such as this that not
only is a considerable saving in weight effected, but also a material
increase in strength and efficiency.
The =T= frame adapted to a “rise-off-ground” fuselage is shown in Fig.
27. A hollow spar should be used for preference, but the spar cut from
the solid is shown, as most amateurs will not be in possession of a
joiner’s plough, which is the tool required for this job. =F= is a
section on the vertical line of the spar.
[Illustration: Fig. 27]
[Illustration: Fig. 28]
[Illustration: Fig. 29]
[Illustration: Fig. 30
Figs. 27 to 30.—Various Forms of Fuselage]
In bracing such frames as those dealt with, fine No. 35 S.W.G.
(Standard Wire Gauge) should be used, fixed to small hooks bound in
suitable places on the spar; the hooks should be made from No. 22 S.W.G.
Fig. 27 shows at =G= a perspective view of a =T=-frame propeller bar
and support. As there shown, the propeller bar fits into a slot cut in
the spar end. If the spar is hollow, the channel should be filled with
hard wood, such as birch, before the slot is cut, to strengthen the
spar at this point. So much for twin-screw fuselages.
Fig. 28 is a perspective view of a boat-shaped tractor fuselage, it
being understood that a tractor machine is one with the air-screw in
front. A model built on such lines is extremely neat in appearance and
has a pleasing aspect in the air. The three longerons are attached to a
three-way brass bearing at the front end, and are simply bound together
at the rear, the hook for the elastic being inserted between the two
top members and turned round the end of one of them for security, as
shown at =H=. The bottom member should be cut 1 in. longer than the
two top ones, to compensate for the shortening due to the curve, which
is effected by compressing the bottom member to the same length as the
top ones. The curved cross members are of bamboo, bent to the required
shape over a lamp flame; or they could be made from piano wire. Their
shape should be drawn full-size to use as a template during the
bending operation. As will be seen, a skid is used to protect the
tractor screw from damage. This should extend for 2 in. beyond the
bearing, and must be attached to the bottom longitudinal directly
beneath the first cross member, so that the latter absorbs the shock of
landing. At the point of intersection between the skid and the axle,
the former should be bound to the latter with fine florist’s wire and
neatly soldered.
A two-membered fuselage can be adapted from this design by omitting the
bottom member and skid. Such a fuselage would be suitable for a light
machine.
It is an essential point with tractor models to fit a chassis; the
purpose thus being twofold. First, it protects the propeller, and
secondly, it obviates the characteristic tendency of tractor machines
to ascend “nose first,” by keeping the weight low (in technical
language, providing a low centre of gravity). Hand-launched tractor
machines that are unprovided with a landing gear are seldom successful
and notoriously troublesome. Furthermore, the centre of thrust
(literally the axis centre, or centre of rotation of the bearing)
should always be above the centre of resistance. The centre of
resistance can usually be taken (although not quite accurate) as being
on a level with the planes.
An exceedingly strong two-membered tractor fuselage of the fusiform
or cigar-shaped type is that shown by Fig. 29, the bearing, which
is bracketed and cut from brass, being shown in detail at =I=. In
this instance the greatest width of the top spar should be disposed
horizontally, the bottom member, with the two cross members, providing
rigidity and a girder-like form of construction. The bottom member need
be only one-half the weight of the top one, as it will be in tension
and so acting as a tie. Silver spruce should be used throughout.
A simple single-spar chassis, consisting of a hollow spar, is shown
by Fig. 30. A kingpost and bracing is fitted underneath the spar, to
counteract the tendency of the twisted skein to bow it. The chassis
should be (and this applies to all models) of piano wire of from No. 17
S.W.G. to No. 18 S.W.G.
A twin-screw propeller-behind fuselage of the cantilever type that
is exceedingly strong, although more difficult than it appears to
construct, is shown by Fig. 31. This can be made exceedingly light from
a hollow spar (packed solid at the point where the kingposts are let
through), and braced with No. 35 S.W.G. piano wire. The bracing is the
difficult operation, as the tension on each wire requires to be very
delicately adjusted to maintain the truth of the spar.
The box-girder type of fuselage shown by Fig. 32 is more suited to
models which aim at an accurate representation of some prototype. It is
intricate in construction yet of neat appearance, the difficulty being
in the adjustment of the large number of bracing wires necessary. The
illustration gives a view of a Blériot type of fuselage, =J= being a
detail of the cross member and compression-strut joint.
Fig. 33 gives some spar sections which are in common use by some of
the crack aero-modellists. All spars should taper in a fore-and-aft
direction, so that it virtually becomes a cantilever. The greatest
cross-section should be one-third of the total length from the front
end of the spar.
[Illustration: Fig. 31]
[Illustration: Fig. 32]
[Illustration: Fig. 33]
[Illustration: Fig. 34
Figs. 31 to 34.—Various Forms of Fuselage]
Another form of spar construction is that given by Fig. 34. =Q= shows a
spar fretted out, the sides being covered with a thin veneer glued and
cramped into place, and =R= the method of making a slotted spar.
Fig. 35 is the simplest possible form of model aeroplane fuselage, if
such it can be called.
Choice of materials and the method of utilising them to the best
advantage, so that the machine is strong without being unduly heavy,
is a phase of model aeroplaning that calls for some care and judgment.
There is a very erroneous impression prevalent among novices that
packing-case wood or similar material is suited to the requirements
peculiar to model aeroplanes. Nothing could be farther from the
truth; and the fact that 50 per cent. of the total marks awarded in
competition are for design and construction should show that this
matter is of primary importance. The true test of any model is the way
it “stands up” to a nose dive, for then the care and forethought of
the builder in providing for anticipated eventualities will manifest
itself. It is to be feared that those who had lavished much care and
infinite pains in the scientific construction of models were woefully
handicapped in competition, the flimsy freak that could flutter aloft
for a minute or so, with three strands of rubber wound to nearly
breaking point, gaining priority over the properly built machine.
There are three salient points to be borne in mind on which the
durability of the machine largely depends: (1) its capacity for
resisting the torque of the rubber motor; (2) of absorbing the shocks
of rough landing; and (3) the provision that has been made for the
rigid attachment of the various parts. If the machine is at fault
with regard to point one, fuselage distortion is likely to occur, and
resulting from this there will be lack of alignment of the surfaces and
attendant troubles. Point two calls for suitable bracing of the spar or
spars, and careful choice of timber. It is inadvisable to use wood of
square cross-section, an oblong section with the greatest measurement
placed vertically being preferable. If no arrangement has been made
to fix rigidly the wings, chassis, etc. (point 3), these parts are
likely to rock or sway when the machine is in the air, and so occasion
bad stability, apart from which a couple of landings would shake the
machine out of truth.
[Illustration: Fig. 35.—Simple Fuselage]
To obviate these difficulties a knowledge of the strength of the
various timbers will be found useful, and there is appended a table of
the weights of various timbers. The writer prefers birch for fuselage
members over 3 ft. 6 in. in length. Although on the heavy side compared
with spruce, it will stand a great amount of rough usage. Spruce is
also suitable for fuselages up to this length, while maple is more
suited for main planes. Bamboo can be used more efficaciously for cross
members, struts, etc. Some model-makers use bamboo for planes, the
joint of the rib to the spar being by means of glue and cross-binding.
Although planes so built are exceedingly strong, it is not possible to
make quite so neat a job of them as with spruce or maple.
Another method of building main planes is to use spruce or birch spars
with piano-wire ribs, these latter being bound to the former.
For single-spar models the main spar should be tapered fore and aft
from a point one-third of the length from the front of the machine.
Where it is necessary to pierce the spar of a model aeroplane for the
reception of a kingpost or other member, silk tape binding should be
used, the joint being soaked with clean, weak glue.
To resist fuselage distortion the spar must be suitably braced in a
lateral direction, the outrigger carrying the bracing wires being
situated just forward of the centre of the spar. No. 35 S.W.G. is quite
strong enough for fuselage bracing. Silk fishing-line or Japanese silk
gut is admirably suited for wing bracing, and is not so liable to
stretch as the tinned-iron or brass wire sometimes used. Piano wire
is generally used for elevators, tail planes, chassis, and propeller
shafts, of a gauge ranging from No. 17 S.W.G. to No. 22 S.W.G. A
clock-spring or piano-wire protector fitted to the nose of a model
aeroplane will also prevent a broken spar should it strike any object
during flight.
[Illustration: Fig. 36.—Built-up Plane]
[Illustration: Fig. 37.—Wing Plans]
[Illustration: Fig. 38.—Types of Bearings]
[Illustration: Fig. 38A.—Twin-screw Bearings]
WEIGHT OF WOODS CHIEFLY USED
Mahogany 35 lb. per cubic foot
Birch 45 ” ” ”
Maple 46 ” ” ”
Spruce 31 ” ” ”
Bamboo 25 ” ” ”
=Building Scale Models.=—Models of well-known machines should be built
to correct proportions, if as perfect a resemblance as possible is
aimed at. The best way to do this is, of course, to adopt a definite
scale. The particular scale will depend principally on the size the
builder requires his model; but the size of the prototype must, of
course, be considered, because the large machines differ so much in
point of size.
Taking the span or width across the planes as the base from which to
start, it is assumed that the width of the model is desired to be from
25 in. to 35 in., which is perhaps the best all-round minimum and
maximum to adopt. Then having decided on the prototype, multiply the
span of the real machine by a fraction, which brings the model span
somewhere between the two figures. For instance, suppose it is desired
to model an Antoinette monoplane, the span of which is about 46 ft.,
and multiplying by ¾ the model span becomes 34½; therefore the scale is
¾ in. to the foot.
If the model is to be a Blériot, then as the original has a span of 28
ft., the model may be built to a scale of 1 in. to the foot. The Wright
machine has a span of 41 ft., so a model to ¾ in. to the foot would
have a span of 30¾ in. In this case, perhaps, 1-in. scale would not
be considered too large. Odd scales such as ⅞ in. to the foot can, of
course, be adopted; but whatever the scale is to be, the model should
be set out full-size on a sheet of cartridge paper, and the scale drawn
accurately at the foot. The ribs should be built up as in Fig. 36.
In designing a rubber-driven model, absolute scale must of necessity be
departed from, except in the principal measurements and in the distance
of centres apart of spars and other important members, which if not
reproduced in their proper form and position would mar the otherwise
correct appearance of the machine. Many of the spars will, of course,
need to be increased in cross-sectional dimension in order to make them
of sufficient strength. Some efficient wing plans are given by Fig. 37.
Stated briefly, there are essentially three kinds of model aeroplanes.
First, the scale model, which is a reproduction to scale of a real
machine; second, a modified copy of a large machine, which is so
designed as to resemble in general form some well-known prototype,
while retaining by means of a suitable motor, generally twisted rubber,
some ability to fly; third, a machine which does not in any way follow
the lines of full-size machines, and is built for flight only.
The first of these is essentially an exhibition model; it is more
often built either to illustrate points in the design and construction
of large machines, or to demonstrate the functions of the various parts
to technical classes, etc.
Scale models, as a rule, are unsatisfactory flyers, and if they fly
at all the flight is so short that little can be learned from their
performance.
Some serviceable types of bearings are given by Figs. 38 and 38A, on p.
31.
CHAPTER IV
Practical Construction: Carving Air-screws
One of the most important units of an aeroplane, whether full-size
or model, is the screw, since excellence of design with regard to
the other portions of the machine are rendered void if the means of
converting the power of the engine into work are inefficient.
The action of an air-screw may be likened to a bolt turning in a nut
(the screw being the bolt and the air the nut), the difference being
that whereas one turn of a bolt with, say, a Whitworth pitch of 14
threads per inch in a nut is bound to advance a distance equal to
the pitch = ¹/₁₄ in., an air-screw may only advance 75 per cent. of
its theoretical pitch, owing to the yielding nature of the air. This
loss in efficiency is called “slip,” and is usually expressed as a
percentage of the theoretical pitch. Thus a screw with a theoretical
pitch of 4 ft., which possesses 75 per cent. efficiency, has an
effective pitch of 3 ft. That is to say, each turn of the screw will
take the aeroplane forward 3 ft. If, however, the screw were working
in a solid, it would advance its theoretical pitch = 4 ft. A greater
efficiency is obtainable with screws working in water, owing to the
difference in density of the two media, namely, air is to water as
800: 1. Probably no air-screw has yet exceeded 80 per cent. efficiency,
70 per cent. being a fair average.
It may, perhaps, not be amiss to outline some of the factors involved
in the design of an air-screw. Having decided on the diameter of it,
the proportions of the block from which the screw is to be carved
are required. It is a very good rule to make the pitch from one and
a half to twice the diameter for single-screw machines, and from two
and a half to three times the diameter for twin-screw machines. It is
possible to use much longer-pitched screws with twin-screw machines
(it being understood that the screws revolve in opposite directions),
since the torque, or tendency of the screw to capsize the machine in
the opposite direction to which it revolves, will be balanced. For the
purposes of this chapter, however, it is presupposed that a screw is
required for a single-screw machine, and a diameter of 12 in. has been
decided on. One and a half times 12 in. gives 18 in. as the pitch.
Remembering the formula for pitch,
(thickness of block)
P = 3¹/₇ × D × --------------------,
(width of block)
where P = pitch, D = diameter of screw, and using a ratio of width of
blade to diameter of screw of 6: 1 (which gives 2 in. as the width of
block) gives
22 12 (thickness of block)
18 = ---- × ---- × ------------------,
7 1 2
61
from which thickness of block = ·954 = ---- approx.
64
[Illustration: Fig. 39]
[Illustration: Fig. 40]
[Illustration: Fig. 41]
[Illustration: Fig. 42]
[Illustration: Fig. 43]
[Illustration: Fig. 44]
[Illustration: Fig. 45]
[Illustration: Fig. 46]
[Illustration: Fig. 47]
[Illustration: Fig. 48
Figs. 39 to 48.—Carving Air-screws.]
The block may now be prepared from these dimensions. American
whitewood, silver spruce, mahogany, or walnut are the most suitable
woods to use. The block should be planed up true and square, and a hole
drilled axially through its geometrical centre. The first operation
is to rough the block out to the shape shown by Fig. 39, which shows
the Chauvière type. Of course, other shapes may be used as desired,
but the method of manufacture is the same. Now, with a flat chisel or
woodworker’s knife pare the wood away (see Fig. 40) until the hollow or
concave side of the blade is formed (see Fig. 41). The obverse side of
the other blade is then similarly treated (see Fig. 42), which clearly
shows how the blade is hollowed out.
Fig. 43 shows the method of forming the boss of the screw, and Fig.
44 how the reverse or convex side of the blade is shaped. Fig. 45
shows the screw roughed out, and Fig. 46 indicates the glass-papering
operation.
At this stage the screw has to be balanced. This is of great
importance, since the screw that is unbalanced loses a great amount
of efficiency owing to the consequent vibration when it rotates. In
full-size practice it would be highly dangerous to use a screw that is
not balanced.
A piece of wire is passed through the hole previously drilled, and the
heavier blade carefully glasspapered down (with No. 00 glasspaper to
finish) until the screw poises in a horizontal plane. Fig. 47 shows the
sort of brush to use for polishing, and Fig. 48 the finished screw.
For models that require a good finish an excellent form of
construction (incidentally it may be remarked that full-size screws
are made in this way) is that shown by Fig. 49, the laminated type.
These laminated screws are exceedingly strong, as the grain, by virtue
of the splayed blanks, follows the blade. Screws carved from the solid
block are a trifle weak near the boss owing to cross grain. The laminæ
could be alternate layers of whitewood and mahogany, which give a
pleasing finish to the screw. =A= is an end view of a carved screw.
[Illustration: Fig. 49.—Laminated Air-screw]
The method of obtaining the pitch angles at various points along a
screw-blade is shown diagrammatically in Fig. 50. It will be obvious
that the pitch of a screw should be constant along the whole length
of blade, so that the air is deflected or driven back at a constant
velocity. An efficient screw will deliver a solid cylinder of air,
whereas an inefficient one delivers a tube of air.
If, for instance, the pitch at the propeller tip is 30 in., whilst at,
say, 3 in. from the centre it is only 25 in., obviously the tip of the
screw will be imparting a higher velocity to the air than the portion
approaching the boss, and thus this latter would be acting as a drag
upon the other portion.
[Illustration: Fig. 50.—Setting Out Pitch Angles]
The method is to lay off a distance, equal to the pitch, to some
convenient scale, and to erect another line vertically and to the
same scale equivalent to the circumference of the disc swept by the
propeller, which may be called the peripheral line. Subdividing this
line into a convenient number of equidistant parts (three or four are
sufficient for screws up to 14 in. in diameter), and connecting up
the points so obtained to the right-hand end of the base line, gives
the pitch angles at the corresponding points of the blade. It is the
subtended angles which are required, as indicated by the arrows.
Templates should be cut to these angles (which, of course, are the
angles made with the axis) with which to check the angles along the
blade during construction. This checking is more necessary with
bentwood screws than with carved ones.
CHAPTER V
Practical Construction: Bending Air-screws
Great diversity of practice exists with regard to the construction
of model air-screws, some aero-modellists favouring small diameter
with long pitch, others long diameter and short pitch, and still
others who adhere to either bentwood or carved screws in either of
the above forms. Generally speaking, a screw with a large diameter
in proportion to short span has a short pitch, say one and a quarter
times the diameter, while those having a short diameter in relation
to span should have a fairly long pitch, from one and a half to twice
the diameter. It is a useful rule to make the diameter approximately
one-third the span of the machine for either single-screw or twin-screw
machines. This relation seems to give a very small effect on lateral
stability, whereas when the diameter is made larger, the machine has a
tendency to capsize laterally in the opposite direction to which the
screw revolves. This force is known as torque.
It can, however, be fairly claimed that, for a given torque or turning
power, better results are usually obtained with carved screws, whether
short or large ones are used. The writer personally prefers a large
diameter and short-pitched screw, because, as the screw thrust is equal
to the weight of air displaced, the larger the screw the greater is the
proportion of air driven back in proportion to diameter. That is to
say, double the diameter and four times the volume of air is displaced
for only a double expenditure of power.
[Illustration: Fig. 51.—Bentwood Screws]
[Illustration: Fig. 52]
[Illustration: Fig. 53
Figs. 52 and 53.—Standard Types of Bentwood Screws]
It is difficult to speak positively on the question of the best speed
at which a screw should rotate, as the loading per square foot of
surface enters into the proposition. If a model has 1 sq. ft. of
surface for every ounce of its weight, there is a speed at which the
main surface will give a maximum of lift for a minimum of power, and
a screw must be fitted whose pitch, multiplied by its revolutions per
minute, equals the distance per minute the model should fly. If a screw
that is too fast is fitted the model will show a tendency to “stall,”
or ascend nose first, and if too slow a one is used the model will
appear to be under-powered.
The writer has outlined these points to emphasise the fact that no
definite rules, but only approximations, can be laid down, owing to the
large number of unknown quantities which would have to be taken into
consideration. As the aero-modellist, however, becomes accustomed to
puzzling out the many little problems connected with model aeroplaning,
he speedily diagnoses the complaint of a refractory machine, and
applies a remedy accordingly.
[Illustration: Fig. 54.—Bentwood Shaft Attachment]
[Illustration: Fig. 56.—Safety Hook]
[Illustration: Fig. 55.—Carved Screw Shaft]
[Illustration: Fig. 57.—Proportions of Camm Air-Screw]
The accompanying illustrations (see page 43) show the method of
making bentwood and screws. Fig. 51 is a view of a finished pair of
propellers. To the left of this illustration is given the method of
setting out the blank in terms of pitch and diameter relations. The
maximum blade width should be located one-third of the radius from the
screw tip, and should be about one-eighth the diameter. This latter,
in turn, should be two-thirds of the pitch. Inversely, therefore, the
pitch should be one and a half timed the diameter. With twin-screw
machines this may be extended to twice the diameter, or even more, but
should never exceed three times the diameter.
Fig. 52 is a view of the Camm type of bentwood screw, which has a high
thrust to power ratio. Birch should be used for bentwood screws, as
this bends easily and yet has a tenacity which is lacking in other
woods. Ash or hickory may be used as an alternative, but neither of
these is as satisfactory as birch. Before bending, the blanks should
be filled with gold size to keep the blade as rigid as possible, and
prevent it from going back or flattening out after bending.
Fig. 53 shows the Twining type of screw, which has long, narrow
tapering blades and fine pitch. Under test this has given extremely
satisfactory results, and can be recommended.
Fig. 54 shows the method of attaching spindles to bentwood screws, a
strap of tin being wrapped round the blank centre to which the shaft
is soldered. Care should be taken to ensure that the shaft is quite
central sectionally and diametrically.
A method of securing carved screw shafts is shown by Fig. 55, and
is self-explanatory. When the elastic skein is in tension it has a
tendency to pull the hooks out straight, so releasing the skein, with
sometimes painful consequences to the hand. The safety hook shown by
Fig. 56 has a brass-tube collar which slides over the end. All the
hooks should be covered with valve tubing, to prevent the elastic
cutting through.
Fig. 57 gives the proportions of the Camm bentwood blank, and will
require no explanation beyond the fact that it is bent along the dotted
lines.
CHAPTER VI
Practical Construction: Planes
There is little difference of opinion regarding the construction of
the planes of a model aeroplane, and the methods of making can be
classified under three headings—cane, wood, and wire.
There are advocates for each form of construction, and it is difficult
to state definitely which is the best practice, each having equally
good results. The wire plane, especially when steel wire of the music
or piano variety is used, is much stronger, offers less resistance
to the air, and has a neater appearance than the others, but it is
slightly the heavier. A wooden plane can also be made extremely neat
and light, although it is a little weak. Birch is the best wood to use
for this purpose, as it is extremely tough and not too heavy. Where
cane is used for the frame, pinning and gluing is out of the question,
hence binding and gluing must be resorted to. A plane so made is very
strong and flexible, and will withstand a great amount of rough usage.
It is, however, not neat in appearance and hardly to be recommended,
although many prizes have been won by models possessing such planes.
Yet another form which can be considered good practice consists of a
combination of umbrella ribbing and piano wire. This gives a very rigid
and almost unbreakable plane, but its weight for small machines is
prohibitive. It should chiefly be used for power-driven machines, power
in this instance meaning any form of motive power other than elastic.
[Illustration: Fig. 58.—Wooden Planes]
=The Wooden Plane.=—In constructing wooden planes it is usual to adopt
the method shown by Fig. 58. The spars are set out to their correct
positions but left overlapping, so that the pinning operation does
not split the ends out. The pins should be driven through to secure
the frame to bench, so that it remains true until the glue has set.
Whereupon it may be prised up with a pocket-knife, and the pins
clinched over as shown in the joint analysis A. The centre rib should
be trimmed up as shown at B, to provide a means of attachment of the
completed plane to the fuselage or body of the machine. Two spars are
sufficient for models up to 36-in. span, but over that three spars
should be used, as in the part plan (Fig. 59), or two spars spaced
closer together, as in Fig. 60, may be used, with a thread trailing
edge. This gives a neat appearance to the finished plane and greater
rigidity.
[Illustration: Fig. 59.—Three Spar Plane]
[Illustration: Fig. 60.—Two Spar Plane]
[Illustration: Fig. 61.—Cane Plane]
[Illustration: Fig. 62.—Umbrella Ribbing Plane]
=The Cane Plane.=—Another form of construction that is very light
is that shown by Fig. 61. Here a length of thin cane is bent to the
form of the outline, the ribs being bent to align with the leading
and trailing edges. Gluing and binding is used here. Such a plane can
be made light, but it always has an appearance anything but neat. It
cannot be advocated for machines over 30 in. in span.
=The Umbrella-ribbing Plane.=—Umbrella ribbing can be utilised, in
conjunction with piano wire, for plane construction as shown by Fig.
62. The channel of the ribbing should be thoroughly cleaned with emery
cloth, so that the leading ends of the ribs can be soldered therein.
Three spars should be used for spans over 30 in.
For the planes of model flying machines steel wire offers exceptional
advantages, as it is practically unbreakable and can be bent to
any desired shape. Another advantage is that it offers a minimum
resistance when travelling in the air. To the uninitiated, the making
of steel-wire planes is a difficult undertaking; but if the following
instructions are carefully carried out the planes will prove very
satisfactory.
First procure a piece of wood about ½ in. thick and slightly larger
than the plane to be made, and draw on it a plan of the plane as
shown in Fig. 63. For example, it will be assumed that a plane 30
in. span and 5 in. wide, having four ribs, is to be made. For planes
approximately this size, No. 17 S.W.G. steel wire is employed. Before
beginning the work the wire should be straightened as much as
possible. Then lay the wire over the plan, beginning at =A= (Fig. 63)
and passing round to =B=. As the wire is bent to the shape of the plan,
it must be fastened down to the board by means of small staples. Then
cut four pieces of wire for the ribs =C=, =D=, =E=, and =F=, allowing ½
in. each end for turning at right angles as in Fig. 63.
[Illustration: Fig. 63]
[Illustration: Fig. 64
Figs. 63 and 64.—Making Wire Planes]
The framework is now ready for soldering together. It is essential that
the wire and soldering bit must be perfectly clean. Apply a little
killed spirits of salt to the parts to be soldered, and then place a
piece of solder in position and touch with the hot soldering bit. Care
must be taken to see that the wires lie close together.
When the plane is soldered together remove all the staples and clean up
all the joints with a file. The joints must now be bound round tightly
with fine iron wire, which must be perfectly clean. The plane must now
be fastened to the board again, and all joints soldered again. When
the soldering is completed the plane is once more removed from the
board, straightened, the dihedral angle given, and the ribs bent to the
desired camber. If the soldering has been carefully accomplished there
is no fear of the joints giving way.
[Illustration: Fig. 65.—Swept-Back Wing]
For covering planes it is far better to purchase a waterproof silk
especially manufactured for the purpose than to attempt to use ordinary
silk. The silk varies in weight from 1 oz. to 1½ oz. per square yard.
When cutting the silk about ½ in. must be allowed for turning over for
fastening. At the curved ends of the plane slits about ½ in. apart
must be cut in the edge of the silk, as shown in Fig. 64. Apply a
thin coating of glue to the silk (use seccotine) to be turned back,
and allow sufficient time for the glue to get tacky. Then stick over
the plane, beginning at =A= (Fig. 64) and finishing at =B=. Allow time
for the glue to set, then fasten the opposite end in the same manner.
Care must be taken to stretch the silk tightly, so that it is free from
wrinkles. Then fasten first one side of the plane and lastly the other.
Another method of covering steel-wire planes is to lace the silk to the
framework. The silk must be cut about ¼ in. larger than the framework,
and the edges hemmed with a sewing machine. The silk cover when hemmed
should be slightly smaller than the framework. First sew the silk
roughly in position, and then carefully sew it, beginning at one end,
following with the other end, and lastly the sides. The stitches should
first be passed through the silk, and then round the wire at intervals
of about ¼ in.
Fig. 65 is the plan of a swept-back wire plane. The plan should
be drawn full-size on a board by means of squares, the contour of
the plane being contiguous in relation to the squares as that in
the illustration and the method outlined above followed. The ribs
should also be fitted up to the outline, being bound and soldered
to the piano-wire frame. For machines above 30-in. span a third
strengthening spar should be fixed in the position of the dotted line
to obtain rigidity. Two central ribs should be fitted to provide a
spar-attachment. The tips require to be set at a slight negative angle
as at =C=.
CHAPTER VII
Simple Twin-Screw Monoplane
The accompanying illustrations show as simple a type of model aeroplane
as it is well possible to make, excluding the now obsolete single-stick
hand-launched 1—1—P1. It is thus a suitable model for beginners,
flights of well over a quarter of a mile being easily obtainable.
The main spar (see Fig. 66) is cut from straight-grained birch, to the
dimensions given, each end of it being tapered down to ³/₁₆ in. square.
The propeller bar is of silver spruce, ⅜ in. by ⅛ in. in cross-section.
The end of the main spar is slotted to receive the propeller bar, this
latter being pinned and glued into position. The propeller bar support
is similarly slotted to take the bar, a pin being driven through the
two and clinched over on the under-side. Fig. 67 clearly shows both
joints. At 6 in. from one end the main spar is mortised to receive two
tenons which are cut on the ends of the bar supports. These tenons
should be so cut that they butt to one another in the centre of the
mortise. An idea of the shape of the tenon will be gathered from Fig.
68, a view of the joint assembled being given.
[Illustration: Fig. 66]
[Illustration: Fig. 67]
[Illustration: Fig. 68]
[Illustration: Fig. 69]
[Illustration: Fig. 70
Figs. 66 to 70.—Details of Simple Monoplane]
Two brass propeller bearings will now be required. They should be cut
from No. 20 gauge brass, a hole being drilled in each to allow the No.
18 gauge propeller shafts to rotate freely. Each bearing is bound on
with three-cord carpet thread, a portion of each being left overhanging
the bar to provide clearance for the revolution of the shaft. These
projections should be bent at an angle of 90° to the skeins of rubber,
so that the bearing faces present true surfaces for the screws to
revolve on. Details of the bearings are given by Fig. 69. Two hooks
bent from one continuous length of wire are bound to the nose of the
machine, to embrace the skeins of rubber. All bindings on the machine
should be smeared with weak glue.
In Fig. 70 details are given of the spar bracing outrigger. The
binding, for the sake of clearness, is omitted. A piece of wire
(hard-drawn brass is suited to the purpose) is passed through the spar,
a portion being bent to align with each side of this. It is then bent
outwards, the ends being pulled round a piece of No. 20 gauge wire
secured in the vice, to form eyes, through which the bracing passes.
The outrigger arm is 2 in. long, the cranked portion of it being bound
to the spar. The bracing is attached to small No. 20 S.W.G. hooks bound
to each end of the spar at the points shown. Care should be taken to
apply equal tension on each wire, or the spar will become warped.
The elevator is built from No. 18 S.W.G. piano wire. All joints are
bound with fine wire and soldered. The centre rib continues over the
leading edge, being bent downwards and backwards as at =A= (Fig. 66).
This projection fits into a hole drilled in the nose of the model;
and, being bent at an angle, the trailing edge binds on the spar with
sufficient friction to retain it in place, but yet permitting it to
swivel should it strike any object when flying.
[Illustration: Fig. 71.—Plane Fastening]
[Illustration: Fig. 73.—Elevation of Main Plane]
[Illustration: Fig. 74.—Finished Screws]
[Illustration: Fig. 72.—Detail of Plane Bracing]
The main planes are built from birch ¼ in. by ¹/₁₆ in. in cross
section, the trailing spar being bent in a jet of steam, so that its
ends sweep forward, as shown in the plan view. The ribs are pinned
and glued to the spars, the pins being clinched on the under-side.
The centre rib is left overhanging the spars, as shown in Fig. 71, to
enable the tin straps (lapped and soldered together as at =A=) to slide
over them and secure the wing to the spar. A section of the joint is
given by =B=. By removing these straps it is thus possible to alter
the disposition of the main surface, when it is desired to adjust the
elevation of the complete model. Fig. 71 is a perspective view of the
centre rib, the strap being shown black. To move the main plane each
clip is forced off the extensions of the centre rib and thus releases
the wing. Each clip is cut from tinfoil to the dimensions given at =A=
(Fig. 71), being bent to a rectangular shape, and soldered up. B is a
section of the joint. Fig. 72 indicates the method of attaching the
diagonal wing bracing, which imparts a dihedral angle of 1½ in. to the
plane. A 1½-in. dihedral means that each wing tip is 1½ in. above the
level of the spar. An elevation of the main plane is given by Fig. 73.
Two propellers, of right-handed and left-handed pitch (for the reason,
see p. 36), must be bent from birchwood 12 in. dia. × 1½ in. wide, by
¹/₁₆ in. thick; a finished view of the two screws is given by Fig. 74.
For more comprehensive details of air-screw construction see chapters
IV and V.
[Illustration: Fig. 75.—Plan of Model]
[Illustration: Fig. 76.—Outrigger Details]
[Illustration: Fig. 77.—View of Elevator
Figs. 75 to 77.—Another Simple Monoplane Design]
Some details are also given of a similar design, the difference being
that the former machine is built for distance, whilst the latter (see
Fig. 75) is built for duration. The main spar is of spruce ⅜ in. ×
¼ in. in cross-section and of the length shown in Fig. 75. The main
plane, as with the previous machine dealt with, is adjusted by means of
the tin clips and extending centre-rib; a plan of the outrigger is also
given in Fig. 76. The main plane has a dihedral angle of 1½ in. =A= is
a side elevation of the elevator, showing its angle of incidence in
relation to the spar, and Fig. 77 a perspective sketch of the elevator.
An idea of a model of this type flying can be gathered from Fig. 78.
[Illustration: Fig. 78.—Finished Model]
CHAPTER VIII
Simple Twin-screw Biplane
According to the type formula, the machine illustrated on p. 63 is
of the 1-2-p2 type, which signifies that it has two superposed main
supporting surfaces and twin screws, and that it flies with the small
plane leading. The writer, in testing the model from which the drawings
were made, found that 300 yards were easily obtainable at every flight
at an altitude of 40 ft. or so. Although its construction is slightly
more complicated than a monoplane, this is amply compensated for by its
majestic appearance in the air.
The central spar is hollow, and measures 36 in. by ⅜ in. by ⁵/₁₆ in.
Fig. 79 shows a cross-section of it. Spruce was used for this, a groove
⅛ in. by ⁹/₃₂ in. being ploughed in the spruce, and a ³/₃₂-in. strip
being glued over the open side. The spar should be tapered off from a
point 12 in. from the front end to ¼ in. by ¼ in., to give additional
strength.
The propeller bar is mortised into one end of the central spar, and is
stayed from a point 6 in. from the rear end to ½ in. in from each end
of the propeller bar, to which projection the bearings, cut from sheet
brass, are lashed. These latter are shown in Fig. 80. To the front of
the spar are bound two hooks, formed from one continuous length of
wire. These embrace the rubber, and should be covered with valve tubing.
Four birch struts should next be cut, ¼ in. by ¹/₁₆ in. in section and
7 in. long, to support the main aerofoils on the spar. They should fit
over the spar in the position shown, small blocks uniting them top and
bottom, and should be so fixed that their upper ends are 3 in. off the
spar. Notches are to be cut in them 2¼ in. below the spar, to form a
convenient fixing for the lower main plane. Fig. 81 clearly shows the
struts united to one of the blocks; the saw cuts or notches will also
be apparent from this.
A cane skid is bound to the lower blocks, and to the spar at the
position indicated in Fig. 82. A half-section of a round cane is quite
suitable for this purpose.
A plan view of the model is given by Fig. 83, from which the relative
position of ribs, planes, etc., will be seen. Fig. 84 is an end
elevation looking through from the propeller end. It will be observed
that the screws rotate from approximately the centre of resistance. The
measurements given should be carefully followed to see that this is
so in the completed model. The dihedral angle of the planes should be
made 1¼ in., which is ample to provide lateral stability. Longitudinal
stability is provided by an angle of incidence on the elevator (Fig.
85).
[Illustration: Fig. 79]
[Illustration: Fig. 80]
[Illustration: Fig. 81]
[Illustration: Fig. 82]
[Illustration: Fig. 83]
[Illustration: Fig. 84]
[Illustration: Fig. 85]
[Illustration: Figs. 79 to 85.—General Arrangement and Details of
Biplane]
The elevator is constructed from one continuous length of No. 18 gauge
piano wire. It is rectangular in plan, the joint being a soldered one
at the centre of the trailing edge. The centre rib projects downwards
for 1½ in., which projection fits into a hole bored in the nose of the
machine with just sufficient friction to retain it in place. It should
also be bent back at an angle to cause the trailing edge to bind on the
spar sufficiently to allow it to swivel in the event of it striking
any fixed object. From this centre rib the elevation of the complete
machine is adjusted. The main planes are constructed from birch ¼ in.
by ¹/₁₆ in. in cross-section, five ribs connecting the longer spars. No
camber is given to the ribs; they should be cut off 1 in. or no longer
than is necessary, pinned to the spars, so that the latter are 5 in.
apart, and cut off flush after the glue is dry. The top plane, it will
be noticed, has an overhang of 3 in. The planes are covered underneath
to eliminate the undulations which would otherwise be caused by the
ribs. Fabric should be sewn to the elevator frame.
The top plane is lashed to the struts in the manner shown in Fig. 86,
the centre rib resting on the small wooden blocks, while the bottom
plane is sprung into the notches previously referred to. Four No. 20
S.W.G. wire stanchions, with eyes bent in them top and bottom, as shown
in Fig. 87, will next be required to form an anchorage for the wing
bracing, and to maintain the “gap” at the tips of the wings. Brass wire
will do for them, and when made their ends should be forced through the
spars in the position shown in the end elevation, and then clinched
over.
Bracing the wings should next be undertaken, and carpet thread should
be requisitioned for this purpose. It is the easiest matter possible
to warp the wings in this operation, so that too much care cannot be
taken in this respect. It should be understood that the bracing is
fixed to holes in the wooden stanchions, where it must be securely
tied, and not continued to the opposite side without, or the wings will
rock laterally, and so cause instability. Sufficient tension should be
placed on the threads which pass to the wing tips of the bottom plane
to impart a 1¼-in. dihedral angle.
[Illustration: Fig. 86]
[Illustration: Fig. 87]
[Illustration: Fig. 88
Figs. 86 to 88.—Details of Biplane]
The last, and perhaps the most important, unit of the model should be
made—the propellers. Cut a pair of blanks, as shown in Fig. 88, to
shape from ¹/₁₆-in. birch to form the propellers. Strips of tinfoil
are wrapped round their centres, to which the spindles are soldered.
Bend the blades at the dotted lines under a jet of steam from a
kettle, making them to revolve in opposite directions. They rotate on
steel-cupped washers placed on the spindles.
Motive power is supplied from eight strands per side of ¼-in. strip
rubber well lubricated with soft soap emulsified with water. These
skeins will stand 650 turns each, which number should be gradually
worked up to on new rubber, and _not_ applied at the first flight. A
perspective view of the finished model is given in Fig. 89.
=Flying the Model.=—Having selected a large open space clear of trees,
give about 100 turns on the propellers in order to adjust the elevator.
If the model points its nose in the air it is elevated too much. If the
model flies too low it is not elevated enough. In each case it requires
adjustment until the precise position is arrived at. The elevator
should never be at a greater angle than 8° or less than 5°. If the
machine still flies too low with the elevator at 5° the main planes
will have to be moved forward slightly; but the exact position is only
found by experiment. If it flies too high with the elevator at 5° the
planes will have to be put back.
[Illustration: Fig. 89.—The Finished Biplane]
Much depends on the way a model is launched. The proper way is to hold
it by the propellers, with the thumb and forefinger along the main
sticks, taking care not to bend the propeller hooks; then hold at about
the angle shown in the photograph (Fig. 90). and launch as near as
possible with the wind.
[Illustration: Fig. 90.—Launching the Model]
It is necessary when flying in windy weather to launch the machine high
and smartly, as the wind has a tendency to beat it to the ground. Both
propellers must be released at exactly the same time. It should be
carefully watched while it is flying, and if it persists in turning,
say to the right, the fault will probably be that the left propeller
is more effective, or the planes on the left side of the machine are
elevated more. For straight flights it is most important that all the
planes should be in perfect alignment; but to succeed in making a
twin-propelled model fly perfectly straight is largely a matter of
perfect construction.
The model can be steered by means of the elevator (looking at the
machine from the propeller end); if the elevator is lowered on the left
side it will fly round to the left, and vice versa. The adjustment
necessary to effect this can be accomplished by means of the slot in
one of the elevator uprights.
CHAPTER IX
Winders for Elastic Motors
To a model aeroplane enthusiast a winder is an enormous acquisition.
The converted egg-beater type of winder, so much in evidence, leaves
much to be desired, the chief fault being that the bearing spindles
wear so quickly, apart from the fact that they are awkward to
manipulate single-handed. A second person is generally required to
support the model.
The winder here illustrated bears the distinct advantage that one
person can wind, keep the model in alignment with, and forced into,
the chuck simultaneously. The construction and general details will be
fairly obvious from the accompanying illustrations, so that it will
only be necessary to give a brief description.
It consists of an ash stump, 13 in. by 1 in. by 1 in., tapered at one
end, as in Fig. 91, to facilitate its being forced into the ground. A
gear and pinion (see Fig. 91), which may be requisitioned from some
of the cheaper type of clockworks, are mounted at the top end of the
stump in a casting of No. 18 S.W.G. brass, which is secured to the ash
by means of two round-headed screws (see Fig. 94). It will be found
that for general purposes a gear ratio of six to one will be most
suitable. Thus the pinion may have ten teeth and the gear sixty. The
handle should be bent to shape after being passed through the stump.
Copper ferrules are used on the spindles to keep the gears central
between the casing, as shown in Fig. 93, and should allow a little play
to ensure easy rotation. The pinion spindle must be flattened out after
the gearing is put together, and the hardwood chuck then driven on.
A glance at Fig. 92 will show clearly what is meant. The slot in the
chuck should be made sufficiently large to take a carved propeller.
[Illustration: Fig. 91]
[Illustration: Fig. 92]
[Illustration: Fig. 93]
[Illustration: Fig. 94
Figs. 91 to 94.—A Model Aeroplane Winder]
[Illustration: Fig. 94A.—The Winder]
[Illustration: Fig. 95.—An Egg Whisk]
[Illustration: Fig. 95A.—Using Twin Winder]
=A Double Winder.=—As each propeller of a twin-screw machine requires
to be wound up 400 to 500 times, it is obviously necessary to use a
geared-up winder. This can easily be constructed out of an ordinary
egg-beater, and one converted into a very useful instrument is shown by
Fig. 94A. The great advantage of using a winder of this type is that
both propellers can be wound simultaneously. Figs. 95 and 95A clearly
show how the alteration is made; it is quite simple, and all the tools
required are a three-cornered file, a drill, and a soldering bit. The
egg-beater can be obtained for a few pence at any ironmonger’s. The
two hooks at the nose of the machine are attached to the cross-pieces
on the winder, and the rubber is wound in the same direction as the
propellers revolve (see Fig. 95A). The winder shown is geared 5 to 1,
so that 100 turns on the winder gives 500 turns on the propellers.
Geared-up winders may be purchased fairly cheap.
CHAPTER X
Collapsible Monoplane
The difficulty of carrying a fairly large model to a convenient flying
ground prevents many would-be makers taking a practical interest in
model flying. The necessity of overcoming this difficulty has resulted
in several excellent designs, one of the best being the monoplane
designed and constructed by Mr. A. B. Clark, the secretary of the
South-Eastern Model Aero Club. When this model monoplane was built the
objects aimed at were extreme reliability and easy conveyance to and
from the flying grounds situated some distance away.
The model is fitted with a chassis to enable it to start off good
ground under its own power; but this starting-gear is so constructed
that the whole model will pack up flat and make a convenient parcel. In
fact, the complete model will easily go into a cardboard box measuring
2 ft. 10 in. by 1 ft. 2 in.
Referring to the accompanying illustrations, Fig. 96 shows the plan
view of the complete machine, Fig. 97 a side view, and Fig. 98 a front
view. The body (fuselage) is made of two pieces of silver spruce, 3
ft. 6 in. long, ⅜ in. deep, and ³/₁₆ in. thick. These gradually taper
towards each end, where they measure ¼ in. by ³/₁₆ in. Two distance
pieces of bamboo are shaped to streamline form and placed at equal
distances along the fuselage; the front piece is 2⅜ in., the other 2
in. These pieces should be pointed at the ends, and fit in a slot made
in the side lengths, as indicated at =A= (Fig. 99), and then bound very
tightly with glued narrow silk tape or ribbon, as indicated at =B=.
This is the neatest and also the strongest method of making joints for
model aeroplane frames. The ends of the two long lengths should be
bound together with strong thread and carefully glued.
The tail (used in place of the familiar elevator) is built on to the
rear end of the fuselage, and is composed of two pieces of yellow
bamboo, 9¾ in. by ⁵/₁₆ in by ⅛ in., tapering to ⅛ in. square at the
end to which the propeller bearings are attached. These bearings are
made of No. 18 or No. 20 S.W.G. piano wire, and their shape is clearly
shown in Fig. 100. The bearings are bound to the inner edge of the wood
with glued thread or fine flower wire. The wide ends of the bamboo
lengths are held over a bunsen (the blue flame of an incandescent
burner is very suitable), and bent to the angle shown in Fig. 96. The
trailing edge of the tail is made of No. 26 S.W.G. piano wire, or a G
banjo string. The wire is taken right through the end of the fuselage,
a small hole being carefully drilled ½ in. from the end. A bead of
solder should be run on the wire on both sides of the hole, to prevent
movement in a lateral direction, and the two ends are taken through the
bearings and bound to the bamboo with fine wire, leaving sufficient
to form hooks for the two bracing wires to be afterwards attached.
The whole of this tail framework is covered with proofed silk on both
sides, thus forming an approximate streamline surface, which has proved
remarkably efficient. Two triangular pieces of silk should be cut out,
just large enough to give sufficient overlap. They should be attached
with fish glue, and stretched as tightly as possible.
[Illustration: Fig. 96]
[Illustration: Fig. 97]
[Illustration: Fig. 98]
[Illustration: Fig. 99]
[Illustration: Fig. 100]
[Illustration: Fig. 101
Figs. 96 to 101.—Arrangement and Details of Collapsible Monoplane]
At the other end of the fuselage is attached the hooks for the elastic,
and the wire forming them is also utilised as a protector. The whole
construction is made from a 10½-in. length of No. 18 S.W.G. piano wire,
reinforced with a strip of thin bamboo, bent to shape over a blue flame
and bound with silk tape. The part is shown unbound in Fig. 101, and
Fig. 102 shows an alternative protector; but it is not so effective.
The main plane is made of bamboo and piano wire of No. 18 S.W.G., and
measures 2 ft. 10 in. by 6¼ in. The leading edge is of bamboo 3 ft. 2
in. long, ³/₁₆ in. wide, tapering to ⅛ in. at the ends, with a uniform
thickness of ⅛ in. When planed down the length should be heated over
a blue flame, and bent to the shape shown in Fig. 96, the outside of
the bamboo being kept on the outside of the curve. The trailing edge
should now be attached to the ends of the leading edge, a length of ½
in. being bent up at each end of the wire, and securely bound to the
bamboo. The ribs should now be cut from the same gauge wire. The ends
should be bent out at an angle so that they may be bound to the leading
and trailing edges, as shown in Fig. 103; the projecting ends should be
about ⅜ in.
[Illustration: Fig. 102]
[Illustration: Fig. 103]
[Illustration: Fig. 104]
[Illustration: Fig. 105]
[Illustration: Fig. 106]
[Illustration: Fig. 107]
[Illustration: Fig. 108]
[Illustration: Fig. 109
Figs. 102 to 109. Details of Collapsible Monoplane]
The two centre ribs are shaped as shown in Fig. 104. It will be seen
that the leading edge is raised ¼ in. above the fuselage, and the rear
edge is level with it. The ends of the ribs are fitted into small holes
drilled in the top of the fuselage, and kept in position by means of
small metal clips, as shown in Figs. 105 and 106. Four of the clips are
required, and they may be easily made from thin tinplate and soldered.
The ribs should be soldered to the trailing edge so as to make them
secure. The framework should be covered with proofed silk, and neatly
glued on all edges.
[Illustration: Fig. 110.—Rear Skid]
[Illustration: Fig. 111]
[Illustration: Fig. 112
Figs. 111 and 112.—Details of Screw]
The chassis is shown by Fig. 107, No. 18 gauge piano wire being used
for the framing, and an ordinary cycle spoke for the axle. Figs. 108
and 109 show the flexible joints of the chassis, which folds up flat
when the hooks at =C= (Fig. 107) are withdrawn.
The wheels are 2 in. in diameter, and are rubber tyred, the ends of the
spoke being burred over to keep them in position.
The rear skid is shown in Fig. 110, and is made of No. 18 gauge piano
wire. A single length is used, being bent to shape, passed through the
end of the fuselage, and held to its work by the projecting end =D=,
which fits in a hole in the under-side of one of the pieces of wood.
When not in use the skid may be folded flat.
The two propellers are of the simple bentwood type, 10 in. in diameter
and 1 ft. 8 in. pitch. They are made of ¹/₁₆-in. birch in the usual
way. The shape of the blades is shown in Fig. 111, and the angle at
which the blades are bent is shown in Fig. 112. Six strands of strip
rubber should be attached to each propeller, ordinary soft soap being
used as a lubricant.
CHAPTER XI
Tractor Monoplane
Some of the competitions arranged by the Kite and Model Aeroplane
Association have been for duration models of a minimum weight of 1
lb., capable of rising from the ground under their own power, carrying
a dead weight of a quarter of their own total weight. Such a model
is that here illustrated and described. It has flown repeatedly
for thirty-eight seconds after rising from the ground, while its
hand-launched duration is about half a minute, by no means a small
accomplishment for a 16-oz. model. It will be seen that it has somewhat
larger dimensions than the ordinary rubber-driven model.
The top main spar is of spruce, 4 ft. 6 in. long and ½ in. by ¼ in.
in cross-section, the bottom one being 2 in. longer and ¼ in. by ³/₁₆
in. in section. The bottom member is to be bent by steam approximately
to the shape shown in the side view of the machine (Fig. 113), and
then fitted to make a clean butt joint to the top spar =B= (Fig. 114).
Bearings for the two ⅝-in. gears, details of which are shown in Fig.
115, are bent to shape from No. 20 gauge sheet-brass, a lug being left
projecting to engage with the bottom member of the fuselage. The holes
for the shafts should be drilled so that the gears make a fairly tight
mesh. The spindles are to be of No. 16 gauge piano wire, on to which
the gears are to be soldered. The propeller shaft is continued forward
of the bearing for 1½ in., and is bent back at an angle (as shown in
detail at =A= in Fig. 116) to grip tightly in the propeller boss. The
gears are kept central between the two bearings by means of pieces of
brass tubing, which are slipped on the spindles at each side of the
bearing and soldered in position.
[Illustration: Fig. 113.—Side Elevation]
[Illustration: Fig. 114.—Spar Joint]
[Illustration: Fig. 115.—Gearing]
[Illustration: Fig. 116.—Shaft Attachment]
[Illustration: Fig. 118.—Chassis]
[Illustration: Fig. 119.—Main Plane Attachment]
Elastic hooks, formed from No. 16 gauge wire, are fixed at the rear end
of the fuselage, and serve the double purpose of providing an anchorage
for the bottom spar, which is whipped and glued with the hooks to the
top spar. Fig. 114 shows clearly what is meant. Bamboo, of ³/₁₆-in. by
₧-in. cross-section, is used for the tail skid, which is secured to the
fuselage by thread binding. Piano wire is used for the landing chassis,
and of the same gauge as hitherto used on the spindles.
The triangular side struts of the chassis should first be framed up
from one continuous length of wire, lugs being bent at the point
where they meet the fuselage, to be bound with fine tinned iron wire
and soldered to the spar. Figs. 113, 117 and 118 clearly show its
construction. Make the axle of such a length that a 12-in. wheel-base
may be left after the wheels are placed on. The writer found that
2-in. rubber-tyred disc wheels left nothing to be desired for rising
off short grass. Care should be taken that the measurement from the
periphery of the wheel to the top spar, measured, of course, in a
vertical direction, shall be 9 in. (see Fig. 113).
[Illustration: Fig. 117.—Plan]
[Illustration: Fig. 121.—Kingpost]
[Illustration: Fig. 122.—Wing Bracing Attachment]
As twin gears of equal size are used, the torques of the oppositely
revolving skeins of rubber will be balanced. Hence no bracing will be
found necessary on the motor spar. The gearing, by the way, is bound to
the top and bottom members of the fuselage with tinned iron wire, and
soldered as shown in Fig. 115.
The main plane is of rather a large span, and it is essential that
birch be used, ½ in. by ³/₃₂ in. in section. The wing spars are bent
at their centres, to impart a tapering wing plan analogous to the
Martinsyde monoplane. Seven ribs connect the spars, and these are
cambered to ¾ in. The wing tapers from 10 in. at the centre to 7 in.
at the tips, the centre rib projecting for ½ in. fore and after of the
wing. The tin clips shown in detail in Fig. 119 slip over these, and so
provide a means of adjustment to the centre of pressure of the complete
machine.
Choice of covering must be left to the builder; but yellow Japanese
silk, proofed with varnish, will be found quite suitable, and of a
rather pleasing amber hue. A birch kingpost passes through the fabric,
and to this the wings are braced by No. 35 gauge music wire. Sufficient
tension should be placed on the top wires to give the wings a 3-in.
dihedral angle, as in Fig. 120. The detail illustration of the kingpost
(Fig. 121) is self-explanatory. The bracing wires are anchored to wire
hooks forced through the wing in the manner shown in Fig. 122.
The correct position of the main plane should be found by trial. The
kingpost can then be permanently fixed to the main spar by pinning and
gluing.
No. 18 gauge wire of the music or piano variety should be used for the
tail and rudder. Draw the plan form of the wing full-size on a board;
pins may then be driven partly home on each side of the line at spaces
of about 3 in. The wire may now be pushed between the pins, cut off,
and lapped for ½ in. The two cross ribs can be soldered to the tail
before the tacks are withdrawn. It will be found on releasing the tacks
that the wire will remain true to the shape of the template. This may
seem a rather laborious process; but it is far quicker and easier than
attempting to guess the correct curvature. The rudder may be made to
any convenient shape, preferably that shown. The two ends should be
sprung outwards after the form of the letter =A= to form a clearance
for the rubber hooks, and then soldered to the tail. A very slight
adjustment of this will be found necessary to obviate propeller torque.
[Illustration: Fig. 120.—Front Elevation]
No provision has been made for the adjustment of the lift on the tail.
Indeed, none was found necessary, it being quite an easy matter to
bend the tail flaps up or down to increase or decrease the elevation.
They should always, however, have a slight negative angle to maintain
longitudinal stability. The tail should be bound to the fuselage with
copper wire.
The propeller, of the usual integral type, is carved from the solid
block, which measures 15 in. by ⁵/₈ in. by 2¼ in. It should be made
of right-handed pitch, and must be placed on the right-hand gear, so
that thrust balances torque. On each side, nine strands of ¼-in. strip
rubber, lubricated with diluted soft soap, supply the motive power.
This will stand approximately 600 turns. Vaseline will suffice to
minimise friction on the gears.
It is advisable to test the model down the wind with about 50 per
cent. of the maximum turns. The main plane should be moved forwards to
increase the elevation, and backwards to decrease it.
[Illustration: Fig. 123.—Complete Tractor Monoplane]
Fig. 123 shows the model in perspective.
CHAPTER XII
Hydro-monoplane
The present machine is capable of making a flight of about sixty
seconds after rising from the water, which it does after travelling
from 8 ft. to 10 ft. under normal conditions. This model is what is
known as the “A” frame (see Chapter III.) type of monoplane fitted with
a loaded elevator.
The framework or fuselage is not constructed in quite the orthodox
manner, but in the manner shown in section at =D= and =E= (Fig. 124).
These side members are made from two pieces of best silver spruce 3
ft. 3 in. long, ⅜ in. by ¼ in. at the forward or elevator end, ½ in.
by ¼ in. in the middle, between the elevator and the main plane, and
⅜ in. by ³/₁₆ in. at the propeller end. This tapering is necessary in
order to make the wood proportionate to the strain to which it will
be subjected. This is called a cantilever. After each stick has been
planed to the above sizes, a hollow chisel is used to channel out the
wood on one side, the finish being given with a woodworker’s file.
The opposite side is rounded off after the inside is finished. The
front ends are bound together and glued, the forward hooks (to which
the rubber is attached), and the protector, shown by Fig. 125, being
incorporated at the same time; these are made of No. 18 and No. 20
S.W.G. piano wire respectively.
The other extremity of the fuselage is held 9½ in. apart by means
of a bamboo distance strut, measuring ³/16 in. by ¹/16 in. This
strut, together with the three others, is carefully shaped, the
ends sharpened, and then fitted into a split in the side members as
indicated in Fig. 126. Considerable care is needed in making this form
of joint; but when the joints are glued and bound over with ¼-in. strip
silk, they are wonderfully strong.
The propeller bearings are made of No. 18 S.W.G. wire, and resemble
a lady’s plain hairpin bent at right angles midway, with a cupped
washer soldered on the round end to take the thrust (see Fig. 127).
These washers, known as French clock collets or cupped washers, may
be procured at any watch repairer’s at 3d. per dozen. The propeller
bearings should be bound to the fuselage at the same time as the end
distance piece is fixed. The frame is trussed with two diagonal bracing
wires; No. 30 S.W.G. piano wire should be used, this being strained
with the aid of hooks as shown in Fig. 128. To tighten the wire, twist
the hooks with a pair of round-nose pliers.
The main plane is 37 in. in span, with a maximum width of 7 in. at
the centre, tapering to 6 in. at 3 in. from the tip. The camber is ¾
in. at the centre and ⅜ in. near the tip. The frame is constructed
of bamboo, the leading edge and the end ribs being one long piece of
selected yellow bamboo, ³/₁₆ in. by ³/₃₂ in., and is bent to the shape
by holding over an incandescent gas burner. The trailing edge is made
of similar material; but is straight when looked at in plan. This piece
measures 34 in. by ³/₁₆ in. by ³/₃₂ in., and is joined to the end ribs
as shown at =C= (Fig. 129), afterwards being bound with strip silk. The
ribs are all ³/₁₆ in. by ¹/₁₆ in., being bent in the manner suggested
above and split-jointed into the spars.
[Illustration: Fig. 124.—Plan]
[Illustration: Fig. 125.—Nose of Model]
[Illustration: Fig. 126.—Cross Member Joint]
[Illustration: Fig. 127.—Bearings]
[Illustration: Fig. 128.—Bracing Anchorage]
[Illustration: Fig. 129.—Rib Attachment]
The main plane has a dihedral angle of 1 in 7; that is, the tips are
about 2⅝ in. higher than the centre of the plane. The plane is covered
with proofed silk secured with fish glue.
The elevator is a miniature edition of the main plane, being 13 in. in
span and provided with a chord of 3½ in. The camber is ⅜ in. at the
centre and ³/₁₆ in. at the tips. It is attached to the framework in the
following manner: Two straight pieces of thin bamboo 5 in. long are
attached to the under-side of the elevator, and run parallel to the
tapered end of the fuselage, and the projecting ends of these pieces
are attached to the frame with elastic bands. To give an increased
angle, blocks of wood are placed under the bamboo strips as shown in
Fig. 125, these pieces being ¼ in. high. A greater or less angle may be
given by moving the blocks either backwards or forwards as required.
[Illustration: Fig. 130.—Side Elevation]
[Illustration: Fig. 131.—Floats]
[Illustration: Fig. 132.—Rear Float]
[Illustration: Fig. 131A.—Rear Float]
[Illustration: Fig. 133.—Screw Eye]
[Illustration: Fig. 134.—Rear Elevation]
The floats are three in number, and are of equal dimensions, 6 in. by
2 in. by ¾ in. at the greatest depth, which is about three parts of
the way from the front. To construct the floats, two side pieces of
¹/₂₀-in. birch are cut to the shape given in Figs. 130 to 131A, and
these are nailed to the ends of a piece of whitewood measuring 2 in.
by ¾ in. by ⅛ in. Joining each end of the side pieces is a piece of
whitewood, cut to fit so as to form a nice entry, the forward piece
being flat on top and the rear piece flat on the bottom. At the place
marked =G= (Fig. 130) there is an additional piece to strengthen the
float and keep the silk covering taut. Running from front to back there
is a ⅛-in. by ¹/₁₆-in. strip of bamboo to keep the silk from sagging
when running on the water. The front floats are nailed to the bamboo
cross-piece, and the adjustment is made by bending the small pieces of
piano-wire =F=. The rear float attachment is shown by Fig. 132, and the
adjustment is made in the same way. The wire is attached to the floats
with small hooks as shown by Fig. 133, these being screwed into the
distance pieces of the float. The connecting wires are bound to the
fuselage with strip silk and glued. The flat portion of the under-side
of the front floats has an angle of incidence of 1 in 6, the rear float
angle being 1 in 8.
Another type of float is quite practicable, if preferred.
[Illustration: Fig. 135.—Model Hydroplane]
The propellers are 10 in. in diameter with a pitch of 23 in., and are
carved from a solid piece of mahogany. The blades are glasspapered to
a thickness of about ¹/₂₀ in., and are strengthened with silk stuck on
one side.
The floats are covered with the same proofed silk as the planes; but to
ensure complete imperviousness to water, coat with a mixture of 2 parts
of boiled oil and 1 part of gold size. Fig. 134 is a rear elevation.
The main plane rests flat on the fuselage, and is held in place by
means of two 9-in. by ⅛-in. by ¹/₁₆-in. pieces of bamboo, which are
secured to the framework with elastic. This method allows the plane to
be readily removed. There are six strands of thick ¼-in. strip elastic
to each propeller, and the number of turns given is about 900 when well
lubricated.
The total weight of the complete model is only 6 oz., and in making the
machine every effort should be made not to exceed this amount.
Fig. 135 shows a model hydro-monoplane in perspective.
CHAPTER XIII
Compressed-air Engine for Model Aeroplane
Signs are not wanting that compressed air as a motive power for model
aeroplanes will become equally as popular as the twisted skein of
rubber, which has practically held the field since it was introduced
about the year 1870 by Alphonse Penaud.
One of the chief disadvantages of the rubber motor is that experiments
of a full-size scale nature cannot be undertaken, owing to the length
of frame required in order that the necessary power and duration of
run may be obtained, and also owing to the disposition of weight,
and consequently of the centre of gravity, not being tantamount to
that obtaining in full-size practice. With a compressed-air plant
these disadvantages are eliminated, since the weight can be kept well
forward, thus making possible the designing of a model which represents
in essential proportions a full-size machine.
Particulars are here given of a highly successful plant, for which the
machine described in the next chapter was especially designed. Several
of the illustrations in this present chapter are exaggerated to render
the construction clear, and it is thought that the details given will
be found comprehensive.
[Illustration: Fig. 136]
[Illustration: Fig. 137]
[Illustration: Fig. 138]
[Illustration: Fig. 139
Figs. 136 to 139.—Arrangement and Details of Engine]
Fig. 136 gives a plan view of the engine, which is rotary with, of
course, a stationary crank-shaft. The five cylinders are soft-soldered
to what may be termed the crank-case, which consists of two circular
brass discs of the gauge indicated. In order that the cylinders may be
accurately located round the plates, a wooden jig should be made with
slots to receive the cylinders, and a recess to take the plate. Five
lightening holes are drilled in the two plates as shown. The front
plate, that is, the one carrying the propeller bolt, is, however, left
off until the pistons, crank-shaft, and sleeve are assembled.
[Illustration: Fig. 140.—Details of Crank]
[Illustration: Fig. 142.—Connecting Rod]
[Illustration: Fig. 141.—Section of Cylinder]
[Illustration: Fig. 143.—Piston Tongues]
Fig. 137 gives a side elevation of the engine, with the crank-shaft and
sleeve shown in section. It will be seen that the sleeve butts to a
shoulder, a slight undercut being given to the shaft when turning this
portion to ensure a good joint. From this figure the inlet and exhaust
principle will be manifest. It will be noticed that as each inlet
pipe coincides with the right angular inlet in the shaft, so does it
receive a charge of compressed air. The pressure on the piston revolves
the engine, thus shutting off inlet to that particular cylinder and
bringing the next cylinder in line with the inlet. As soon as the
first cylinder nears the bottom of its stroke it begins to exhaust
through the diametrically opposed exhaust port. Needless to say, the
crank-shaft and sleeve must be turned a good running fit, otherwise
there will be considerable waste of power. The best method to employ is
to turn the shaft a push fit within the sleeve, and then to grind it in
with rottenstone. When soldering the inlet pipes into the sleeve, care
must be taken to ensure that they do not become “choked” with solder.
The sleeve should afterwards be reamed out to remove all superfluous
solder. When soldering the sleeve into the back plate care must also be
exercised to ensure that it is truly at right angles to the plate.
It must be clearly understood that the engine revolves with the sleeve
as a bearing. The five holes which are drilled round the sleeve to
receive the inlet pipes must be equidistant, so that the periods of
inlet are synchronous.
Fig. 138 gives an enlarged view of the crank-shaft and sleeve, and is
self-explanatory. Observe that the exhaust port is larger in diameter
than the inlet.
Details of the pistons are shown by Fig. 139. The connecting-rods
are soldered to tubular distance pieces, which rock on the ¹/₃₂-in.
silver-steel gudgeon-pins, which pass through the pistons, being cut
shorter than the outside diameter of the piston to avoid possible
scoring of the bore of the cylinders. The gudgeon-pins are soldered
into position, the superfluous solder being scraped from the piston
walls. To ensure airtightness of the pistons and cylinders, cupped
leather washers are fixed to the piston-heads by means of tin tongues
soldered to them, and which are forced through the washer and bent
over. The ordinary cycle-pump washer is admirably suited to the
purpose, but the height of the washer when within the cylinder should
not exceed ⅛ in.
[Illustration: Fig. 144.—Container]
Fig. 140 gives dimensions of the crank and throw, to exaggerated scale,
to avoid crowding the details. The important point to bear in mind when
beginning this portion of the construction is to obtain the correct
stroke, since the cylinders are designed to take a stroke of ½ in.
only. See also that the crank-pin revolves truly, that is, at 180° to
the shaft.
Fig. 141 is a longitudinal section of the cylinder. As there shown, the
cylinder-head is “let in” the head and soldered there. The inlet pipes
should be packed with resin prior to bending, this being afterwards
melted out. The connecting-rods are shown by Fig. 142, the important
dimension, obviously, being the centre distance of the holes for the
crank-pin and gudgeon-pin respectively. These are of No. 20 B.W.G.
brass. The tin clips used to secure the cupped leather washers to the
piston head (four of which are used for each piston, so that twenty in
all will be required) are shown by Fig. 143. They are of No. 30 S.W.G.,
and are bent along the dotted line to a right angle, the ⅛-in. portion
being the end to be soldered to the piston.
The compressed-air container shown by Fig. 144 is made from copper
foil of the thickness shown. This is folded round a wooden former of
circular cross-section, and tied tightly in place while the lapped
joint is being soldered. The two faces of the joint that are in contact
should first be tinned, using Fluxite or resin as a flux; spirits of
salt should on no account be used, as this has a deleterious effect on
metal of so fine a gauge; and a mediumly heated iron should be used to
solder the joint.
Wind the body with the No. 35 S.W.G. piano wire, soldering each spiral
at each revolution so that it maintains its correct pitch. Now attach
one of the half-balls (which for preference should be provided with a
stepped flange as shown) while the body is still on the wooden former,
first tinning the two surfaces in contact, and then “running” the
solder round with a mediumly heated soldering bit, and so sealing the
joint. Prior to attaching the second half-ball to the other end, a
tension wire must be attached to the flange, either of the valve or the
tap (according to which half-ball was attached first), by soldering.
This is then passed through the body of the container (the wooden
former, of course, now having been removed), and threaded through a
hole drilled in the half-ball at a convenient point near the centre.
Tension is now applied to the wire and the second half-ball eased into
position, and while still pulling on the wire it is soldered into the
hole through which it passes, afterwards being cut off sufficiently
long to form a coil on the end.
It will, of course, be clear that the valve (of the Lucas type) and
tap are soldered to the half-ball before the latter are affixed to the
container body.
The container should be inflated and immersed in paraffin to test for
leakages, and when these are stopped up the container and engine may
be connected by a short length of tubing. The engine is then ready for
running. Thin machine oil should be used for lubricating purposes, and
where necessary the connecting-rods must be staggered for clearance.
In conclusion, it should be pointed out that the plant should not weigh
more than 10 oz. complete, and is capable of flying a machine weighing
2 lb., provided that it is efficiently constructed. The container
should be inflated to a pressure of not less than 100 lb. Fig. 145
shows a similar compressed-air model aeroplane engine complete.
[Illustration: Fig. 145A.—Compressed-air Plant for Model Aeroplane]
The accompanying photographic reproduction shows a model compressed-air
plant for model aeroplanes which is similar in general design to the
one illustrated. The difference is that inlet takes place through
hollow connecting-rods, which are ball-ended and fit into ball
seatings. The cylinders oscillate, and the connecting-rods, being
rigidly attached to the pistons, by their angularity during revolution
form the inlet and exhaust mechanism. The propeller is geared up in the
ratio 2: 1.
[Illustration: Fig. 145.—Three-cylinder Engine]
The design is analogous to a very early French engine much in evidence
in the early days in model experiments.
It is thought that the photograph will give the reader an idea of the
general arrangement of the plant previously described.
=Driving Small Biplane=.—Fig. 145 is a front elevation, with the front
plate removed, of a 3-cylinder engine on similar lines, which would
be sufficiently powerful for models up to 12 oz. weight. As will be
obvious, the three cylinders are fixed (by solder) to two circular
discs of No. 20 gauge brass forming the crank chamber. The cylinders
and various component parts should be assembled _before_ the front
plate is fixed. A length of brass tube is soldered into the back plate,
and equidistant round its periphery three ⅛-in. holes must be drilled
to receive feed pipes which pass to the cylinder heads. A piece of
brass rod to form the crank-shaft must be turned to make a good running
fit within the tube and inlet, and exhaust holes drilled as indicated
by the dotted lines. The pistons should be made an easy fit. Pieces of
by-pass tubing are soldered into the small ends of the connecting-rods.
Through these tubes pass the gudgeon-pins which are anchored to the
piston walls. The position of the connecting-rods in relation to the
piston is thus maintained. The container (into which air is compressed
with a foot pump to from 100 lb. to 120 lb. per square inch) is
constructed from copper foil of three-thousandths (·003) of an inch
thickness, and is of the same dimensions as the five-cylinder one.
CHAPTER XIV
Biplane Driven by Compressed-air Engine
The model aeroplane illustrated by Fig. 148 has been designed to
suit the compressed-air plant fully illustrated and described in the
preceding chapter. It is from the results obtained from the testing
of the plant that the dimensions of a suitable model for it are
determined; and while the design may suit the majority of the plants
constructed from the illustrations shown in pp. 95 to 101, it is
chiefly given to show the correct method of designing a “power-driven”
machine, since the power unit (unlike the elastic motor) cannot be
varied, and recourse to some established line of reasoning becomes
essential.
The first thing to do, then, once the plant has been “tuned up,” is to
ascertain the thrust obtainable from it. This is found by suspending
the plant by the valve on a balance, with a container fully inflated,
the weight registered being carefully noted. The container pressure
should now be released, and the weight registered when the motor is
running observed. By subtracting the former from the latter the thrust
is obtained.
Thus, assuming the plant, at rest, to weigh 8 oz., and when running 12
oz., it is clear that the thrust is equal to 4 oz. Now, it is necessary
to know the _average_ thrust developed, since, as hitherto explained,
the thrust is not constant, but gradually diminishes as the density of
the air in the container approaches normal atmospheric conditions; that
is, 14·6 lb. per square inch (known as an atmosphere). It is possible
to obtain some very interesting data by plotting a graph of the thrust
given off at various moments from the release of the pressure in the
container. Meanwhile it can be taken as a good rule that the thrust
registered after one-third of the effective run of the motor represents
approximately the average thrust; and the figure given above (4 oz.)
will serve for the purpose of illustration.
It is next necessary to know the weight of the model it will lift. It
is well established that a plant will fly a machine weighing from four
to six times the weight of the thrust it develops, although, of course,
much depends on the efficiency of the model; the greater the complexity
of frame members the lower the lift drag ratio, and consequently
the lower the ratio between the thrust and the weight of the model.
Compromising, and taking 5: 1 as the ratio, 20 oz. is obtained as the
total weight of the plant and model.
The next point to be decided on is the loading, and as the model is to
be a biplane a comparatively light loading can be used. In the case of
the machine shown in side elevation by Fig. 146 and in front elevation
by Fig. 147, 4 oz. has been taken as the loading per square foot. So
that the total area of the wings will be
20 = 5 sq. ft. = 720 sq. in.
----
4
A span of 54 in. for the top plane and 46 in. for the bottom one has
been decided on, and by using a chord of 8¾ in. the total area of the
wings vies very approximately with this figure, allowing a small margin
for excess weight. The area of the tail, which is non-lifting, need not
be taken into account. Although the “gap” is given as being equal to
the chord, it could be made, if anything, ½ in. greater.
Now with regard to actual materials. Birch is to be used for the
longitudinals, straight in the grain and of the cross-sections
illustrated. The lower member is bent under steam to the curvature
shown—of 6½-in. radius. Two vertical struts support the wings, and
these should be cut from hickory. A short tie-strut secures the bottom
longitudinal to the front inter-strut, the joint being made by means of
side angle-plates bound into place. It will be found good practice to
make a full-size drawing of the machine in side elevation, so that it
can be used as a template to fit up the cross members—particularly with
regard to the cutting of the angles.
The joint of the longeron to the cross member is shown separately at
=A= (Fig. 146). The usual fish-plates are employed, so made that a
small wiring plate is left protruding from the binding, to which the
cross-sectional and longitudinal-sectional wires are made off.
[Illustration: Fig. 148.—Plan View of Biplane Driven by Compressed Air]
The plant itself is slung into the framework by means of eight wires,
each being made off to the wiring plates. Each should also be provided
with a small ¾-in. wire strainer to enable the plant to be fixed quite
rigidly—albeit permitting of its being removed for inspection or
repairs. The wires from the engine itself are taken off from the four
small eyes soldered to the stationary portion of the crank-shaft. Great
care should be taken to ensure that the plane of rotation of the screw
is at right angles to the main planes. A 1¼-in. dihedral is given to
the bottom plane by means of the bracing wires passing between the
inter-struts, and shown on the preceding page.
It has been thought advisable to attach a small rear wheel, to enable
the model to rise off the ground with as little loss of power as
possible. Such a wheel, with attachments, need weigh no more than ¼
oz., and is a great improvement over the cane skid usually employed.
[Illustration: Fig. 146.—Side Elevation of Biplane Driven by Compressed
Air]
In bracing the outriggers, or longerons, some care will be required to
ensure their being quite true. It will be easier to finish each section
off first, so that they are quite parallel at the joints.
The part plan view of the model (Fig. 148) will make the relative
position of the various component parts quite plain. The two top tail
outriggers pass through the fabric at the point where the spar is
located, their front ends being pinned and cross bound to the wing
spar, which is made of greater cross-section in the centre, so that its
strength is not materially impaired through the piercing of it. Birch
is to be used for the wing spars and ribs of the sections indicated.
The planes are ribbed at periods of 6 in. and given a camber of ¾ in.,
the greatest depth of which is 2¼ in. from the leading edge. It is far
easier to impart the camber after the wing framework is made than to
camber each rib separately. Each rib should be cut 1 in. longer than
necessary, and pinned and glued to the spars, with ½ in. overlapping
each of these latter. When the glue is quite set, the pins may be
clinched over by supporting the wing on an iron weight and tapping them
back flush to the spars.
The full-size section of the camber should be drawn upon a board, with
which to check the accuracy of the first rib to be cambered (the end
rib).
The ribs are cambered in a jet of steam, the convex or top sides being
placed nearest to it. Having cambered the end rib carefully to agree
with the drawing, the others may be matched to it. It will thus be easy
to ensure that every rib is of the correct curvature, as any mistake in
the steaming of the rib will distort the wing spar at the point of its
attachment.
If, however, it is thought advisable to camber the ribs first, a
wooden bending jig should be made, to enable several ribs to be bent
at one operation. The ribs should be tied down to the jig with string,
and thus held under the steam jet, being well dried in front of the
fire before they are detached from the jig. All three spars pass
_underneath_ the ribs.
A very light fabric should be chosen, such as can be obtained from the
model-aero accessory warehouses, or an unproofed Japanese silk can be
used and varnished when on the wing. If this latter is used, it will
be found advantageous to use a yellow hue, as this colour is least
affected by the action of the varnish. But the covering of the wings
must be left for the time being, for the reason that the sockets to
which the inter-struts are made fast must first be attached. Further,
the top plane must be covered after the tail outriggers have been
assembled, as it is so much easier to make the joint between the wing
spars and these latter before the fabric is attached.
To render it unnecessary to refer to the point further, it may be
noted that the fabric is brought over the leading spar of each wing to
pocket it out. It is much neater to sew the fabric along on the leading
edge, as when glue is used an unsightly black smear shows through. The
fabric should be stretched from end to end first, the fabric overlaps
being glued on the bottom face of each end rib. Drawing-pins should be
partially pressed into the ribs to secure the fabric until the glue is
set.
At =B= in Fig. 148 is shown the method of securing the bottom plane to
the inter-struts. Convenient notches are cut in the struts into which
the plane is sprung. It will have been noticed from the side elevation
(Fig. 146) that the width of the inter-struts increases towards the
bottom or lower ends, and also that they incline slightly; this is
to provide for the entry of the lower plane, since the top plane is
attached _outside_ the struts, while the bottom is placed _inside_
them. At =C= is shown the method of attaching the inter-struts.
The tail is built up from split bamboo, ⅒ in. by ³/₃₂ in. in
cross-section, and the rudders are framed up from No. 20 gauge piano
wire. The ends of the rudder frames are forced through the longerons,
and the ends bent back in alignment with them; they are then bound to
the longerons with black three-cord carpet thread. The rudders are
covered after being fixed to the outriggers. When it is necessary to
adjust them, the piano wire will be found sufficiently ductile to admit
of a warp being placed thereon.
[Illustration: Fig. 147.—Front Elevation of Compressed-air-driven Model]
Eleven ribs connect the spars of the top plane and nine those of the
lower, the camber of each being the same; that is, the same depth of
camber is maintained throughout. Before the wings are covered, the
angle-plates to which the inter-struts are fixed must be bound on; and
these are cut from No. 30 gauge sheet tin. They should be cut less in
width than the spar to which they are attached, in order that their
sharp edges shall not cut through the binding. To prevent the plates
from moving, they should be lightly sunk into the wing spar with two
centre-punch dots, and a film of glue should also be spread over the
face of the plate coming in contact with the spar.
The inter-struts are streamlined in cross-section (see Fig. 149); but
they are to be left rectangular in section at their ends, to provide a
flat surface for the plates to bed home on. The ends of the plates are
turned back over the binding, which may be of the light machine variety.
The lower ends of the inter-struts are cut off to the same angle as the
dihedral on the lower plane, to avoid distortion of the plane. Spruce
or American whitewood may be used for them, the greatest cross section
being ½ in. by ⅛ in. The greatest cross-section is situated at the
middle of the strut, whence it tapers to ³/₁₆ in. by ⅛ in. Fig. 149
shows the attachment of the inner strut to the wing spar. In Fig. 150
are shown the brackets forming the guides for the axle, and also the
supports for the rubber shock-absorbers. Piano wire is used for them.
The width of the guide should be such that the umbrella-ribbing, which
constitutes the inner portion of the axle, rides freely within it. The
wheel axles are cut from No. 16 gauge piano wire, and they are soldered
to the umbrella-ribbing, being sunk into the channel of the latter,
bound with No. 30 gauge tinned iron wire, and then soldered.
Ordinary elastic, as used for a rubber-driven model, can be used as the
shock-absorber, and it should be neatly and fairly loosely bound to the
vertical guide, the axle of course being first seated therein. In order
that the absorber brackets may maintain a vertical position, their ends
are shaped to a form similar to the letter =U=. They are wire-bound to
the skids and lightly soldered.
[Illustration: Fig. 149.—Details of Inter-struts.]
[Illustration: Fig. 150.—Detail of Axle Guides.]
[Illustration: Fig. 151.—Outrigger and Wing-spar Joint.]
[Illustration: Fig. 152.—Tail Crossbar Joint.]
[Illustration: Fig. 153.—Detail of Front Skid Angle-plates.]
[Illustration: Fig. 154.—Tail Incidence-quadrant.]
[Illustration: Fig. 155.—Details of Axle.]
[Illustration: Fig. 156.—Rear Chassis Details.]
[Illustration: Fig. 157.—Inter-strut and Longeron Joint.]
The bottom plane only must be covered; it will be easier to cover the
top plane when the machine is assembled, for it would be a difficult
matter to secure the top outriggers to the spars were the fabric
attached.
Having assembled the outriggers and completed the bracing of it, it
will be possible to attach it to the wings.
Small elliptical holes are cut in the fabric of the lower wing, through
which the central supports or stanchions pass, and the bottom plane
is seated home in the notches alluded to in Fig. 148. Next, the top
outrigger ends are fitted up, being cut off to correct length and
halved on to the wing spars, as shown in Fig. 151. The vertical support
is then glued, pinned, and cross-bound to the outrigger.
Great care will be necessary to ensure that the outriggers are quite
central with the planes. A point to be made clear is that if in the
fitting of the top outriggers one is cut even ¹/₃₂ in. short, the
tail end of the machine may be ¾ in. out of centre. In order to check
inaccuracy in this direction it would be advisable to mark the centre
of the horizontal tail member, insert a drawing-pin, and take the
measurement to the corner of the wing tip, on both sides of the model;
the outriggers should be temporarily lashed to the wing spars, and
gradually adjusted until they are located centrally with the planes.
Perhaps it may be interesting to here mention that this is the method
employed in locating the fuselage of full-size machines.
The bracing of the planes should now be undertaken. All lift wires
should first be fixed, beginning from the wing tips. Just sufficient
tension should be placed on each wire to ensure rigidity. A wooden
straightedge should be used to reveal any distortion of the spar. The
top plane must be given a slight dihedral, so that when the anti-lift
wires are inserted it assumes a perfectly parallel position.
[Illustration: Fig. 158.—Biplane Driven by Compressed Air]
The upper and lower longerons are spanned at the tail end with light
spruce cross-bars, ⅛-in. by ¼-in. section, which are let into mortises
cut in the longerons; and two vertical posts are halved on to these
cross members (see Fig. 152), to provide the fulcrum about which the
tail swings in the quadrant, to be referred to presently. They are
spaced 4 in. apart, which is equivalent to the distance between two
ribs; and on the outside of them a groove is cut in the centre of each
to provide a seating for the two central tail ribs. These grooves must
be cut =V=-shaped, the apex of the =V= facing the trailing edge of the
post. The object of the groove is to form a guide for the tail when it
is desired to alter the angle of it. A pin should be driven through
the rib and into the groove to constitute the pivot on which the tail
swings; and the ribs must be bound with fine thread on each side of
the pin to prevent the rib from splitting. It will be found that it is
better to bind the ribs before inserting the pin.
The central inter-struts are attached to the skids by angle-plates,
and in Fig. 153 the form of these is given. It must be understood that
there are two plates to each joint, one on each side of it, and for
neatness and simplicity they can be cut from one piece of tin, both
plates being thus formed in the one. No. 30 gauge tinplate is suitable.
The plates are pinned, clinched, and bound into place, and constitute
an exceedingly rigid piece of construction, which is needed in this
portion of the machine, bearing as it does the impact of landing. Glue
should be neatly brushed into all the joints. The tie-strut is to be
streamlined as far as practicable without materially impairing the
strength of it.
A very neat finish can be given to the binding if it is just brushed
round with japan black, which shows up in pleasing contrast to the
light brown varnish with which the framework is coated.
Fig. 154 gives the shape of the quadrant, which makes possible the
variation to the angle on the tail. It is cut to a radius of 4 in.,
and is pinned into position. The pitch of the teeth is ⅛ in., and this
facilitates a very fine adjustment. It should be so fixed that the
tail springs tightly into notches, but not so tightly as to render
adjustment difficult. Trial and error will be found the best method of
locating its position.
[Illustration: Fig. 159.—Compressed-air-driven Monoplane]
It was mentioned in the preceding chapter (see pp. 94 to 103) that the
axle is composed of two portions, umbrella-ribbing and piano wire,
and Fig. 155 shows the construction. It will be seen that the piano
wire beds into the channel (which is fixed in a trailing position),
wherein it is bound and soldered. The wheels are spaced apart by means
of small brass-tube collars, soldered to the piano-wire axles in their
respective positions. The axle itself, as mentioned earlier in the
chapter, is attached between the shock-absorber brackets, being held
there by means of suitable radius wires secured to any convenient part,
the rubber binding forming the absorber. The radius wires are essential
in order to maintain the lateral position of the axle relatively to the
planes. Sufficient rubber binding is to be used to absorb the shocks
the model is bound to receive, the exact quantity, of course, being
impossible to define.
The rear wheel members are fixed to the longerons in the following
manner. The ends are bent back parallel to align with the frame member.
The apices of the =V= chassis members are soldered to the short axle
carrying the back wheel, the axle being cut a length suitable to the
hub of the wheel. No. 20 gauge wire is used for all portions of the
rear chassis. Fig. 156 makes this clear.
Fig. 157 shows the joint of the trailing central inter-struts to the
top longeron. It will be seen that the joint is a halved one, pinning
and binding forming the security.
All woodwork may be polished by filling the grain with gold size, and
finishing with a good varnish.
In flying the model the writer would point out that full pressure
should not be given to the plant until adjustment has been completed,
also the importance of tuning the machine by starting it from the
ground, thus obviating many vexing smashes. Further, the rudder must
be set to counteract torque; if the screw is of left-hand pitch, then
torque will tend to bank the machine to the right, and the rudder must
therefore be set to the left, and vice versa.
A sketch of the finished machine is given by Fig. 158, and a design for
a monoplane driven by the same plant in Fig. 159.
CHAPTER XV
General Notes on Model Designing
=Calculations in Designing a Plant for a Model Aeroplane.=—The correct
method to adopt in designing a model is to build the machine exclusive
of the motor, weigh it, and design the plant to suit; or to build the
plant first, determine the thrust it develops, and vary the dimensions
of the machine (and hence the weight) to suit.
It can now be taken as a general rule that a plant will fly a machine
three times the weight of its thrust. Hence a plant developing 3-oz.
thrust would fly a machine weighing 9 oz. or 10 oz. But, since
the thrust of a compressed-air engine is not constant, gradually
diminishing as the pressure in the container grows less, for a machine
weighing 9 oz. (assuming the machine to be built first) the plant will
require to develop about 5 oz. initial thrust. The diameter of the
propeller is dependent on the most efficient speed of the particular
motor employed.
Assuming that the model does not weigh more than 17 oz., a
four-cylinder engine constructed would answer admirably. It would
require a container 24 in. long and 3 in. in diameter, constructed
of copper or hard-drawn brass foil ·002 in. thick. Should, however,
the reader particularly desire to fit a rotary motor, doubtless the
five-cylinder rotary previously described will suit.
=Building Scale Models.=—The great difficulty in building scale models
to fly with rubber motors is to get the centre of gravity in the same
relative position that it holds in the prototype. This is due to the
long length of rubber motor required to give the requisite power, and
also to ensure a reasonable time-length of flight.
It is in connection with the fixing or arranging of the rubber motor
that the most radical departures in the design of the prototype will
have to be made, although the writer has evolved an arrangement
whereby even this need not entail much departure from the lines of the
original. This arrangement consists in providing a separate strut or
frame to take up all strain from the rubber, it only being necessary
to arrange suitable fastenings for the strut, which may take the form
of clips, so making it possible either to remove the motor for the
purpose of changing or repairing the rubber, or substituting a motor
of different power or length. This is a great advantage on tractor
monoplanes (with the main plane in front), where the rubber is more
or less inaccessible by reason of the closed-in frame or fuselage of
the machine. Another important advantage to be gained from the use of
a detachable motor is that its position fore and aft on the machine
can be varied, in order to bring the centre of gravity into the proper
position to obtain correct balance, or, speaking with more technical
accuracy, to make the centres of pressure and gravity coincide.
With regard to the type of motor to adopt, this depends very largely on
the machine which is being modelled. Whenever possible it should be of
the simplest possible kind, consisting of the main strut to take the
tensile strain, compressive, of course, so far as it affects the strut,
and the torsional strain put on by the twisting of the rubber. At one
end of this strut a hook of wire or other form of metal is formed
to hold the rubber skein, whilst at the other end is fixed a plain
bearing. Through this the propeller spindle is passed, having a hook at
its end, over which the other end of the rubber is placed.
On certain types such a simple motor is not possible. In order to
concentrate the weight more at one point, the rubber and its struts
have to be shortened; and to get the necessary number of revolutions of
the propeller a gearing of two to one, three to one, or four to one,
as the case may be, must be used. By this means the small number of
turns which can be got on a short thick skein of rubber of great power
will still give the number of propeller revolutions required to make a
good flight, just the same as with a motor of ordinary thickness and of
great length. Of course, some power is lost in the gearing.
To resist fuselage distortion the spar must be suitably braced in a
lateral direction, the outrigger carrying the bracing wires being
situated just forward of the centre of the spar. No. 35 S.W.G. is quite
strong enough for fuselage bracing. Silk fishing-line or Japanese silk
gut is admirably suited for wing bracing, and is not so liable to
stretch as the tinned iron or brass wire sometimes used. Piano wire
is generally used for elevators, tail planes, chassis, and propeller
shafts, of a gauge ranging from No. 17 S.W.G. to No. 22 S.W.G. A
clock-spring or piano-wire protector fitted to the nose of a model
aeroplane will also prevent a broken spar should it strike some object
such as a tree or wall during flight.
The Kite and Model Aeroplane Association, which is the paramount body
to observe and control model flying in England, and which is recognised
by the Royal Aero Club, stipulate that protectors must be fitted to all
machines competing in their contests.
CHAPTER XVI
General Notes
=Stability.=—The principles underlying the design of a successful
flying model aeroplane are almost, if not quite, as complex as those
involved in the planning of full-size machines. In some respects
perhaps this is more so, owing to the fact that models must be
practically automatically stable both longitudinally and laterally,
since they are not under any sort of control after they have left
the hands of the person flying them. The adjustments to produce this
automatic stability must be made before the machine is launched, and
the fact that there are models which are capable of flying distances of
several hundreds of yards, and high up in the air, is evidence that it
is quite possible to make this adjustment accurately.
A useful rule to remember is that to produce a longitudinally stable
effect, the leading plane should make a greater angle (that is, a
positive angle) than the following plane. And that conversely a
condition of instability is set up when the leading plane makes a
negative angle to the trailing one (see Fig. 160).
Referring now to lateral stability, the same principle applies,
although in the case of biplanes stability to a great extent can be
obtained in another way.
Monoplanes and sometimes biplanes are made stable by what is known as a
dihedral angle shown by Fig. 161, which represents the usual method of
shaping the planes. This last-mentioned figure is intended to represent
a front (or back) view of the machine. In aeroplanes of this type the
elevator or tail, whichever is employed, may also be given a dihedral,
though this is not often done.
[Illustration: Fig. 160.—Machine without Stability]
[Illustration: Fig. 161.—Dihedral Angles]
=Lubricant.=—A good lubricant can be made from pure soft soap 4 parts,
pure glycerine 2 parts, water 6 parts, these constituents being boiled
together to the consistency of syrup.
Another excellent lubricant is made from castile soap 1 part, boiled
in water 3 parts. Add black lead or plumbago sufficient to make a thin
paste.
[Illustration: Fig. 162.—Fixing Skids]
[Illustration: Fig. 162A.—Fixing Skids]
[Illustration: Fig. 163.—Fitting Wheels]
=Fitting Skid.=—Steel-wire skids can be fixed to model aeroplanes as in
Figs. 162 and 162A. The wheels can be obtained of any model aeroplane
firm, and can be fixed to the machine as shown in Fig. 163. This would
increase the weight of the machine, and more rubber may have to be used
and the main planes adjusted to suit. Use No. 21 B.W.G. wire (steel).
=Elevator Adjustment.=—Elevators are sometimes fixed to the fuselage as
shown in the accompanying sketch (Fig. 164), which shows it fixed above
the spar. It is claimed that, in this position, the elevator is more
efficient.
[Illustration: Fig. 165.—Fixing Struts to Biplane]
[Illustration: Fig. 164.—Elevator Adjustment]
=Loading per Square Foot.=—Generally speaking, the loading per square
foot of supporting surface for rubber-driven models should not exceed 6
oz., nor be less than 3 oz., while to obtain good stability the ratio
of machine length to span should be somewhere in the neighbourhood of
3: 2.
=Fixing Struts to Biplane.=—One very suitable method of attaching the
inter-struts to wings of model biplanes, that admits of dismantling
the model for packing, is shown at =A= (Fig. 165). A short length of
brass tubing of ³/₃₂-in. bore is bound with fine florist’s tinned iron
wire to the wing spar, the inter-struts being bent to the shape given
at =B=, so that they spring tightly into the sockets. A simpler method
is illustrated by =C=. Here the inter-struts are bent at right angles
on the ends, bound to the wing spar, and soldered. Much will depend
on whether a fuselage is one-or two-membered, but a frame attachment
capable of adaptation to either is given by =D= and =E=. Wire crutches
are bent to take the cross-section of the frame member or members, and
fixed by binding and solder to the central inter-struts. Yet another
method is shown at =F=, which is self-explanatory.
[Illustration: Fig. 166.—Water-Surface Hydroplane]
=Water-surface Hydroplane.=—For a water-surface hydroplane 3 ft. long
try a breadth of 11 in., same beam all the way. Make the depth 2½ in.
at the stem and 3 in. at the step. The step may be ¾ in. deep, and
would be placed about 15 in. from the stern. The writer would suggest
fins as shown by dotted lines in the accompanying illustration (Fig.
166) for steering purposes, one fin on each side at the stem, and one
on the centre line forward. The fins should be made of thin aluminium
and be quite sharp on the edges. Canvas would be too rough a surface,
producing too much skin friction; oiled silk would be better, but the
writer would recommend thin wood, french polished. The power needed to
make these boats “plane” is very great, and considerable difficulty
might be experienced in getting any distance out of it with clockwork.
[Illustration: Farman This Section is inefficient]
[Illustration: Streamline An efficient Section]
[Illustration: Wedge This formation cannot be recommended]
[Illustration: Stepped
Fig. 167.—Sections of Hydroplane Floats]
=Calculating Capacity of Hydroplane Floats.=—The floats of a model
hydroplane must be made sufficiently large to displace about three
times the model’s weight of water, since it is necessary that they
should be only one-third immersed. A cubic foot of water weighs 1,000
oz. approximately. Then
1728 8
---- × --- = 13·8 cub. in.
1000 1
must be displaced to float 8 oz. Multiplying this by three gives 42
cub. in. as the total cubic capacity of the floats. Two front floats,
each 5 in. by 2 in. by 1 in. maximum depth, and a rear float 8 in. by 3
in. by 1 in. would be about the correct size to use. A slightly larger
diameter and pitched propeller would be necessary on a hydroplane to
develop more thrust to overcome the resistance of the floats. Some
well-known hydroplane sections are given in Fig. 167.
[Illustration: Showing Marking of Block]
[Illustration: Finished Screw of Truly Helical Formation]
[Illustration: Setting out the Angles]
[Illustration: Cross-Section]
[Illustration: Showing Halving of Block
Fig. 168.—Four-bladed Screws]
=Waterproofing Silk for Model Aeroplanes.=—A waterproofing solution can
be made of pure coach varnish reduced in consistency with turpentine in
the proportion 2: 1. It is, however, more important to make the fabric
airtight than waterproof. This solution accomplishes both. Rubber
lubricant is made of 1 part of graphite, 6 parts of pure soft soap,
1 part of glycerine, 4 parts of water, and 1 part of salicylic acid,
boiled together and allowed to cool.
=Making Four-bladed Air-screws.=—Fig. 168 on the preceding page shows
how truly helical and also four-bladed screws are carved. Four-bladed
screws are not so efficient for models as two-bladed ones. The drawing
also shows the method of marking out and also of halving the blocks
together at the centre, so that the four blades are at right angles to
one another. A view of an ordinary twin-blade screw of similar design
is appended to give some idea of the finished shape of the blades. The
pitch should not exceed the circumferential measurement of the disc
swept by the propeller; that is to say, the pitch angle, or the angle
made by the propeller tip with the axis, should not exceed 45 degrees.
The angles along the blade are determined in the manner illustrated
by Fig. 168. A line is laid off to any convenient scale equal to the
circumference of the propeller disc = π × diameter. The tip angle (or
pitch angle) may now be produced and the triangle completed by erecting
a line at right angles to the first, or the pitch may be erected
perpendicularly to the circumferential line to the spiral scale and
the pitch angle line drawn in. The circumferential line may now be
divided into a number of equal parts and the points connected up. In
the illustration three points have been taken, which will be enough for
a small propeller. Cardboard templates should be cut to these angles
and the blades checked at the points corresponding. The balance of the
screw must be attended to and is a most cogent factor in such a small
design. Marine-screws are exceedingly inefficient when working in air,
and their use for driving model aeroplanes, etc., is to be strongly
deprecated. The formula for propeller pitch is: p = πd tan =A=, where
p = pitch, π = 3·14, d = diameter of propeller, tan =A= = tangent of
pitch angle.
[Illustration: Fig. 169.—Securing Wooden Planes]
=Fixing Planes.=—Fig. 169 shows a neat and effective method of securing
the planes.
Very little wood is now used for the planes of model aeroplanes; but to
build a plane of veneer, it should first be cut to the shape required
and then a strip of birch pinned and glued on the under-side of the
leading edge for strength. The veneer is then pinned and glued to the
ribs which have previously been bent to the correct camber. The reader
may be reminded that birch is the most suitable wood for bentwood
propellers, the wood being first cut to shape with a fret-saw, then
soaked in hot water and bent over a bunsen burner to the desired pitch.
This requires considerable experience, but can be done quite quickly by
an experienced workman. For propellers up to 12 in. in diameter, use
¹/₁₆-in. wood; this should gradually taper off to ¹/₃₂ in. at the tips.
[Illustration: Fig. 170.—Geared Motor for Model Aeroplane]
[Illustration: Fig. 171.—Fixing Shaft of Carved Screw]
=Making Motor for Model Aeroplane.=—A geared motor suitable for a model
aeroplane is given by Fig. 170. A cage =E= for the gears =A= can be
made from ¼-in. by ¹/₃₂-in. strip iron, and the propeller shafts =B=
can be made from cycle spokes. The illustration also shows a method
of fixing the propeller to the shaft. The piece =C= is soldered to
the shaft =B=, and engages with two holes drilled in the propeller
boss, the propeller being secured by the nut =D=. The gears =A= have
an equal number of teeth. There is no advantage in using a geared-up
motor. =F= is the fuselage of the model.
=Fixing Propeller of Model Aeroplane.=—With a carved propeller, the
motor hook can be secured to the propeller as in Fig. 171. If a
bentwood propeller, the best way is to fasten a strip of tin round the
centre and solder the motor hook to this.
CHAPTER XVII
Easily-made Tailless and Box Kites
One of the difficulties that beset the juvenile kite-flyer is the
inability to get the kite to rise from the ground unless there is a
fairly good wind blowing. Even then the services of an assistant are
required, whilst numerous trials to adjust the amount of weight on the
tail, the position of the carrying thread on the kite, and the distance
required to be run to get the artificial wind to enable the kite to
rise above the surrounding buildings, are difficulties that damp the
enthusiasm of the most inveterate kite-flyer.
To the kites sold in the shops the foregoing remarks apply. They
are too heavy, and are suitable for flying at the seaside only. To
make the sport interesting, the kite should fly in the least wind,
and should be so made as to fly from the hand, the thread or string
being paid out as the kite rises. It should be perfectly balanced
so as to be stable in the most erratic wind. The kites shown in the
accompanying illustrations, if made to the directions, will fulfil all
the above conditions, and at the same time be easy of construction.
The dimensions can be varied in proportion; but the would-be maker is
advised to stick to the given sizes for the first attempt.
[Illustration: Fig. 172.—Hargreaves Kite]
[Illustration: Fig. 173.—Frame of Hargreaves Kite]
[Illustration: Fig. 174.—Measuring Bend]
[Illustration: Fig. 175.—Box Kite]
[Illustration: Fig. 177.—Joint of Stretcher Ribs]
[Illustration: Fig. 176.—Covering with Paper]
The Hargreaves kite (Fig. 172) is the simplest. The frame (Fig. 173)
consists of a straight length of yellow pine =A=, to which is attached
a cross piece of cane =B=. This cross piece is made by cutting a thin
cane and splitting it, trimming the edges off until nearly square in
section, and then bending it over a small gas flame. As it gets hot,
bend it gently by grasping the ends in each hand. Pass it to and fro
in the low flame, and it will be found to give. Bend it evenly and
allow to cool, when it will remain in this shape. Find the centre and
ascertain if the bend is equal on each side by measuring as shown in
Fig. 174. This being so, notch =A= and =B= (Fig. 173) slightly where
they cross, and assemble, applying a little glue and lapping with
cotton. Join the ends of the bent limb with linen thread, and also take
a thread round the frame, securing it at each extremity in a notch with
the addition of glue.
When dry the frame is covered with paper. The very thin coloured paper
used for making artificial flowers is just the thing. Gum it on with a
thin flour paste, and allow very little overlap, keeping the paper as
free from crinkles as possible. Cut a disc of stout paper, and gum it
over the frame at the crossing. This is to strengthen the paper at this
point, as the attached thread is liable to make a wide hole. Secure a
yard of strong thread at the crossing, and when dry the kite can be
tested. If all is right the result will be eminently satisfactory, the
kite flying from the hand like a bird, no tail whatever being needed.
For decorative purposes a short fish tail and fins can be added, or a
rubber balloon can be used as a tail.
The box kite shown by Fig. 175 is not more difficult to construct than
the kite previously described. It flies with equal facility, and is
quite as steady. Being made collapsible it can be rolled in a small
compass.
To make the kite, paste two of the coloured sheets together to form a
continuous length, and spread on the floor. Cut the four uprights of
yellow pine to the correct length and thickness, and having liberally
glued them, place them on the paper as shown in Fig. 176, and paste a
long strip over them on to the paper underneath. This is to strengthen
the paper round the uprights. When dry and firm, which does not take
long if the glue is put on tacky, the loose end of the paper can be
joined to the upright to form a hollow long box.
Now cut four thin lengths of wood to form cross-pieces, secured
together with a pin through the centre (see Fig. 177). Notch the ends
and open them out so as to fit inside the box between the uprights. Fit
one to each end, and then secure the upright ends with a linen thread
pulled tightly from one to the other, and tied and glued at each end.
This takes the strain of the cross piece off the paper, and at the same
time provides a support for the paper, which is clapped round it and
gummed on to itself.
The kite having been completed, a short length of thin string is
attached about 3 in. from one end of one of the uprights. To this the
thread for flying is attached. This kite will fly quite well with
the thread attached to the very end of the upright, so that there
is no fixed position for it to a fraction of an inch. The nearer it
is brought to the centre, the more unstable the kite becomes. In a
moderate wind, with the thread attached to the top, the kite tends to
ascend more vertical than it would otherwise.
[Illustration: Fig. 177A]
[Illustration: Fig. 177B
Figs. 177A and B.—Details of Twin Cellular Box Kite]
Fig. 177A shows another simple form of box kite. It is made from four
strips of straight grained wood (preferably spruce), 2 ft. 6 in. by ⅜
in. by ½ in. Obtain also four other pieces, each 1 ft. 7½ in. long, but
¹/₁₆ in. wider and thicker than the foregoing, and halve their ends to
a depth of ⅛ in. by ¼ in., in order that when the false end =A= (Fig.
177B) is tightly bound on, these cross sticks will firmly grip the long
pieces edgewise; the sides of the cell are indicated by dotted lines.
It is advisable to make the cross-pieces a trifle long, to ensure their
straining the kite to its correct form.
CHAPTER XVIII
Building a Model Airship
It has not for obvious reasons been possible, in the design here
presented, to rigidly adhere to the lines of the prototype, as in
the adaptation of the design to rubber-driven model form several
modifications have necessarily been introduced. It has been the aim of
the writer to bring the model within the constructional capabilities
of the amateur; indeed, it is hardly possible to have simplified the
construction further. Now, the success of a dirigible, whether full
size or model, depends primarily on the observance of the fact that
an airship is lighter than air, and thus, unlike the aeroplane, does
not rely on speed to obtain lift. Secondarily, the lifting power of
hydrogen must be remembered, and although this varies according to
temperature and the purity of it, it may be taken as a general rule
that hydrogen will lift 80 lb. per 1,000 cubic feet. It is a good rule
to adopt a lower figure, say 70 lb. lift per 1,000 cubic feet, to allow
for discrepancies. In the design here submitted, aluminium (or what is
equally as good, magnalium) tube forms the framework of the body or
envelope. The general arrangement will be apparent from Figs. 178 and
179, which show the model in plan and side elevation respectively. The
framework is of hexagonal cross section, the longitudinal members of
which terminate at each end in a brass cap, to which they are riveted
with soft brass pins. It will be necessary to anneal the tubes before
bending them to impart the conical shape to each end of the frame, and
this can best be effected in a weak spirit flame, care being taken
to keep them on the move in it, to obviate fusing them. Where it is
necessary in the construction to rivet the tubes, solder should be run
over the pin to take up any play. Fig. 180 shows the method of securing
the longitudinal members to the brass end-caps. The tubes are first
flattened out, as at =B=, and then riveted to the caps. This figure
also shows the method of adjusting the angle on the rudder. A piece
of brass tube is soldered to the end-cap; and it should be of such a
bore that the No. 18 gauge wire of which the rudder is constructed
makes a bare fit through it. Two similar pieces of tube, ⅛ in. long,
are soldered to the rudder, to maintain the position of it. Thirteen
hexagonal cross members will be required, and each is formed from ³/₁₆
in. aluminium tube. In order that they may not become out of truth,
they are cross-braced with No. 35 S.W.G. piano wire, the ends of each
wire being made off in a small hole drilled through the tube. Fig.
180 will make the detail clear; each cross member is riveted to the
longitudinal, and the latter is flattened out at those points where the
cross member is attached.
[Illustration: Fig. 178.—Side Elevation]
[Illustration: Fig. 179.—Plan]
The model is driven by four elastic motors, transmitting their power
through equal gearing to the twin propellers. The four motor rods may,
for preference, be hollow spars, of the same cross-section as the
solid ones indicated. If such are used, they may be made by ploughing
a groove in a length of wood of suitable cross-section so that it
represents the letter =U=. Two will be required for each spar, so glued
together that a hollow tube is formed. At the point where it will be
necessary to pierce the spar to admit the bracing outrigger, small
packing pieces of birch should be placed in the grooves previous to
assembling the two halves of the spar. The spars, or, more correctly,
motor rods, are suspended from the envelope framework by aluminium
tube outriggers of ¼-in. diameter. Fig. 181, which shows the machine
in end elevation, and Fig. 182, showing the central cross-section,
indicate the form they are to take. Eight of them will be required. The
angles must be cleanly and accurately formed, so that the two centres
of thrust lie in the same plane. It will be found good practice, when
forming both the outriggers and the cross-sectional members, to make
a full-size drawing of them to use as a template. More especially is
this needed in the construction of the cross-sectional members, for the
purpose of ensuring that, in the operation of embracing them, they are
not strained in any way so as to become out of truth.
Fig. 183 shows the joint of the motor-rod outrigger to the motor rod
itself. As there shown, the tube is flattened out partly to engirdle
the spar, to which it is attached by pinning and clinching.
[Illustration: Fig. 180.—Joint of Tube to End-Caps]
[Illustration: Fig. 181.—End Elevation]
[Illustration: Fig. 182.—Central Section]
[Illustration: Fig. 183.—Joint of Outrigger to Motor Rod]
The gearing (see Fig. 184) consists of a brass framework (bound to the
ends of the motor rods) which provides bearings for the gears. The
use of gears is obvious; they eliminate torque on the spars, or the
tendency which a single skein would have to twist the spar. Pieces
of tube are passed over the shafts to bear between the gears and the
gear bearing. In order to counteract the tendency of the rubber hooks
to pull out straight when the rubber is in tension, the hook ends
are secured as shown at =A=. When it becomes necessary to detach the
skeins, it is only necessary to slide the tubes along the shaft to open
the hook.
In Fig. 185 the kingpost attachment is shown. The spar is mortised, the
kingpost forced through, glue having previously been brushed into the
slot, and a pin tapped through from the side to secure it. Birch should
be used for the kingposts, and their widest cross-section should be ⅜
in. by ³/₃₂ in., tapering off towards the extremities to ⅛ in. by ¹/₁₆
in.
To the ends of each motor rod are attached small No. 22 S.W.G. piano
wire hooks, to which the spar bracing is made fast. A suitable length
of wire is passed through the spar, and the hooks then formed. The
bracing is fastened to the kingpost by a couple of turns being taken
round it.
Next, four twin hooks should be made, of the form shown in Fig. 186.
Sixteen gauge wire is to be used for them, bent tightly to clip the
spar ends, to which they are bound. All the hooks should be covered
with valve tubing to prevent them from cutting through the rubber when
this latter is in tension.
The joint of the cross members of the envelope to the longeron is given
by Fig. 187, from which the bend in the longitudinal, hitherto referred
to, will be clear.
To the tail of the machine a pair of superposed surfaces or elevators
are fixed. These are fastened at their foremost extremities to the
motor-rod outriggers, and at the rear they are supported by the two
cross ribs of the rudder. Small slits are to be cut in the fabric with
which the rudder framework is covered to enable any slight alteration
in the angle of incidence of the elevators to be effected.
[Illustration: Fig. 184.—Arrangement of Gearing]
[Illustration: Fig. 185.—Kingpost Attachment]
[Illustration: Fig. 186.—Detail of Twin Hooks]
[Illustration: Fig. 187.—Cross Member Joint]
[Illustration: Fig. 188.—End View of Screw]
Attention may now be given to the propellers. As twin screws are used,
they may be made of fairly long pitch, since their torques will be
opposite and consequently balanced. Fig. 189 gives a perspective view
of the propeller block marked out ready for carving, and Fig. 188 shows
the screw in end elevation. The screw is first carved as a true helix,
and then shaped up to the form shown in the sectional view (Fig. 182).
American whitewood should be used for the blocks, or, failing this,
poplar would do. Circular tin discs are pinned to each side of the
propeller boss, to which the shaft is soldered. Great care is essential
to ensure that both propellers are of the same weight, and that each
is poised; that is, assumes an angle of 180° when balanced on a shaft.
They may be finished with a coat of gold size and one of varnish. Each
gear is driven by six strands of ¼-in. strip elastic, well lubricated
with soft soap and glycerine.
[Illustration: Fig. 189.—Propeller Block before Carving]
[Illustration: Fig. 190.—A Model Airship]
The covering is to be yellow Japanese silk, proofed with varnish
diluted with 20 per cent. of linseed oil and 10 per cent. of turpentine;
two coats should be given before the fabric is applied, and two
afterwards. A covering strip of fabric should be glued over all
seams to make the envelope as impervious as possible. And where the
outriggers pass through further pieces should be glued over, and
flanged on to the tube itself, being afterwards well doped. It is well
to impress here the importance of making the envelope as gasproof
as possible, and although a fabric entirely impervious has yet to
be invented, it is possible, with care, to reduce loss of gas by
percolation to a very low figure indeed.
A Lucas cycle valve is soldered into the brass end-cap constituting the
rear of the envelope, for the purpose of inflation, which is effected
in the following manner: A =T=-piece is fixed to a cycle pump, and
a pipe from the gas container to the =T=-piece. A further tube is
connected from the remaining arm of the =T=-piece to the valve. To
inflate the envelope, release the pressure from the container until
the pump handle is forced out to its full extent, and then shut off
pressure and force the pump down, thereby causing ingress of gas to the
envelope. Continue thus until the envelope “swells.” Of course, this
method could be adopted if hydrogen from the gas jet is used, although
this does not possess the lifting capacity of hydrogen procured from
the balloon manufacturers.
In conclusion, it may be pointed out that the weight of the complete
model inflated should not exceed 2 lb. A view of the appearance of the
model in flight is given by Fig. 190.
For those who wish to design model airships of their own it may be
stated that coal-gas lifts about 35 lb. per 1,000 cub. ft., or half
the weight lifted by an equal volume of hydrogen. A small model would
not be very successful, as the volume, and hence the lifting power,
decreases as the cube of the diameter. Thus assuming a model to be
built of half the dimensions given in this chapter, the weight of it
could only be ½ × ½ × ½ = ⅛ of the original model, and it would be
found difficult to work to this limit. The writer would point out
that little success can be expected from an airship of such small
dimensions, as the following elementary calculation will show. It
takes 35,000 cub. ft. of hydrogen to support 1 ton. Then 1 cub. ft. of
hydrogen supports ²²⁴⁰/₃₅₀₀₀ lb. Now, the cubic contents of a model
dirigible, we will assume, is
22 5 5 36 1
---- × --- × --- × ---- × ------ cub. ft.,
7 2 2 1 1728
and therefore the total weight it is capable of supporting is
22 5 5 36 1 2240
---- × --- × --- × ---- × ------ × ------ = .026 lb., or .4 oz.
7 2 2 1 1728 35000
From this it will be seen that it is extremely improbable that a model
can be built to this weight.
No more suitable covering than gold-beater’s skin exists. If the model
is of the rigid type, then the covering should be stretched over the
framework that imparts the ichthyoid shape to the envelope. Strips of
the fabric must be attached with mucilage over all seams to make the
envelope as impervious as possible. It is worthy of note that full-size
airships have as many as three or four coverings to eliminate loss of
hydrogen by escape through the pores of the fabric. If, however, the
reader contemplates building a model of the nonrigid type, a wooden
hull should be cut and the fabric fitted up to this. The hull should
represent the shape of the inflated envelope.
Compressed air also lends itself to model airship propulsion.
In conclusion, a word of warning: do not place the model near any fire,
gas, match, etc., as any small leaks may cause an explosion.
In building model airships, it is advisable to remember that by
_doubling_ the diameter of the envelope we get _four_ times the
capacity and hence four times the lift for only double the weight. This
will be seen from the following calculation:
Lift of airship 3 ft. long 2½ in. diameter equals, as we have just
seen, ·026 lb.
By doubling the diameter the calculation becomes:
22 5 5 36 1 2240
---- × --- × --- × ---- × ------ × ------ = ·052 lb.
7 1 1 1 1728 35000
From this it will be seen that if it is desired to lift a certain
weight with a lighter-than-air craft, the minimum capacity should first
be calculated and the diameter (which should always be as large as
possible) then varied to obtain the required capacity.
Although it has been stated that the lift of hydrogen is 70 lb.
per 1,000 cub. ft., it will be in order to explain two important
considerations. It will be clear that, firstly, a full-size balloon
must be inflated to a much higher pressure than a model, owing to
the heavier mass of material to be forced out to form. As a direct
adjunct to this fact it will be seen that percolation will be high.
Consequently the lift weight ratio 70: 1,000 is somewhat a low estimate
for a model, since, firstly, it will only require to be inflated to
less than half the pressure of a full-size balloon; and, secondly,
percolation will be considerably less. The writer intentionally gave
the full-size limit in order that, should the builder’s model fail to
come within the prescribed limit of 2 lb., its flying capability would
not be appreciably impaired.
Further, the envelope is kept to its shape by a framework inside, and
is consequently full of air. Now if we start pumping hydrogen in it
will make the contrivance heavier instead of lighter, because no outlet
is provided for the escape of air. This could easily be remedied by
standing the model on its end with the cycle valve at the top, with a
valve or tap on the bottom brass cap. Open this bottom tap to allow
the air to escape, and allow the hydrogen to fizz in through the cycle
valve. Hydrogen will not escape through the bottom tap until all the
air has been displaced. If the hydrogen cannot be had at a high enough
pressure to operate the valve itself a valve should not be used.
It may in such a case be necessary to fit an induction valve to the
pump. The envelope should be inflated to as low a pressure as possible,
otherwise the lift is correspondingly reduced. In all cases it is
exceedingly difficult with model airships to prescribe any definite
formula, and although the lift coefficient for models exceeds the
figure stated, the conjectural nature of the purity (and hence of lift)
of hydrogen in various parts of the country renders it a safe one to
use for all practical purposes.
INDEX
Action and reaction, 5
Aerofoil, flow of air round, 10
——, lift of, 10
Aeroplane, speed of, 4
——, types of, 12-16
——: Why does it fly? 1-11
=A= frame, 20, 22, 23
Air, density of, 35, 36
—— flow, direction of, 5
—— pressure, 2
Air-screw, action of, 35
—— balancing, 38
——, calculations for, 36
——, efficiency, 35
——, four-bladed, making, 133
——, laminated, 39
——, suitable woods for, 36, 38
Bamboo planes, 30
Bending air-screws, 42-46
Biplane, fitting struts to, 128
——, sample twin-screw, 61-68
Blade width, maximum, 45
Blériot type monoplane, 18
Box kite, 138, 139, 140
Bragg-Smith, 15
Building airship, 141, 154
Calculations in designing aeroplane plant, 120
Camm air-screw, proportions of, 46
Canard, meaning of, 8
Carving air-screws, 35-41
Chauvière type air-screw, 38
Clarke, T. W. K., 14
Compressed-air engine, 94-102
—— ——, container for, 99
—— ——, crank for, 97, 98
—— ——, inlet and exhaust, etc., of, 97
—— ——, pistons for, 98
Compressed-air engine, testing the container, 100
—— ——, three-cylinder, 94-103
—— ——, valve and tap for, 100
Compressed-air-driven biplane, 104-114
—— ——, bracing planes of, 115
—— ——, calculations for, 104-109
—— ——, flying, 118, 119
—— ——, inter-struts for, 112
—— ——, materials for, 106
—— ——, planes for, 109
—— ——, tail for, 111
Designing, general notes on, 120, 123
Dihedral angle, 7
Double-winder, 70-72
Drift and lift, 3, 6
Efficiency air-screw, 35
—— formula, 19
Elastic motor, disadvantages of, 94
Elevator adjustment, 127
Equilibrium, 2
Fairey, 12
Force of gravity, 2
Forces acting on kite, 2, 3
Formula, efficiency, 19
——, type, 18
Four-bladed air-screws, making, 133
Fuselages, 19-34
——, boat-shaped, 24
——, box-girder, 26
——, distortion, 30
——, simple, 28, 29
——, single-spar, 26
Fuselages, twin-screw, 26
——, two-membered, 25
Geared motor for model aeroplane, 134
General notes, 124-135
Gravity and pressure, 6, 7, 8
Gravity, force of, 2
Hargreaves kite, 132
Hollow spar, 22
Houlberg, 14
Hydro-biplane, 17
Hydro-monoplane, 87-93
——, bearings for, 88
——, elevator for, 90
——, floats for, 90
——, fuselage for, 87
——, mainplane for, 88
——, weight of, 93
Hydroplane floats, calculating capacity of, 130
——, water surface, 128
Kite, box, 138, 139, 140
——, forces acting on, 2-3
——, Hargreaves’, 132
——, speed of, 4
——, tailless, 136-141
Lateral stability, 7
Lift and drift, 3, 6
—— of aerofoil, 10
Loading, 44
—— for aeroplanes, 127
Longitudinal stability, 8
Lubricant, 125
Mann, R. F., 20
Materials, choice of, 28
Monoplane, Blériot type, 18
——, collapsible, 73-79
——, ——, chassis for, 78
——, ——, mainplane for, 75
——, ——, propellers for, 79
——, ——, tail for, 74
——, hydro, 87-93
——, Ridley, 12
——, ——, twin-screw, 54-60
——, ——, mainspar for, 54
——, ——, propellers for, 58
——, tractor, 80-86 (_see_ Tractor monoplane)
Penaud, Alphonse, 94
Piano wire, uses of, 30
Pitch, 35, 36, 39, 42, 44, 45
—— angles, setting out, 40
“Plane” and “aeroplane,” meaning of terms, 8
Planes, bamboo, 30
——, cane, 50
——, constructing, 47-53
——, fixing, 133
——, swept-back, 52
——, types of, 47
——, umbrella ribbing, 50
——, wire, making, 50, 51
——, wooden, 48
Plant, calculations for designing, 120
Pressure and gravity, centres of, 6, 7, 8
Resistance, air, 3
Ridley monoplane, 12
Scale models, building, 32, 34, 121, 122, 123
Silk, waterproof, 130
Skid, fitting, 126
Slip, 35
Small biplane, driving, 102-103
Spar sections, 26, 27
Speed of aeroplane, 4
Spindles, attaching, 46
Stability, 29, 124, 125
——, lateral, 7, 42
——, longitudinal, 8
Tailless kites, easily made, 136-141
=T= frame, 20, 22, 23
Thrust, 42
—— required for aeroplane, 11
Timber, strength of, 29, 30, 32
Torque, 29, 36, 42
Tractor hydro-biplane, 17
——, monoplane, 80-86
——, ——, chassis for, 82
——, ——, mainplane for, 84
——, ——, mainspar for, 80
——, ——, twin-gears for, 84
Twining, E. W., 14, 17
—— air-screw, 46
Twin-screw biplane, simple, 61-68
—— —— ——, elevator for, 62
—— —— ——, flying the, 66-68
—— —— ——, mainspar for, 61
Type formula, 18
—— of aeroplanes, 12-18
Waterproof silk, 130
Water-surface hydroplane, 128
Winders for elastic motors, 69-72
——, double, 70-72
——, push-in-the-ground, 69-70
Wing bracing, 30
PRINTED BY CASSELL & COMPANY, LIMITED,
LA BELLE SAUVAGE, LONDON, E.C.4
50.220
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