Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
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.