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

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.

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].

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.

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.

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.

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.

Fig. 5.—Disposition of Angles

Fig. 6.—Air-flow Round Plane

Fig. 7.—Air-flow Round Cambered Surface

Fig. 8.—Air-flow Round Streamline Strut

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)].

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.

Fig. 13.—Clarke Type Monoplane

Fig. 12.—The Fairey Monoplane

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.

Fig. 14.—Tractor Monoplane

Fig. 15.—Twining Monoplane

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.

Fig. 18.—Tandem Monoplane

Fig. 17.—Tractor Hydro-Biplane

Fig. 21.—Fuselage Biplane

Fig. 22.—Blériot Type Tractor Monoplane

Fig. 20.—Tractor Biplane

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

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.

Fig. 23.—A Frame

Fig. 26.—T Frame

Fig. 25.—A and T Frame

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.

Fig. 27

Fig. 28

Fig. 29

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.

Fig. 31

Fig. 32

Fig. 33

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.

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.

Fig. 36.—Built-up Plane

Fig. 37.—Wing Plans

Fig. 38.—Types of Bearings

Fig. 38A.—Twin-screw Bearings

Weight of Woods Chiefly Used

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,

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

Fig. 39

Fig. 40

Fig. 41

Fig. 42

Fig. 43

Fig. 44

Fig. 45

Fig. 46

Fig. 47

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.

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.

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.

Fig. 51.—Bentwood Screws

Fig. 52

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.

Fig. 54.—Bentwood Shaft
Attachment

Fig. 56.—Safety Hook

Fig. 55.—Carved Screw Shaft

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.

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.

Fig. 59.—Three Spar Plane

Fig. 60.—Two Spar Plane

Fig. 61.—Cane Plane

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].

Fig. 63

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.

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.

Fig. 66

Fig. 67

Fig. 68

Fig. 69

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.

Fig. 71.—Plane Fastening

Fig. 73.—Elevation of Main Plane

Fig. 74.—Finished Screws

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].

Fig. 75.—Plan of Model

Fig. 76.—Outrigger Details

Fig. 77.—View of Elevator