HINTS ON THE BUILDING OF MODEL
AEROPLANES.

§ 1. The chief difficulty in the designing and building of model aeroplanes is to successfully combat the conflicting interests contained therein. Weight gives stability, but requires extra supporting surface or a higher speed, i.e. more power, i.e. more weight. Inefficiency in one part has a terrible manner of repeating itself; for instance, suppose the aerofoil surface inefficient—badly designed—this means more resistance; more resistance means more power, i.e. weight, i.e. more surface, and so on ad infinitum.

It is because of circumstances like the above that it is so difficult to design really good and efficient flying models; the actual building of them is not so difficult, but few tools are required, none that are expensive or difficult to use.

In the making of any particular model there are special points that require special attention; but there are certain general rules and features which if not adhered to and carefully carried out, or as carefully avoided, will cause endless trouble and failure.

§ 2. In constructing a model aeroplane, or, indeed, any piece of aerial apparatus, it is very important not to interrupt the continuity of any rib, tube, spar, etc., by drilling holes or making too thinned down holding places; if such be done, additional strength by binding (with thread, not wire), or by slipping a small piece of slightly larger tube over the other, must be imparted to the apparatus.

§ 3. Begin by making a simple monoplane, and afterwards as you gain skill and experience proceed to construct more elaborate and scientific models.

§ 4. Learn to solder—if you do not know how to—it is absolutely essential.

§ 5. Do not construct models (intended for actual flight) with a tractor screw-main plane in front and tail (behind). Avoid them as you would the plague. Allusion has already been made in the Introduction to the difficulty of getting the centre of gravity sufficiently forward in the case of BlÉriot models; again with the main aerofoil in front, it is this aerofoil and not the balancing elevator, or tail, that first encounters the upsetting gust, and the effect of such a gust acting first on the larger surface is often more than the balancer can rectify in time to avert disaster. The proper place for the propeller is behind, in the wake of the machine. If the screw be in front the backwash from it strikes the machine and has a decidedly retarding action. It is often contended that it drives the air at an increased velocity under (and over) the main aerofoil, and so gives a greater lifting effect. But for proper lifting effect which it can turn without effort into air columns of proper stream line form what the aerofoil requires is undisturbed air—not propeller backwash.

The rear of the model is the proper place for the propeller, in the centre of greatest air disturbance; in such a position it will recover a portion of the energy lost in imparting a forward movement to the air, caused by the resistance, the model generally running in such air—the slip of the screw is reduced to a corresponding degree—may even vanish altogether, and what is known as negative slip occur.

§ 6. Wooden or metal aerofoils are more efficient than fabric covered ones. But they are only satisfactory in the smaller sizes, owing, for one thing, to the smash with which they come to the ground. This being due to the high speed necessary to sustain their weight. For larger-sized models fabric covered aerofoils should be used.

§ 7. As to the shape of such, only three need be considered—the (a) rectangular, (b) the elongated ellipse, (c) the chamfered rear edge.

§ 8. The stretching of the fabric on the aerofoil framework requires considerable care, especially when using silk. It is quite possible, even in models of 3 ft. to 4 ft. spread, to do without "ribs," and still obtain a fairly correct aerocurve, if the material be stretched on in a certain way. It consists in getting a correct longitudinal and transverse tension. We will illustrate it by a simple case. Take a piece of thickish steel pianoforte wire, say, 18 in. long, bend it round into a circle, allowing ½ in. to 1 in. to overlap, tin and solder, bind this with soft very thin iron wire, and again solder (always use as little solder as possible). Now stitch on to this a piece of nainsook or silk, deforming the circle as you do so until it has the accompanying elliptical shape. The result is one of double curvature; the transverse curve (dihedral angle) can be regulated by cross threads or wires going from A to B and C to D.

The longitudinal curve on the camber can be regulated by the original tension given to it, and by the manner of its fixing to the main framework. Suitable wire projections or loops should be bound to it by wire, and these fastened to the main framework by binding with thin rubber cord, a very useful method of fastening, since it acts as an excellent shock absorber, and "gives" when required, and yet possesses quite sufficient practical rigidity.

§ 9. Flexible joints are an advantage in a biplane; these can be made by fixing wire hooks and eyes to the ends of the "struts," and holding them in position by binding with silk or thread. Rigidity is obtained by use of steel wire stays or thin silk cord.

§ 10. Owing to the extra weight and difficulties of construction on so small a scale it is not desirable to use "double surface" aerofoils except on large size power-driven models.

§ 11. It is a good plan not to have the rod or tube carrying the rubber motor connected with the outrigger carrying the elevator, because the torque of the rubber tends to twist the carrying framework, and interferes with the proper and correct action of the elevator. If it be so connected the rod must be stayed with piano wire, both longitudinally (to overcome the pull which we know is very great), and also laterally, to overcome the torque.

§ 12. Some builders place the rubber motor above the rod, or bow frame carrying the aerofoils, etc., the idea being that the pull of the rubber distorts the frame in such a manner as to "lift" the elevator, and so cause the machine to rise rapidly in the air. This it does; but the model naturally drops badly at the finish and spoils the effect. It is not a principle that should be copied.

§ 13. In the Clarke models with the small front plane, the centre of pressure is slightly in front of the main plane.

The balancing point of most models is generally slightly in front, or just within the front edge of the main aerofoil. The best plan is to adjust the rod carrying the rubber motor and propeller until the best balance is obtained, then hang up the machine to ascertain the centre of gravity, and you will have (approximately) the centre of pressure.

§ 14. The elevator (or tail) should be of the non-lifting type—in other words, the entire weight should be carried by the main aerofoil or aerofoils; the elevator being used simply as a balancer.[39] If the machine be so constructed that part of the weight be carried by the elevator, then either it must be large (in proportion) or set up at a large angle to carry it. Both mean considerably more resistance—which is to be avoided. In practice this means the propeller being some little distance in rear of the main supporting surface.

§ 15. In actual flying models "skids" should be used and not "wheels"; the latter to be of any real use must be of large diameter, and the weight is prohibitive. Skids can be constructed of cane, imitation whalebone, steel watch or clock-spring, steel pianoforte wire. Steel mainsprings are better than imitation whalebone, but steel pianoforte wire best of all. For larger sized models bamboo is also suitable, as also ash or strong cane.

§ 16. Apart from or in conjunction with skids we have what are termed "shock absorbers" to lessen the shock on landing—the same substances can be used—steel wire in the form of a loop is very effectual; whalebone and steel springs have a knack of snapping. These shock absorbers should be so attached as to "give all ways" for a part side and part front landing as well as a direct front landing. For this purpose they should be lashed to the main frame by thin indiarubber cord.

§ 17. In the case of a biplane model the "gap" must not be less than the "chord"—preferably greater.

In a double monoplane (of the Langley type) there is considerable "interference," i.e. the rear plane is moving in air already acted on by the front one, and therefore moving in a downward direction. This means decreased efficiency. It can be overcome, more or less, by varying the dihedral angle at which the two planes are set; but cannot be got rid of altogether, or by placing them far apart. In biplanes not possessing a dihedral angle—the propeller can be placed slightly to one side—in order to neutralise the torque of the propeller—the best portion should be found by experiment—unless the pitch be very large; with a well designed propeller this is not by any means essential. If the propeller revolve clockwise, place it towards the right hand of the machine, and vice versa.

§ 18. In designing a model to fly the longest possible distance the monoplane type should be chosen, and when desiring to build one that shall remain the longest time in the air the biplane or triplane type should be adopted.[40] For the longest possible flight twin propellers revolving in opposite directions[41] are essential. To take a concrete case—one of the writer's models weighed complete with a single propeller 8½ oz. It was then altered and fitted with two propellers (same diameter and weight); this complete with double rubber weighed 10¼ oz. The advantage double the power. Weight increased only 20 per cent., resistance about 10 per cent., total 30 per cent. Gain 70 per cent. Or if the method of gearing advocated (see Geared Motors) be adopted then we shall have four bunches of rubber instead of two, and can thereby obtain so many more turns.[42] The length of the strands should be such as to render possible at least a thousand turns.

The propellers should be of large diameter and pitch (not less than 35° at the tips), of curved shape, as advocated in §22 ch. v.; the aerofoil surface of as high an aspect ratio as possible, and but slight camber if any; this is a very difficult question, the question of camber, and the writer feels bound to admit he has obtained as long flights with surfaces practically flat, but which do, of course, camber slightly in a suitable wind, as with stiffer cambered surfaces.

Wind cambered surfaces are, however, totally unsuitable in gusty weather, when the wind has frequently a downward trend, which has the effect of cambering the surface the wrong way about, and placing the machine flat on the ground. Oiled or specially prepared silk of the lightest kind should be used for surfacing the aerofoils. Some form of keel, or fin, is essential to assist in keeping the machine in a straight course, combined with a rudder and universally jointed elevator.

The manner of winding up the propellers has already been referred to (see chap. iii., §9). A winder is essential.

Another form of aerofoil is one of wood (as in Clarke's flyers) or metal, such a machine relying more on the swiftness of its flight than on its duration. In this the gearing would possibly not be so advantageous—but experiment alone could decide.

The weight of the machine would require to be an absolute minimum, and everything not absolutely essential omitted.

It is quite possible to build a twin-screw model on one central stick alone; but the isosceles triangular form of framework, with two propellers at the base corners, and the rubber motors running along the two sides and terminating at the vertex, is preferred by most model makers. It entails, of course, extra weight. A light form of skid, made of steel pianoforte wire, should be used. As to the weight and size of the model, the now famous "one-ouncers" have made some long flights of over 300 yards[43]; but the machine claiming the record, half a mile,[44] weighs about 10 oz. And apart from this latter consideration altogether the writer is inclined to think that from 5 oz. to 10 oz. is likely to prove the most suitable. It is not too large to experiment with without difficulty, nor is it so small as to require the skill of a jeweller almost to build the necessary mechanism. The propeller speed has already been discussed (see ch. v., §15). The model will, of course, be flown with the wind. The total length of the model should be at least twice the span of the main aerofoil.