The Theory and Practice of Model Aeroplaning

by V. E. Johnson

§ 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.