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.
