Calculations in Designing a Plant for a Model Aeroplane.—The correct method to adopt in designing a model is to build the machine exclusive of the motor, weigh it, and design the plant to suit; or to build the plant first, determine the thrust it develops, and vary the dimensions of the machine (and hence the weight) to suit.
It can now be taken as a general rule that a plant will fly a machine three times the weight of its thrust. Hence a plant developing 3-oz. thrust would fly a machine weighing 9 oz. or 10 oz. But, since the thrust of a compressed-air engine is not constant, gradually diminishing as the pressure in the container grows less, for a machine weighing 9 oz. (assuming the machine to be built first) the plant will require to develop about 5 oz. initial thrust. The diameter of the propeller is dependent on the most efficient speed of the particular motor employed.
Assuming that the model does not weigh more than 17 oz., a four-cylinder engine constructed would answer admirably. It would require a container 24 in. long and 3 in. in diameter, constructed of copper or hard-drawn brass foil ·002 in. thick. Should, however, the reader particularly desire to fit a rotary motor, doubtless the five-cylinder rotary previously described will suit.
Building Scale Models.—The great difficulty in building scale models to fly with rubber motors is to get the centre of gravity in the same relative position that it holds in the prototype. This is due to the long length of rubber motor required to give the requisite power, and also to ensure a reasonable time-length of flight.
It is in connection with the fixing or arranging of the rubber motor that the most radical departures in the design of the prototype will have to be made, although the writer has evolved an arrangement whereby even this need not entail much departure from the lines of the original. This arrangement consists in providing a separate strut or frame to take up all strain from the rubber, it only being necessary to arrange suitable fastenings for the strut, which may take the form of clips, so making it possible either to remove the motor for the purpose of changing or repairing the rubber, or substituting a motor of different power or length. This is a great advantage on tractor monoplanes (with the main plane in front), where the rubber is more or less inaccessible by reason of the closed-in frame or fuselage of the machine. Another important advantage to be gained from the use of a detachable motor is that its position fore and aft on the machine can be varied, in order to bring the centre of gravity into the proper position to obtain correct balance, or, speaking with more technical accuracy, to make the centres of pressure and gravity coincide.
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.