[Fig. 157] shows the joint of the trailing central inter-struts to the top longeron. It will be seen that the joint is a halved one, pinning and binding forming the security.
All woodwork may be polished by filling the grain with gold size, and finishing with a good varnish.
In flying the model the writer would point out that full pressure should not be given to the plant until adjustment has been completed, also the importance of tuning the machine by starting it from the ground, thus obviating many vexing smashes. Further, the rudder must be set to counteract torque; if the screw is of left-hand pitch, then torque will tend to bank the machine to the right, and the rudder must therefore be set to the left, and vice versa.
A sketch of the finished machine is given by [Fig. 158], and a design for a monoplane driven by the same plant in [Fig. 159].
CHAPTER XV
General Notes on Model Designing
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.