Fig. 179.—Plan
The model is driven by four elastic motors, transmitting their power through equal gearing to the twin propellers. The four motor rods may, for preference, be hollow spars, of the same cross-section as the solid ones indicated. If such are used, they may be made by ploughing a groove in a length of wood of suitable cross-section so that it represents the letter U. Two will be required for each spar, so glued together that a hollow tube is formed. At the point where it will be necessary to pierce the spar to admit the bracing outrigger, small packing pieces of birch should be placed in the grooves previous to assembling the two halves of the spar. The spars, or, more correctly, motor rods, are suspended from the envelope framework by aluminium tube outriggers of ¼-in. diameter. [Fig. 181], which shows the machine in end elevation, and [Fig. 182], showing the central cross-section, indicate the form they are to take. Eight of them will be required. The angles must be cleanly and accurately formed, so that the two centres of thrust lie in the same plane. It will be found good practice, when forming both the outriggers and the cross-sectional members, to make a full-size drawing of them to use as a template. More especially is this needed in the construction of the cross-sectional members, for the purpose of ensuring that, in the operation of embracing them, they are not strained in any way so as to become out of truth.
[Fig. 183] shows the joint of the motor-rod outrigger to the motor rod itself. As there shown, the tube is flattened out partly to engirdle the spar, to which it is attached by pinning and clinching.
Fig. 180.—Joint of Tube to End-Caps
Fig. 181.—End Elevation
Fig. 182.—Central Section
