Choice of covering must be left to the builder; but yellow Japanese silk, proofed with varnish, will be found quite suitable, and of a rather pleasing amber hue. A birch kingpost passes through the fabric, and to this the wings are braced by No. 35 gauge music wire. Sufficient tension should be placed on the top wires to give the wings a 3-in. dihedral angle, as in [Fig. 120]. The detail illustration of the kingpost ([Fig. 121]) is self-explanatory. The bracing wires are anchored to wire hooks forced through the wing in the manner shown in [Fig. 122].
The correct position of the main plane should be found by trial. The kingpost can then be permanently fixed to the main spar by pinning and gluing.
No. 18 gauge wire of the music or piano variety should be used for the tail and rudder. Draw the plan form of the wing full-size on a board; pins may then be driven partly home on each side of the line at spaces of about 3 in. The wire may now be pushed between the pins, cut off, and lapped for ½ in. The two cross ribs can be soldered to the tail before the tacks are withdrawn. It will be found on releasing the tacks that the wire will remain true to the shape of the template. This may seem a rather laborious process; but it is far quicker and easier than attempting to guess the correct curvature. The rudder may be made to any convenient shape, preferably that shown. The two ends should be sprung outwards after the form of the letter A to form a clearance for the rubber hooks, and then soldered to the tail. A very slight adjustment of this will be found necessary to obviate propeller torque.
Fig. 120.—Front Elevation
No provision has been made for the adjustment of the lift on the tail. Indeed, none was found necessary, it being quite an easy matter to bend the tail flaps up or down to increase or decrease the elevation. They should always, however, have a slight negative angle to maintain longitudinal stability. The tail should be bound to the fuselage with copper wire.
The propeller, of the usual integral type, is carved from the solid block, which measures 15 in. by ⁵/₈ in. by 2¼ in. It should be made of right-handed pitch, and must be placed on the right-hand gear, so that thrust balances torque. On each side, nine strands of ¼-in. strip rubber, lubricated with diluted soft soap, supply the motive power. This will stand approximately 600 turns. Vaseline will suffice to minimise friction on the gears.
It is advisable to test the model down the wind with about 50 per cent. of the maximum turns. The main plane should be moved forwards to increase the elevation, and backwards to decrease it.
Fig. 123.—Complete Tractor Monoplane