Some of the competitions arranged by the Kite and Model Aeroplane Association have been for duration models of a minimum weight of 1 lb., capable of rising from the ground under their own power, carrying a dead weight of a quarter of their own total weight. Such a model is that here illustrated and described. It has flown repeatedly for thirty-eight seconds after rising from the ground, while its hand-launched duration is about half a minute, by no means a small accomplishment for a 16-oz. model. It will be seen that it has somewhat larger dimensions than the ordinary rubber-driven model.

The top main spar is of spruce, 4 ft. 6 in. long and ½ in. by ¼ in. in cross-section, the bottom one being 2 in. longer and ¼ in. by ³/₁₆ in. in section. The bottom member is to be bent by steam approximately to the shape shown in the side view of the machine (Fig. 113), and then fitted to make a clean butt joint to the top spar B (Fig. 114). Bearings for the two ⅝-in. gears, details of which are shown in Fig. 115, are bent to shape from No. 20 gauge sheet-brass, a lug being left projecting to engage with the bottom member of the fuselage. The holes for the shafts should be drilled so that the gears make a fairly tight mesh. The spindles are to be of No. 16 gauge piano wire, on to which the gears are to be soldered. The propeller shaft is continued forward of the bearing for 1½ in., and is bent back at an angle (as shown in detail at A in Fig. 116) to grip tightly in the propeller boss. The gears are kept central between the two bearings by means of pieces of brass tubing, which are slipped on the spindles at each side of the bearing and soldered in position.

Elastic hooks, formed from No. 16 gauge wire, are fixed at the rear end of the fuselage, and serve the double purpose of providing an anchorage for the bottom spar, which is whipped and glued with the hooks to the top spar. Fig. 114 shows clearly what is meant. Bamboo, of ³/₁₆-in. by ₧-in. cross-section, is used for the tail skid, which is secured to the fuselage by thread binding. Piano wire is used for the landing chassis, and of the same gauge as hitherto used on the spindles.

The triangular side struts of the chassis should first be framed up from one continuous length of wire, lugs being bent at the point where they meet the fuselage, to be bound with fine tinned iron wire and soldered to the spar. Figs. 113, 117 and 118 clearly show its construction. Make the axle of such a length that a 12-in. wheel-base may be left after the wheels are placed on. The writer found that 2-in. rubber-tyred disc wheels left nothing to be desired for rising off short grass. Care should be taken that the measurement from the periphery of the wheel to the top spar, measured, of course, in a vertical direction, shall be 9 in. (see Fig. 113).

As twin gears of equal size are used, the torques of the oppositely revolving skeins of rubber will be balanced. Hence no bracing will be found necessary on the motor spar. The gearing, by the way, is bound to the top and bottom members of the fuselage with tinned iron wire, and soldered as shown in Fig. 115.

The main plane is of rather a large span, and it is essential that birch be used, ½ in. by ³/₃₂ in. in section. The wing spars are bent at their centres, to impart a tapering wing plan analogous to the Martinsyde monoplane. Seven ribs connect the spars, and these are cambered to ¾ in. The wing tapers from 10 in. at the centre to 7 in. at the tips, the centre rib projecting for ½ in. fore and after of the wing. The tin clips shown in detail in Fig. 119 slip over these, and so provide a means of adjustment to the centre of pressure of the complete machine.

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.

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 shows the model in perspective.