The next point to be decided on is the loading, and as the model is to be a biplane a comparatively light loading can be used. In the case of the machine shown in side elevation by [Fig. 146] and in front elevation by [Fig. 147], 4 oz. has been taken as the loading per square foot. So that the total area of the wings will be
| 20 | ||
| — | = 5 sq. ft. | = 720 sq. in. |
| 4 |
A span of 54 in. for the top plane and 46 in. for the bottom one has been decided on, and by using a chord of 8¾ in. the total area of the wings vies very approximately with this figure, allowing a small margin for excess weight. The area of the tail, which is non-lifting, need not be taken into account. Although the “gap” is given as being equal to the chord, it could be made, if anything, ½ in. greater.
Now with regard to actual materials. Birch is to be used for the longitudinals, straight in the grain and of the cross-sections illustrated. The lower member is bent under steam to the curvature shown—of 6½-in. radius. Two vertical struts support the wings, and these should be cut from hickory. A short tie-strut secures the bottom longitudinal to the front inter-strut, the joint being made by means of side angle-plates bound into place. It will be found good practice to make a full-size drawing of the machine in side elevation, so that it can be used as a template to fit up the cross members—particularly with regard to the cutting of the angles.
The joint of the longeron to the cross member is shown separately at A ([Fig. 146]). The usual fish-plates are employed, so made that a small wiring plate is left protruding from the binding, to which the cross-sectional and longitudinal-sectional wires are made off.
Fig. 148.—Plan View of Biplane Driven by Compressed Air
The plant itself is slung into the framework by means of eight wires, each being made off to the wiring plates. Each should also be provided with a small ¾-in. wire strainer to enable the plant to be fixed quite rigidly—albeit permitting of its being removed for inspection or repairs. The wires from the engine itself are taken off from the four small eyes soldered to the stationary portion of the crank-shaft. Great care should be taken to ensure that the plane of rotation of the screw is at right angles to the main planes. A 1¼-in. dihedral is given to the bottom plane by means of the bracing wires passing between the inter-struts, and shown on the preceding page.
It has been thought advisable to attach a small rear wheel, to enable the model to rise off the ground with as little loss of power as possible. Such a wheel, with attachments, need weigh no more than ¼ oz., and is a great improvement over the cane skid usually employed.
