The rear of the model is the proper place for the propeller, in the centre of greatest air disturbance; in such a position it will recover a portion of the energy lost in imparting a forward movement to the air, caused by the resistance, the model generally running in such air—the slip of the screw is reduced to a corresponding degree—may even vanish altogether, and what is known as negative slip occur.
§ 6. Wooden or metal aerofoils are more efficient than fabric covered ones. But they are only satisfactory in the smaller sizes, owing, for one thing, to the smash with which they come to the ground. This being due to the high speed necessary to sustain their weight. For larger-sized models fabric covered aerofoils should be used.
§ 7. As to the shape of such, only three need be considered—the (a) rectangular, (b) the elongated ellipse, (c) the chamfered rear edge.
Fig. 48.—(a), (b), (c).
§ 8. The stretching of the fabric on the aerofoil framework requires considerable care, especially when using silk. It is quite possible, even in models of 3 ft. to 4 ft. spread, to do without "ribs," and still obtain a fairly correct aerocurve, if the material be stretched on in a certain way. It consists in getting a correct longitudinal and transverse tension. We will illustrate it by a simple case. Take a piece of thickish steel pianoforte wire, say, 18 in. long, bend it round into a circle, allowing ½ in. to 1 in. to overlap, tin and solder, bind this with soft very thin iron wire, and again solder (always use as little solder as possible). Now stitch on to this a piece of nainsook or silk, deforming the circle as you do so until it has the accompanying elliptical shape. The result is one of double curvature; the transverse curve (dihedral angle) can be regulated by cross threads or wires going from A to B and C to D.
Fig. 49.
Fig. 49a.—Mr. T.W.K. Clarke's 1 oz. Model.