Fig. 38.—"Venna" Propeller.
A 20 per cent. more efficient propeller than that shown in Fig. 41; 14 per cent. lighter; 6 per cent. better in dynamic thrust—14 in. diam.; weight 31 grammes.
Metal propellers of constant angle, as well as wooden ones of uniform (constant) pitch, were tested; the former gave good results, but not so good as the latter.
The best angle of pitch (at the tip) was found to be from 20° to 30°.
In all cases when the slip was as low as 25 per cent., or even somewhat less, nearly 20 per cent., a distinct "back current" of air was given out by the screw. This "slip stream," as it is caused, is absolutely necessary for efficiency.
§ 21. Fabric-covered screws did not give very efficient results; the only point in their use on model aeroplanes is their extreme lightness. Two such propellers of 6 in. diameter can be made to weigh less than 1/5 oz. the pair; but wooden propellers (built-up principle) have been made 5 in. diameter and 1/12 oz. in weight.
§ 22. Further experiments were made with twin screws mounted on model aeroplanes. In one case two propellers, both turning in the same direction, were mounted (without any compensatory adjustment for torque) on a model, total weight 1½ lb. Diameter of each propeller 14 in.; angle of blade at tip 25°. The result was several good flights—the model (see Fig. 49c) was slightly unsteady across the wind, that was all.
In another experiment two propellers of same diameter, pitch, etc., but of shape similar to those shown in Figs. 28 and 29, were tried as twin propellers on the same machine. The rubber motors were of equal weight and strength.
The model described circled to the right or left according to the position of the curved-shaped propeller, whether on the left or right hand, thereby showing its superiority in dynamic thrust. Various alterations were made, but always with the same result. These experiments have since been confirmed, and there seems no doubt that the double-curved shaped blade is superior. (See Fig. 39.)
§ 23. The Fleming-Williams Propeller.—A chapter on propellers would scarcely be complete without a reference to the propeller used on a machine claiming a record of over a quarter of a mile. This form of propeller, shown in the group in Fig. 36 (top right hand), was found by the writer to be extremely deficient in dynamic thrust, giving the worst result of any shown there.