whence d A r = 0·07 about.
| Fig. 32. | Fig. 33. |
In Fig. 31 set off A B equal to the pitch of the propeller (42 in.), one-eighth scale. Set off B C at right angles to A B and equal to
π × diameter = 22/7 × 14 = 44 in. to scale 5½ in.
Divide B C into a convenient number of equal parts in the figure; five only are taken, D, E, F, G, H; join A D, A E, A F, A G, A H and produce them; mark off distances P O, S R, Y T ... equal to the width of the blade at these points (H P = H O; G S = G R ...) and sketch in the sections of blade as desired. In the figure the greatest concavity of the blade is supposed to be one-third the distances P O, S R ... from PS.... The concavity is somewhat exaggerated. The angles A H B, A G B, A F B ... represent the pitch angle at the points H, G, F ... of the blade.
Similarly any other design may be dealt with; in a propeller of 14 in. diameter the diameter of the "boss" should not be more than 10/16 in.
§ 20. Experiments with Propellers.—The propeller design shown in Figs. 32 and 33, due to Mr. G. de Havilland,[35] is one very suitable for experimental purposes. A single tube passing through a T-shaped boss forms the arms. On the back of the metal blade are riveted four metallic clips; these clips being tightened round the arm by countersunk screws in the face of the blade.
The tube and clips, etc., are all contained with the back covering of the blade, as shown in Fig. 35, if desired, the blade then practically resembling a wooden propeller. The construction, it will be noticed, allows of the blade being set at any angle, constant or otherwise; also the pitch can be constant or variable as desired, and any "shape" of propeller can be fitted.