§ 19. The Blades, two in number, and hollow faced—the maximum concavity being one-third the distance from the entering to the trailing edge; the ratio of A T to O P (the width) being 0·048 or 1 : 21, these latter considerations being founded on the analogy between a propeller and the aerofoil surface. (If the thickness be varied from the entering to the trailing edge the greatest thickness should be towards the former.) The convex surface of the propeller must be taken into account, in fact, it is no less important than the concave, and the entire surface must be given a true "stream line" form.
| Fig. 28. | Fig. 29. |
If the entering and trailing edge be not both straight, but one be curved as in Fig. 28, then the straight edge must be made the trailing edge. And if both be curved as in Fig. 29, then the concave edge must be the trailing edge.
§ 19. Propeller Design.—To design a propeller, proceed as follows. Suppose the diameter 14 in. and the pitch three times the diameter, i.e. 52 in. (See Fig. 30.)
Take one-quarter scale, say. Draw a centre line A B of convenient length, set of half the pitch 52 in.— ¼ scale = 5¼ in. = C - D. Draw lines through C and D at right angles to C D.
With a radius equal to half the diameter (i.e. in this case 1¾ in.) of the propeller, describe a semicircle E B F and complete the parallelogram F H G E. Divide the semicircle into a number of equal parts; twelve is a convenient number to take, then each division subtends an angle of 15° at the centre D.
Divide one of the sides E G into the same number of equal parts (twelve) as shown. Through these points draw lines parallel to F E or H G.
And through the twelve points of division on the semicircle draw lines parallel to F H or E G as shown. The line drawn through the successive intersections of these lines is the path of the tip of the blade through half a revolution, viz. the line H S O T E.
S O T X gives the angle at the tip of the blades = 44°.