Let the shape of the blade be rectangular with rounded corners, and let the breadth at the tip be twice that at the boss.
Then the area (neglecting the rounded off corners) is 10½ sq. in.
Fig. 30.—Propeller Design.
One quarter scale. Diameter 14 in. Pitch 52 in. Angle at tip 44°.
The area being that of a rectangle 7 in. × 1 in. = 7 sq. in. plus area of two triangles, base ½ in., height 7 in. Now area of triangle = half base × height. Therefore area of both triangles = ½ in. × 7 in. = 3½ sq. in. Now the area of the disc swept out by the propeller is
π/4 × (diam.)2 (π = 22/7)
Fig. 31.—Propeller Design.
Scale one-eighth for A B and B C; but sections of blade are full-sized.
And if d A r = the "disc area ratio" we have
(d A r) × π/4 × (14)2 = area of blade = 10½,