A, B, C, and D, Actual pitch at points tested.
I, Pitch angle at point tested nearest to centre of propeller.
E, Circumference at I.
J, Pitch angle at point tested nearest to I.
F, Circumference at J.
K, Pitch angle at next point tested.
G, Circumference at K.
L, Pitch angle tested at point nearest tip of blade.
H, Circumference at L.
At each point tested the actual pitch coincides with the specified pitch: a satisfactory condition.
A faulty propeller will produce a diagram something like this:
At every point tested the pitch angle is wrong, for nowhere does the actual pitch coincide with the specified pitch. Angles A, C, and D, are too large, and B is too small. The angle should be correct to half a degree if reasonable efficiency is to be maintained.
A fault in the pitch angle may be due to (1) faulty manufacture, (2) distortion, or (3) the shaft hole through the boss being out of position.
2. Straightness.—To test for straightness the propeller must be mounted upon a shaft. Now bring the tip of one blade round to graze some fixed object. Mark the point it grazes. Now bring the other tip round, and it should come within 1/8 inch of the mark. If it does not do so, it is due to (1) faulty manufacture, (2) distortion, or (3) to the hole through the boss being out of position.