s are different, but their pitch (in this
case altitude reached in a given time) is the same.

[Illustration]

In order to test the pitch angle, the propeller must be mounted upon
a shaft at right angles to a beam the face of which must be perfectly
level, thus:

[Illustration]

First select a point on the blade at some distance (say about 2 feet)
from the centre of the propeller. At that point find, by means of a
protractor, the angle a projection of the chord makes with the face of
the beam. That angle is the pitch angle of the blade at that point.

Now lay out the angle on paper, thus:

[Illustration]

The line above and parallel to the circumference line must be placed
in a position making the distance between the two lines equal to the
specified pitch, which is, or should be, marked upon the boss of the
propeller.

Now find the circumference of the propeller where the pitch angle is
being tested. For example, if that place is 2 feet radius from the
centre, then the circumference will be 2 feet x 2 = 4 feet diameter,
which, if multiplied by 3.1416 = 15.56 feet circumference.

Now mark off the circumference distance, which is represented above by
A--B, and reduce it in scale for convenience.

The distance a vertical line makes between B and the chord line is
the pitch at the point where the angle is being tested, and it should
coincide with the specified pitch.

You will note, from the above illustration, that the actual pitch line
should meet the junction of the chord line and top line.

The propeller should be tested at several points, about a foot apart, on
each blade; and the diagram, provided the propeller is not faulty, will
then look like this:

[Illustration:
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:

[Illustration]

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