he
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 dine 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:

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.

3. LENGTH.--The blades should be of equal length to inch.

4. BALANCE.--The usual method of testing a propeller for balance is as
follows: Mount it upon a shaft, which must be on ball-bearings. Place
the propeller in a horizontal position, and it should remain in that
position. If a weight of a trifle over an ounce placed in a bolt-hole on
one side of the boss fails to disturb the balance, then the propeller is
usually regarded as unfit for use.

The above method is rather futile, as it does not test for the balance
of centrifugal force, which comes into play as soon as the propeller
revolves. It can b