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.
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