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 1/16 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.

[Illustration]

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 be tested as follows:

[Illustration]

The propeller must be in a horizontal position, and then weighed at
fixed points, such as A, B, C, D, E, and F, and the weights noted. The
points A, B, and C must, of course, be at the same fixed distances from
the centre of the propeller as the points D, E, and F. Now reverse the
propeller and weigh at each point again. Note the results. The first
series of weights should correspond to the second series, thus:

Weight A should equal weight F.

Weight B should equal weight E.

Weight C should equal weight D.

There is no standard practice as to the degree of error permissible, but
if there are any appreciable differences the propeller is unfit for use.

5. SURFACE AREA.--The surface area of the blades should be equal. Test
with calipers thus:

[Illustration]

The distance A--B should equal K--L.

The distance C--D should equal I--J.

The distance E--F should equal G--H.

The points between which the distances are taken must, of course, be at
the same distance from the centre in the case of each blade.

There is no standard practice as to the degree of error permissible. If,
however, there is an error of over 1/8 inch, the propeller is really
unfit fo