r use.

6. CAMBER.--The camber (curvature) of the blades should be (1) equal,
(2) decrease evenly towards the tips of the blades, and (3) the greatest
depth of the curve should, at any point of the blade, be approximately
at the same percentage of the chord from the leading edge as at other
points.

It is difficult to test the top camber without a set of templates,[18]
but a fairly accurate idea of the concave camber can be secured by
slowly passing a straight-edge along the blade, thus:

[Illustration]

The camber can now be easily seen, and as the straight-edge is passed
along the blade, the observer should look for any irregularities of the
curvature, which should gradually and evenly decrease towards the tip of
the blade.

7. THE JOINTS.--The usual method for testing the glued joints is by
revolving the propeller at greater speed than it will be called upon to
make during flight, and then carefully examining the joints to see if
they have opened. It is not likely, however, that the reader will have
the opportunity of making this test. He should, however, examine all the
joints very carefully, trying by hand to see if they are quite sound.
Suspect a propeller of which the joints appear to hold any thickness of
glue. Sometimes the joints in the boss open a little, but this is not
dangerous unless they extend to the blades, as the bolts will hold the
laminations together.

8. CONDITION OF SURFACE.--The surface should be very smooth, especially
towards the tips of the blades. Some propeller tips have a speed of over
30,000 feet a minute, and any roughness will produce a bad drift or
resistance and lower the efficiency.

9. MOUNTING.--Great care should be taken to see that the propeller
is mounted quite straight on its shaft. Test in the same way as for
straightness. If it is not straight, it is possibly due to some of the
propeller bolts being too slack or to others having been pulled up too
tightly.

FLUTTER.--Propeller "flutter," or vibration, may be due to faulty pitch
angle, balance, camber, surface area, or to bad mounting. It causes a
condition sometimes mistaken for engine trouble, and one which may
easily lead to the collapse of the propeller.

CARE OF PROPELLERS.--The care of propellers is of the greatest
importance, as they become distorted very easily.

1. Do not store them in a very damp or a very dry place.

2. Do not store them where the sun will shine upon them.

3. Never leave them long i