her.

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, or surface area. 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 in a horizontal position or leaning up against
a wall.

4. They should be hung on horizontal pegs, and the position of the
propellers should be vertical.

If the points I have impressed upon you in these notes are not attended
to, you may be sure of the following results:

1. Lack of efficiency, resulting in less aeroplane speed and climb than
would otherwise be the case.

2. Propeller "flutter" and possible collapse.

3. A bad stress upon the propeller shaft and its bearings.

TRACTOR.--A propeller mounted in front of the main surface.

PUSHER.--A propeller mounted behind the main surface.

FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when
the pitch is comparatively large.

For a given pitch, and having regard to "interference," they are not so
efficient as two-bladed propellers.

The smaller the pitch, the less the "gap," i.e., the distance, measured
in the direction of the thrust, between the spiral courses of the
blades.

If the gap is too small, then the following blade will engage air
which the preceding blade has put into motion, with the result that the
following blade will not secure as good a reaction as would otherwise be
the case. It is very much the same as in the case of the aeroplane gap.

For a given pitch, the gap of a four-bladed propeller is only half
that of a two-bladed one. Therefore the four-bladed propell