n 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.

[Illustration:
SPIRAL COURSES OF TWO-BLADE TIPS.
SPIRAL COURSES OF FOUR-BLADE TIPS.
Pitch the same in each case.]

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 (see illustration on preceding page).

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 propeller is only
suitable for large pitch, as such pitch produces spirals with a large
gap, thus offsetting the decrease in gap caused by the numerous blades.

The greater the speed of rotation, the less the pitch for a given
aeroplane speed. Then, in order to secure a large pitch and consequently
a good gap, the four-bladed propeller is usually geared to rotate at a
lower speed than would be the case if directly attached to the engine
crank-shaft.

[Footnote 18: I have heard of temporary ones being made quickly by
bending strips of lead over the convex side of the blade, but I should
think it very difficult to secure a sufficient degree of accuracy in
that way.]

CHAPTER V

MAINTENANCE

CLEANLINESS.--The fabric must be kept clean and free from oil, as that
will rot it. To take out dirt or oily patches, try acetone. If that will
not remedy matters, then try petrol, but use it sparingly, as otherw