e Drift, produced by all the rest of the propeller, i.e., by its
detrimental surface.

Skin Friction, produced by the friction of the air with roughnesses of
surface.

Eddies attending the movement of the air caused by the action of the
propeller.

Cavitation (very marked at excessive speed of revolution). A tendency of
the propeller to produce a cavity or semi-vacuum in which it revolves,
the thrust decreasing with increase of speed and cavitation.

THRUST-DRIFT RATIO.--The proportion of thrust to drift is of paramount
importance, for it expresses the efficiency of the propeller. It is
affected by the following factors: Speed of Revolution.--The greater the
speed, the greater the proportion of drift to thrust. This is due to
the increase with speed of the passive drift, which carries with it no
increase in thrust. For this reason propellers are often geared down to
revolve at a lower speed than that of the engine.

Angle of Incidence.--The same reasons as in the case of the aeroplane
surface.

Surface Area.--Ditto.

Aspect Ratio.--Ditto.

Camber.--Ditto.

In addition to the above factors there are, when it comes to actually
designing a propeller, mechanical difficulties to consider. For
instance, the blades must be of a certain strength and consequent
thickness. That, in itself, limits the aspect ratio, for it will
necessitate a chord long enough in proportion to the thickness to make
a good camber possible. Again, the diameter of the propeller must be
limited, having regard to the fact that greater diameters than those
used to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage to keep the
propeller off the ground, and such undercarriage would not only produce
excessive drift, but would also tend to make the aeroplane stand on
its nose when alighting. The latter difficulty cannot be overcome by
mounting the propeller higher, as the centre of its thrust must be
approximately coincident with the centre of aeroplane drift.

MAINTENANCE OF EFFICIENCY.

The following conditions must be observed:

1. PITCH ANGLE.--The angle, at any given point on the propeller, at
which the blade is set is known as the pitch angle, and it must be
correct to half a degree if reasonable efficiency is to be maintained.

This angle secures the "pitch," which is the distance the propeller
advances during one revolution, supposing the air to be solid.