stration overleaf).

The Thrust is opposed to the Drift of the aeroplane, and must be equal
and opposite to it at flying speed. If it falls off in power, then the
flying speed must decrease to a velocity, at which the aeroplane drift
equals the decreased thrust. The Drift of the propeller may be
conveniently divided into the following component values:

_Active Drift_, produced by the useful thrusting part of the propeller.

_Passive 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 roughness 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.

_Aspect Ratio._--Ditto.

_Camber._--Ditto.

[Illustration:
M, Direction of motion of propeller (rotary).
R, Direction of reaction.
T, Direction of thrust.
AD, Direction of the resistance of the air to the passage of the
aeroplane, _i.e._, aeroplane drift.
D, Direction of propeller drift (rotary).
P, Engine power, opposed to propeller drift and transmitted to
the propeller through the propeller shaft.]

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 al