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
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.
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