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A propeller
is just a driven wing. The diagram illustrates the helical path of the
propeller tip through the air as the vehicle moves with a constant velocity.
The efficiency of the propeller depends upon the ratio of lift to drag for
the device as the drag dissipates energy that does not then contribute to the
propeller thrust.
The thrust depends on the pressure differential across
the propeller. This gives rise to a force, F = (pout - pin)A,
where A is the area swept by the blades. The interaction of the propeller
with the fluid increases its kinetic energy and this determines the power required
to drive the propeller.
In the steady state cruise condition the
propeller will rotate at a constant angular velocity which implies that the
speed of the blade through the air increases linearly with its radial distance
from the hub. Propellers frequently have a twist along their length to change
the local angle of attack and, hence, the local lift (thrust) of the blade.
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