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Rotorcraft Flying Menu >Aerodynamics
of Flight >Forward
Flight > Induced Flow
As the rotor blades rotate they generate what
is called rotational relative wind. This airflow is characterized
as flowing parallel and opposite the rotor’s plane of
rotation and striking perpendicular to the rotor blade’s
leading edge. This rotational relative wind is used to generate
lift. As rotor blades produce lift, air is acceler-ated over
the foil and projected downward. Anytime a helicopter is producing
lift, it moves large masses of air vertically and down through
the rotor system. This downwash or induced flow can significantly
change the efficiency of the rotor system. Rotational relative
wind combines with induced flow to form the resultant relative
wind. As induced flow increases, resultant rel-ative wind becomes
less horizontal. Since angle of attack is determined by measuring
the difference between the chord line and the resultant relative
wind, as the resultant relative wind becomes less horizontal,
angle of attack decreases. [Figure 3-13]
Figure 3-13. A helicopter in forward
flight, or hovering with a headwind or crosswind, has more molecules
of air entering the aft portion of the rotor blade. Therefore,
the angle of attack is less and the induced flow is greater
at the rear of the rotor disc.
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