dissymmetry of lift

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dissymmetry of rotor thrust

dissymmetry of rotor thrustclick for a larger image
View from rear of helicopter.
A condition in which one side of a rotor disc produces more thrust than the other. The speed of relative airflow over the advancing blade is higher than that of the retreating blade. The rotor thrust of the advancing blade also is higher than that of the retreating blade and thus the dissymmetry. This results in the helicopter rolling toward the retreating side. Suitable methods, like decreasing the angle of attack of the advancing blade and increasing the angle of attack of the retreating blade, are used to prevent this rolling tendency. Also called dissymmetry of lift.

flapping to equality

flapping to equalityclick for a larger image
During the movement of a helicopter rotating blade, the speed of airflow over the advancing blade is higher than the retreating blade, resulting in production of an unequal amount of lift. This is called a dissymmetry of lift. To correct this aerodynamic imbalance, the blades are allowed to perform a series of controlled movements to produce equal amounts of lift. This process of equalization is called flapping to equality. See also dissymmetry of lift.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
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It also reduces the dissymmetry of lift due to rotors moving in opposite directions, it increases payload for the same engine power and reduces the noise.