Refer to figure.
Because of the difference of the relative airflow velocity across the rotor disc area in forward flight or in hover with wind conditions, the advancing blade experiences the highest flow velocity and the retreating blade the lowest.
Since, the lift produced is proportional to the squared velocity, then the advancing blade will produce significantly more lift than the retreating one.
As each rotor blade enters the advancing side of the rotor disc area, it begins to generate progressively more lift, which further causes the blade to flap up. As it flaps up, its angle of attack reduces, thus reducing the amount of lift it generates.
Conversely, as the rotor blade enters the retreating side of the rotor disc area, it begins to generate less lift, which further causes the blade to fall. As it flaps down, its angle of attack increases, thus increasing the amount of lift it generates and lift is partially restored.
The overall effect of the loss and gain in lift across the rotor disc area, as the blades pass from the advancing side to the retreating, is called “flapped to equality” and can be achieved either by using flapping hinges or by using flexible blade able to flap without hinges.
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