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Why does a flare during an autorotation generate higher rotor RPM?

  • A

    The pitch angle increases.

  • B

    The vertical airflow through the rotor disc increases.

  • C

    The tangential force tilts backwards.

  • D

    The ground effect leads to better performance.

Refer to figure.
Flare is the most effective method to decelerate from forward speed to zero.

It is initiated by moving the cyclic control rearwards. This tilts the disc and thus the Total Rotor Thrust (TRT) vector rearwards. As a result, the horizontal component of TRT is rapidly reoriented and now points backwards instead of forwards.

Although the flare is mostly associated with the final stages of autorotation, it is actually used in every landing, when you reduce forward speed to zero.

It can slow you down horizontally and vertically because, not only is the horizontal component of TRT reversed, but the airflow from underneath increases the angle of attack and creates more TRT as well.

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