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In airplanes, longitudinal stability is maintained if centre of lift lies aft of center of gravity.

  • Like in the airplane shown in the figure below, do helicopters also have stability criteria?
  • They have main and tail rotors. How is stability affected by their location?
  • Does the centre of the main rotor always correspond to the centre of lift?

I have read that airplanes are more stable than helicopters. A diagram showing point that affects stability on helicopters is much appreciated.

enter image description here

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  • $\begingroup$ The diagram there is really describing trim forces, not static stability forces. In any case, the helicopter has neither. $\endgroup$
    – John K
    Jun 13, 2021 at 21:17

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In hover, helicopters are unstable and there is nothing that can be done about it. Being banked does not create any restoring moment as lift is still along the shaft, but center of drag is below center of thrust, causing destabilizing moment.

In forward flight, the advancing blade creates more lift (unless countered by cyclic), which causes the rotor pitch up (the blade reaches highest point about 90° later due to gyroscopic effect), creating stabilizing moment that overcomes the destabilizing moment of low center of drag above certain speed. However there is no aerodynamic way to trim for speed like in airplanes (there can of course be adjustable springs in the control system).

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