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Which statement describes correctly the definition "lateral lift dissymmetry"?

  • A

    During cruise flight the advancing rotor blade has more airflow than the retreating rotor blade.

  • B

    The displacement of the driving forces to the advancing rotor blade in a vertical autorotation.

  • C

    The greater lift in the outer region of the rotor as opposed to the inner region.

  • D

    The difference in required performance during hover in and out of ground effect.

Refer to figure.
Dissymmetry of lift is unequal amounts of lift generated on the advancing side versus the retreating side of the main rotor during forward flight. A good way to understand the dissymmetry of lift is to understand the Lift formula and the relation of the rotor speed for advancing and retreating sides.

  • L = CL x 1/2 𝜌 x V2 x S

Where:

  • L = lift
  • CL = coefficient of lift
  • 1/2 𝜌 = half the density of the air
  • V2 = velocity of the rotor squared
  • S = surface of the blade

When a helicopter is hovering in still air, then the airflow velocity around the blade will be equal to the rotational flow. As the helicopter gains forward speed or if when hovering experiences wind, then the airflow velocity around the blade will change depending on its angular position:

  • When the blade is advancing into the wind (advancing blade), the airflow velocity around the blade will be greater than the rotational flow (Circumferential velocity plus helicopter forward speed).
  • Conversely, when the blade is retreating away from the wind (retreating blade), the airflow velocity around the blade will be smaller (Circumferential velocity minus helicopter forward speed).

Thus, during cruise flight the advancing rotor blade has more airflow than the retreating rotor blade.

For a helicopter with an anti-clockwise rotation rotor disc hovering with headwind, the advancing blade will be the blade on the pilot’s right side and the retreating blade will be on the pilot’s left side.

If the same helicopter hovers in tailwind, the position of the advancing and retreating blades is completely reversed relative to the pilot.

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