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For a helicopter in straight and level forward flight, the angle of attack of the blades of a main rotor will be...

  • A
    The same throughout the disc.
  • B
    Greatest on the retreating side.
  • C
    Greatest at the rear of the disc.
  • D
    Greatest at the front of the disc.

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
  • 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

Knowing the above we can derive that the advancing blade speed is increased in forward flight and therefore, lift is increased while the retreating blade speed is reduced as the helicopter moves forward in flight leading to a reduction of lift.

If the condition was allowed to exist the helicopter would flap back and roll to the retreating blades side. This is prevented by allowing for blade flapping leading to equalised lift on both advancing and retreating sides.

As the blade moves thru air on the advancing side, the blade flaps up leading to a lowering of angle of attack (pitch angle reduces). Therefore, there is a reduction of the coefficient of lift as it is affected by angle of attack and the shape of the blade. The coefficient of lift is reduced while the velocity of the blade is increased, keeping the lift more constant.

As the blade moves through the air on the retreating side, the blade flaps down leading to an increased angle of attack (pitch angle increases). Therefore, there is an increase in the coefficient of lift and a reduction of the velocity, keeping the lift more constant.

Blade flapping is achieved on the different rotor systems:

  • Fully articulated - by flapping hinges.
  • Semi rigid - by teetering hinge.
  • Rigid - blade bending.

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