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Centrifugal turning moment on a blade is a function of…

  • A

    Lead-lag movement of the rotor blade.

  • B

    Flapping movement of the rotor blade.

  • C

    Coning movement of the rotor blade.

  • D

    Feathering movement of the rotor blade.

Refer to figures.

The coning angle is a function of the radial component of centrifugal force (CF). This force radiates from the centre of the hub like the spokes of a bicycle wheel throughout the plane of rotation.

The CF has a radial component R parallel to the blade axis trying to pull the blade from the hub. At the leading and trailing edge of the blade, there is a tangential component T of the CF.

If one blade is viewed in profile form the tip, it can be seen that the component T acts at a distance X from the feathering hinge. This is the Centrifugal Turning Moment (CTM) or feathering/pitching moment chordwise, which will cause a decreased pitch angle.

For the pilot to increase pitch angle by use of collective, the CTM will produce a force opposing the pilot increasing control forces.

Modern helicopters will incorporate powered flying controls, which overcome this problem. However, the use of another principle of centrifugal force can be used to counter CTM.

A counterweight is fitted at the rotor blades root so that, as it tries to align with the rotational plane, it increases the pitch angle. This will be in opposition to CTM and will reduce the pilot’s control load.

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