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What is the purpose of a free-wheel unit on a helicopter?
  • A
    Allows for fluctuations in engine speed.
  • B
    Prevents the engine from dragging the rotors.
  • C
    Isolates the main rotor from the engine and the tail rotor.
  • D
    Relieve torsional stresses in the gearbox and rotor drive when starting up and accelerating.

Refer to figure.

In case of engine failure, the helicopters are able to continue in a controlled descent, known as “Autorotation”. In this case, the drive to the rotors from the engine is disengaged by a free-wheel unit.

The purpose of the free-wheel unit is to insure that, if an engine fails, it doesn't drag the speed of the rotor down below the minimum required for a controlled flight, i.e. removes any resistance to the rotor rotation caused by a dead engine.

Two common designs include:

  • The sprag clutch used at higher speeds.
  • The ramp and roller used for lower speed applications.

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