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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