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On which type of helicopters are clutches located in the drive between the engine and the main transmission?

  • A

    All helicopters.

  • B

    Helicopters with piston or direct drive gas turbine engines.

  • C

    Helicopters fitted with piston engines only.

  • D

    Helicopters with free power turbine engines only.

Refer to figures.
Generally, the clutch facilitates the engine starting with minimal inertial loading, ensuring that the gearbox engages only after reaching a certain RPM.

The clutch is located between the engine and the main transmission.

It operates within an oil environment essential for cooling and proper functioning, consisting of a set of shoes that connect to a drum casing and are driven by the engine.

Centrifugal force keeps the shoes engaged with the drum, enabling power transmission.

As a result, the engine experiences reduced load during the startup process, apart from driving the standard components such as generators and pumps.

More specifically, the centrifugal clutch operates automatically and becomes more effective as the speed increases.

On the other hand, when the engine's rotational speed falls below the equivalent rotational speed of the rotor, the centrifugal clutch disengages, allowing the rotor to spin freely, which is crucial for autorotation during an emergency landing.

Finally, free turbine engines don’t need clutches, but fixed turbine and piston-engined helicopters do, and it is in the drive between the engine and the main transmission.

Therefore, the correct answer is “Helicopters with piston or direct drive gas turbine engines”.


  • In a free turbine engine, the gas generator and power turbine can rotate independently of each other. The gas generator produces the high-pressure gases that drive the power turbine, which in turn generates power for the helicopter's rotor system. Since the rotations are independent, there's no need for a clutch to engage or disengage the power transfer between the gas generator and the power turbine.

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