Drive train overview

The conventional helicopter drive train is designed around a series of mechanical gearboxes that convert the high rotational speed of the power unit, into the comparatively low speed required to drive the main rotor and tail rotor. These mechanical gearboxes vary from helicopter to helicopter, but the internal configurations are very similar. A helicopter may be equipped with the following types of gearboxes/drive train:


  • Combiner gearbox is typically used on helicopters that have two (2) engines and one (1) main rotor, where drive must be combined in a common gearbox. This gearbox contains a unique differential arrangement that permits the different engine speeds. Ideally these engines would be synchronized to share the same torque, but inevitably this is not feasible. These gearboxes contain an over – running clutch for preventing power from being fed back into a disabled engine.
  • Main gearbox transmits power from the engine to the main rotor and tail rotor. This gearbox steps – down the speed of the engine so that it does not rotate at the same speed as the engine shaft.
  • Tailrotor gearbox is also known as the 90° gearbox, located at or near the top of the vertical fin. Its primary purpose is to provide a 90° change in the drive and a speed reduction between the input and output drive shaft. This gearbox also houses the pitch change mechanisms connected to the anti-torque pedals.
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