Transmissions
The main gearbox and rotor-load path, the freewheel unit and autorotation, tail-rotor drive and the rotor brake.
Transmissions — summary notes
Free- The MAIN GEARBOX (transmission) reduces engine speed to rotor speed through a large gear reduction, and simultaneously drives the tail-rotor shaft and the accessories (generators, hydraulic pumps, oil pumps).
- Crucially, the main gearbox and its mounts carry the ENTIRE rotor lift/thrust load into the airframe — it is the most safety-critical single drive component, so its oil temperature/pressure and magnetic CHIP DETECTORS are closely monitored (and trended by HUMS).
- The FREEWHEEL (sprag/overrunning) unit in the drive lets the rotor overrun the engine — disconnecting a failed or idling engine so the rotor can autorotate.
- The tail-rotor drive runs from the main gearbox along tail-rotor drive shafts (with flexible couplings and hanger bearings), through an intermediate gearbox at the tail-boom bend, to the tail (90°) gearbox that drives the tail rotor.
- A ROTOR BRAKE stops the rotor after shutdown; the transmission has its own lubrication system, sometimes with an emergency-lubrication provision for a limited run-dry time.
- ⚠ Exam trap: the main gearbox carries the full rotor lift into the airframe and is the most critical drive item (chip detectors/HUMS); the FREEWHEEL unit disengages a stopped/slowed engine to permit autorotation.
Why is a magnetic chip detector in the main gearbox regarded as one of the most important warnings on a helicopter?
The main gearbox carries the entire rotor load and has no redundant path — a gearbox failure is catastrophic. Metal particles captured by the chip detector are an early sign of internal gear or bearing deterioration, giving warning before failure. Because the consequence is so severe, a chip-detector indication is treated very seriously (often a land-as-soon-as-possible action) and gearbox condition is continuously trended by HUMS.
Transmissions concept map
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