Transmissions
The main gearbox and rotor-load path, the freewheel unit and autorotation, tail-rotor drive and the rotor brake.
Transmissions, summary notes
- 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
Transmissions
Transmissions quiz
Transmissions, quiz
1. The main gearbox primarily
Reduces engine speed to rotor speed and drives the tail rotor/accessoriesGenerates electrical power only, driving the alternators through an accessory drive padStores fuel2. A safety-critical feature of the main gearbox is that it
Carries the entire rotor lift load into the airframeHas a redundant duplicateIs non-load-bearing3. The freewheel (sprag) unit allows the
Rotor to overrun/disconnect a stopped or slowed engineEngine to overspeed the rotorTail rotor to stop4. Metal particles on the main-gearbox chip detector indicate
Possible internal gear/bearing deteriorationNormal operationLow fuel5. The tail-rotor drive runs from the main gearbox through
Drive shafts, an intermediate gearbox and a tail (90°) gearboxA single direct belt drive, tensioned to slip if the tail rotor is obstructed on the groundThe fuel system, which drives a hydraulic motor mounted at the base of the tail pylon6. A rotor brake is fitted to
Stop the rotor after shutdownSlow the aircraft in flightDrive the tail rotor7. Because the main gearbox has no redundancy, its condition is
Continuously monitored (chip detectors, oil temp/pressure, HUMS)Ignored between scheduled overhauls, since the gearbox is designed to run to its life limitChecked only at the annual inspection, when the magnetic plugs and filters are examined8. An engine fails and its RPM falls below rotor RPM. The freewheel unit
Disengages automatically so the rotor is not dragged downLocks the engine to the rotorStops the rotor