Rotor Drive and Freewheeling Units
Rotor Drive and Freewheeling Units tests the mechanism that lets the rotor turn without the engine and prevents the engine from being driven by the rotor. The freewheel unit is central to autorotation safety.
Core rules
- A freewheel unit uses sprags, rollers or a similar one-way clutch between the engine output and the main transmission.
- It transmits torque from engine to rotor but overruns when the rotor tries to drive the engine.
- During autorotation the main rotor drives the tail rotor and accessories, but not the engine gas generator.
- Some designs use a centrifugal clutch or a combination of clutch and freewheel for starting and engagement.
- A seized or locked freewheel prevents autorotation and can cause rapid rotor RPM decay after engine failure.
- Rotor brake systems must be interlocked so the brake cannot be applied in flight.
Worked example
After an engine failure at 300 ft, the rotor is at 320 RPM and engine N2 is decaying. If the freewheel releases correctly, the rotor keeps turning and the pilot lowers collective to maintain RPM. If the freewheel is locked, the rotor must drive the engine compressor and accessories; rotor RPM decays faster, glide range falls and the autorotation may not reach a safe landing. Preflight and maintenance checks of the freewheel are therefore not cosmetic.
Traps
- Believing the freewheel drives the engine during autorotation.
- Testing a freewheel only by hand without the approved procedure.
- Applying rotor brake before the rotor has stopped.
- Ignoring abnormal noise or vibration from the main gearbox input.
- Assuming a twin-engine helicopter has no freewheel; each engine has one.