FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
During an autorotation, the engine has failed and the main rotor is being driven solely by upward airflow through the disk. What component allows the main rotor to continue turning freely without being restrained by the now-stopped engine?
- AFreewheeling unit
- BTail rotor gearbox
- CPitch change link
- DSwashplate
Show answer & explanationAnswer & explanation
Correct answer: A. Freewheeling unit
The freewheeling unit (a one-way sprag clutch located in the drive system) automatically disconnects the engine from the main rotor system when engine RPM drops below rotor RPM, allowing the rotor to continue turning freely and drive the transmission during autorotation without engine drag.
Why the other options are wrong
- B. The tail rotor gearbox transmits drive to the tail rotor, not related to disengaging the engine from the main rotor.
- C. The pitch change link connects the swashplate to the blade pitch horn, unrelated to engine disconnection.
- D. The swashplate transmits cyclic and collective pitch inputs to the rotor blades; it does not disconnect the engine.
Freewheeling Unit
A one-way clutch in the transmission that automatically disengages the engine from the rotor system whenever engine RPM is lower than rotor RPM, essential for autorotation.
- Also called a sprag clutch or overrunning clutch
- Engages automatically—no pilot action required
- Allows rotor to continue turning via aerodynamic forces after engine failure
Memory trick: Freewheeling = 'free' the rotor from a dead engine automatically.