FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A flight instructor explains that helicopters with low-inertia (lightweight) rotor systems require the pilot to react more quickly when entering autorotation after an engine failure, compared to helicopters with high-inertia (heavier) rotor systems. What is the primary reason for this difference?
- ALow-inertia rotors have a larger disk area, requiring faster collective input
- BHigh-inertia rotors are more susceptible to retreating blade stall during autorotation
- CLow-inertia rotors store less kinetic energy and lose RPM more quickly after power loss
- DLow-inertia rotors automatically increase pitch to compensate for RPM loss
Show answer & explanationAnswer & explanation
Correct answer: C. Low-inertia rotors store less kinetic energy and lose RPM more quickly after power loss
Rotor blades store kinetic energy proportional to their mass and rotational speed. Low-inertia (lightweight) rotor systems store less energy, so rotor RPM decays more rapidly after an engine failure, requiring the pilot to lower the collective more quickly to maintain rotor RPM within safe limits during autorotation entry.
Why the other options are wrong
- A. Disk area is unrelated to rotor blade mass/inertia and does not determine collective timing requirements.
- B. Retreating blade stall relates to forward airspeed and load factor, not rotor inertia during autorotation entry.
- D. Rotor systems do not automatically adjust pitch; the pilot must manually lower collective.
Rotor Inertia and Autorotation
The kinetic energy stored in rotating blades, proportional to blade mass and rotational speed, which determines how quickly rotor RPM decays after an engine failure.
- Low-inertia rotors lose RPM quickly, requiring rapid collective reduction
- High-inertia rotors decay more slowly, giving the pilot more time to react
- Blade mass and radius of gyration both affect stored rotational energy
Memory trick: Light blades spin down fast—like a small flywheel losing spin quickly