FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
During autorotation, which region of the rotor disk produces the forward-acting aerodynamic force along the blade chord that supplies the energy needed to keep the main rotor turning at the correct RPM?
- AReverse flow region, on the retreating side
- BStall region, near the blade root
- CDriven region, near the blade tip
- DDriving region, the middle portion of the blade
Show answer & explanationAnswer & explanation
Correct answer: D. Driving region, the middle portion of the blade
The rotor disk in autorotation is divided into the stall region (innermost), the driving region (middle), and the driven region (outer). The driving region's angle of attack produces a small forward component of aerodynamic force in the plane of rotation, which accelerates or sustains rotor RPM — this is the region that actually 'drives' the autorotation.
Why the other options are wrong
- A. Reverse flow occurs on the retreating blade in forward flight, not a distinct autorotation region.
- B. The stall region has excessive AOA and produces drag, not driving force.
- C. The driven region produces a rearward (decelerating) force that opposes rotation.
Regions of the Rotor Disk in Autorotation
During autorotation the rotor blade is divided into three regions—stall, driving, and driven—based on angle of attack and the resulting aerodynamic force direction.
- Stall region: innermost, high AOA, blade stalled
- Driving region: middle, produces forward (accelerating) force
- Driven region: outer, produces rearward (decelerating) force
- Balance of these regions maintains autorotative RPM
Memory trick: Root stalls, Middle drives, Tip drags — the 'driving' middle keeps you spinning.