During a practice autorotation at high density altitude, a pilot allows the rotor RPM to decay below the normal operating range. The primary performance risk of this RPM decay is:
- AImproved autorotative glide distance due to reduced blade drag
- BBlades approaching a stalled angle of attack, reducing lift and increasing rate of descent
- CAn increase in rotor RPM beyond redline due to reduced drag
- DA sudden increase in main rotor thrust that causes an uncommanded climb
Show answer & explanationAnswer & explanation
Correct answer: B. Blades approaching a stalled angle of attack, reducing lift and increasing rate of descent
In autorotation, rotor RPM must be maintained within limits to keep blade angle of attack in an efficient range. If RPM decays too low, blade angle of attack increases toward the stall point (especially at the blade roots), reducing lift, increasing rate of descent, and risking loss of rotor control—an effect worsened at high density altitude where thinner air already reduces available lift.
Why the other options are wrong
- A. Low RPM increases drag and reduces glide performance, it does not improve it.
- C. Low RPM does not cause an RPM increase; if anything, low RPM tends to self-perpetuate unless corrected by lowering collective.
- D. Low RPM reduces thrust and lift; it does not cause an uncommanded climb.
Autorotation RPM Decay
Allowing rotor RPM to decay below the normal range during autorotation increases blade angle of attack toward stall, reducing lift and increasing rate of descent, especially critical at high density altitude.
- Low RPM increases blade angle of attack, risking blade stall
- High density altitude compounds the lift deficit
- Pilots must monitor and manage RPM using collective pitch throughout autorotation
Memory trick: Low RPM = blades 'gasping' for lift, stalling and dropping you faster