FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
During an autorotation, a pilot notices the main rotor RPM has decreased below the normal operating range. What is the correct initial corrective action?
- AApply aft cyclic to slow the descent and reduce airflow through the rotor
- BRaise the collective slightly to increase blade pitch and generate more lift
- CLower the collective further to reduce blade drag and allow rotor RPM to increase
- DIncrease forward airspeed to its maximum to increase rotor RPM
Show answer & explanationAnswer & explanation
Correct answer: C. Lower the collective further to reduce blade drag and allow rotor RPM to increase
In autorotation, rotor RPM is maintained by balancing collective pitch against the upward airflow through the rotor disc. If RPM decays below the normal range, lowering the collective reduces blade angle of attack and drag, allowing the rotor to accelerate back into the proper operating range; raising the collective would further increase drag and decrease RPM.
Why the other options are wrong
- A. Aft cyclic alone does not directly correct low rotor RPM and can reduce airflow needed to drive the rotor.
- B. Raising the collective increases blade drag, further decreasing rotor RPM — the opposite of the needed correction.
- D. Excessive forward airspeed does not directly restore rotor RPM and may create other control issues.
Autorotation RPM Control
During autorotation, rotor RPM is controlled by collective pitch; lowering collective decreases blade drag and increases RPM, while raising collective increases drag and decreases RPM.
- Low rotor RPM in autorotation is corrected by lowering collective
- Raising collective in this situation worsens RPM decay
- Rotor RPM must stay within the green arc for a safe autorotation and flare
Memory trick: Low RPM? Lower the collective — less drag lets the rotor spin free.