FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A pilot in a non-governed, non-correlated helicopter raises the collective pitch control to increase lift without adding throttle. What is the immediate effect on main rotor RPM, and why?
- ARPM increases because higher blade angle increases lift and reduces drag
- BRPM decreases because raising collective automatically reduces throttle input
- CRPM decreases because increased blade pitch angle increases induced drag, requiring more engine power to maintain speed
- DRPM remains unchanged because collective pitch has no effect on rotor drag
Show answer & explanationAnswer & explanation
Correct answer: C. RPM decreases because increased blade pitch angle increases induced drag, requiring more engine power to maintain speed
Increasing collective pitch increases the angle of attack of the blades, which increases induced drag; without a corresponding throttle increase, engine power cannot overcome the added drag, and rotor RPM decays.
Why the other options are wrong
- A. Increasing blade angle increases drag, not decreases it, so RPM does not increase.
- B. Throttle is a separate control from collective unless linked by a correlator/governor, which this scenario excludes.
- D. Collective pitch directly affects blade angle of attack and thus drag on the rotor.
Collective Pitch and RRPM Relationship
Raising collective pitch increases blade angle of attack and induced drag, which decreases rotor RPM unless engine power (throttle) is simultaneously increased.
- Pitch angle changes directly affect drag on rotor blades
- Without throttle correlation, RPM decays when collective is raised
- Governors/correlators automatically compensate for this in modern helicopters
- Manual throttle control requires pilot to coordinate collective and throttle
Memory trick: More pitch, more drag, less RPM—unless you feed more fuel.