FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A pilot is flying a fully-loaded helicopter at high forward airspeed in turbulent air at high density altitude. The pilot notices abnormal vibration and a nose-up pitching tendency with a roll toward the retreating blade side. Which combination of factors most likely caused this condition?
- ALow airspeed, low gross weight, and low density altitude
- BHigh rotor RPM, low forward airspeed, and smooth air
- CLow collective pitch and steep autorotative descent
- DHigh airspeed, low rotor RPM, high gross weight, high density altitude, and turbulence
Show answer & explanationAnswer & explanation
Correct answer: D. High airspeed, low rotor RPM, high gross weight, high density altitude, and turbulence
Retreating blade stall is most likely to occur with high forward airspeed, low rotor RPM, high gross weight, high density altitude, steep or abrupt turns, and turbulence — all factors that increase the angle of attack needed on the retreating blade to produce equal lift, eventually exceeding its critical angle of attack and stalling.
Why the other options are wrong
- A. These conditions reduce the likelihood of retreating blade stall, not increase it.
- B. High RPM and low airspeed reduce, rather than promote, retreating blade stall.
- C. This describes a normal autorotation setup, unrelated to retreating blade stall onset.
Retreating Blade Stall
A stall of the retreating blade at high forward airspeed when it must operate at a high angle of attack to produce lift equal to the advancing blade, exceeding its critical angle of attack.
- Aggravated by high airspeed, high gross weight, high density altitude, low RPM, turbulence
- Symptoms: abnormal vibration, pitch-up, and roll toward the retreating side
- Recovery: reduce collective, reduce airspeed, adjust attitude smoothly
Memory trick: Fast, heavy, thin air, slow blades = stall waves.