Which of the following is a factor contributing to retreating blade stall?

• A. Airspeed beyond VNE
• B. High rotor RPM
• C. Low density altitude
• D. Turbulent air

Answer: (A)

The key idea is that retreating blade stall becomes a risk as forward airspeed climbs toward the helicopter’s maximum safe speed. In forward flight the rotor disk handles dissymmetry of lift: the advancing blade sees more relative wind and tends to generate more lift, while the retreating blade has less relative wind and must work harder to supply the same overall rotor thrust. As you push toward high forward speed, the retreating blade is required to operate at a higher angle of attack to keep up, which narrows the margin before it stalls. Hitting the airspeed limit reduces that margin even further, making retreating blade stall more likely.

The other factors don’t directly create this stall tendency. Rotor RPM, when kept within limits, doesn’t by itself push the retreating blade toward stall and can even help margins. Dense air (low density altitude) actually improves lift, reducing stall risk rather than increasing it. Turbulent air can cause controllability issues, but the fundamental stall mechanism here comes from high forward speed and the associated lift distribution on the rotor blades.