Operating at what flight speed is retreating blade stall most likely to occur?

• A. Very low airspeed
• B. Excessive airspeed
• C. Moderate airspeed
• D. High altitude

Answer: (B)

In forward flight the rotor experiences dissymmetry of lift: the advancing blade works through more air and tends to produce more lift, while the retreating blade has less relative wind and must work harder to supply the required lift. As forward speed increases, the retreating blade is forced to operate at a higher angle of attack to keep the rotor balanced. If the speed is too high (excessive forward speed) the retreating blade can reach its critical angle of attack and stall. When the retreating blade stalls, lift on that side drops abruptly, causing a rapid roll toward the retreating side and potential loss of control.

This is why very low airspeed or moderate airspeed are not the conditions that typically induce retreating blade stall, and high altitude mainly affects density rather than the likelihood of stall by itself. The scenario that makes retreating blade stall most likely is excessive forward speed, especially with heavy loading or high power settings.