What is blade stall of a helicopter rotor?

• A. A condition of flight in which the retreating blade is operating at an angle of attack higher than will allow for the air to flow over its upper surface without turbulence.
• B. A temporary loss of rotor speed during hover.
• C. When the retreating blade stalls due to high speed.
• D. When the rotor yaw changes direction.

Answer: (A)

Blade stall happens when the retreating rotor blade reaches an angle of attack that the air cannot flow over smoothly, causing the flow to separate and lift to drop rapidly. In forward flight, the retreating blade has less relative wind, so it must operate at a higher angle of attack to produce the needed lift. If that angle becomes too steep, the air can’t stay attached and stall occurs, leading to a sudden loss of lift on that blade and potentially challenging control.

This matches the description that the retreating blade is operating at an angle of attack high enough that the air cannot flow smoothly over the upper surface, causing turbulence and stall. The other options describe unrelated issues like loss of rotor speed, stall caused by speed rather than angle of attack, or rotor yaw changes, which are not what blade stall is.