For a controllable-pitch propeller, under what condition can it provide reverse thrust without changing shaft rotation direction?

• A. Pitch must be set to 0 degrees
• B. Engine must be at maximum RPM
• C. Aircraft must be on the ground only
• D. Pitch can be set to negative values

Answer: (D)

Controllable-pitch props work by changing blade angle to control how much air the blades push and in which direction. When the blade pitch is set to negative values, the blades are angled so that, as they rotate, they accelerate air forward relative to the aircraft. The propeller keeps turning in the same direction, but the reaction force on the airplane points backward, giving reverse thrust. This lets the airplane slow down after touchdown or in some cases during flight, without changing shaft rotation direction. Pitch at zero degrees would not produce reverse thrust, and engine RPM or ground-only usage are not what enables the reverse effect—the blade angle is. The negative-pitch setting is what makes reverse thrust possible.