With the aircraft stationary, how does propulsive efficiency behave?

• A. is minimum.
• B. depends on RPM.
• C. is maximum.
• D. cannot be defined.

Answer: (A)

Propulsive efficiency is the ratio of useful thrust power (thrust times forward speed) to the shaft power input. When the aircraft is stationary, the forward speed is zero, so the useful power is zero. This makes the propulsive efficiency equal to zero, the minimum possible value. Increasing engine RPM doesn’t change that at zero airspeed—the numerator remains zero—so it isn’t defined by RPM and isn’t maximum. It’s simply zero because there’s no forward work being done.