AASA Aviation Practice Exam

In a turboprop, the fuel economy (SFC) is lowest when the propeller is operating at its most efficient point, which happens at relatively low speeds. At low speeds the propeller works with a favorable advance ratio, so the aircraft can produce the needed thrust with less fuel per unit thrust. The engine can run at an efficient power setting without pushing the propeller into less efficient regimes, giving the best fuel efficiency.

Raising speed moves the propeller away from that optimum: blade speeds and flow conditions become less favorable for propulsive efficiency, and more power is required to overcome drag, so more fuel is burned per unit of thrust. Takeoff demands high power, so fuel flow—and thus SFC—is not low. High altitude and thinner air can alter efficiency, but turboprops rely on propeller efficiency in the air, and the overall effect typically does not produce the lowest SFC at high speed or during takeoff.