Flying at speed X, which is the Max L/D, what speed will you fly for maximum endurance?

• A. Same as X
• B. Lower than X
• C. Higher than X
• D. At stall

Answer: (A)

Endurance is how long you can stay in the air for a given amount of fuel, so you want to minimize the engine power (and thus fuel flow) needed to stay aloft. The speed of maximum lift-to-drag ratio is the most aerodynamically efficient point: at that speed the airplane needs the least thrust to maintain level flight, which means the lowest fuel burn for the altitude and weight. Therefore, flying at that same speed keeps the power requirement and fuel use as low as possible, maximizing time in the air. Slower speeds increase induced drag and require more power; higher speeds increase parasite drag and also require more power. In this context, staying at the speed that gives maximum L/D yields the best endurance.