At what airspeed is maximum endurance reached for a prop aircraft?

Maximum endurance for a propeller aircraft is achieved at the airspeed corresponding to the maximum lift-to-drag ratio, known as L/Dmax. This point is the most efficient flying speed for the aircraft, where it experiences the least amount of drag for a given amount of lift. At this airspeed, the aircraft can stay aloft the longest for a particular amount of fuel consumed, optimizing the endurance of the flight.

Flying at L/Dmax maximizes the time the aircraft can remain airborne without descending quickly, as the ratio of lift (which counters weight) to drag (which opposes forward motion) is at its peak. This means that for every unit of thrust used, the aircraft gains the maximum possible lift while minimizing fuel consumption. Hence, reaching L/Dmax provides the best performance for sustaining flight and thus achieving maximum endurance.

In contrast, flying at speeds below or above L/Dmax would result in either increased drag relative to lift or require more power to maintain altitude, which would ultimately reduce the aircraft's endurance.