Why do slower aircraft need larger propulsors than faster aircraft?

Slower aircraft require larger propulsors because of the fundamental relationship between jet velocity and propulsive efficiency. To achieve high propulsive efficiency, the velocity of the air exiting the propulsor (jet velocity) must be matched as closely as possible to the aircraft's flight speed.12

Because the thrust generated by an engine is proportional to the difference between the jet velocity and the flight speed, a slow aircraft cannot rely on a high-velocity, low-mass jet stream—which would be highly inefficient—to generate the necessary thrust. Instead, to produce sufficient thrust while maintaining a low jet velocity that matches its slower flight speed, the engine must accelerate a much larger mass of air. This requirement necessitates the use of a larger propulsor, such as a helicopter rotor, which can move the required volume of air at the necessary lower velocity. Conversely, as design flight speeds increase, the required jet velocity increases, allowing the propulsor size to decrease.12