Endurance with turbo-jet propulsion

For a turbo-jet, the fuel flow rate is approximately proportional to the thrust produced by the engine, regardless of speed or altitude. The best endurance will thus occur at the minimum thrust setting because this will give the lowest fuel flow. The lowest possible thrust, and hence best endurance, will be obtained when the aircraft is flying at its minimum drag speed rather than the minimum power condition.

The maximum time for which the aircraft can be kept airborne will be approximately independent of both wing loading and altitude, because the magnitude of the minimum drag is not influenced by these parameters. How­ever the speed at which minimum drag is obtained, and hence the speed for best endurance, increases with both wing loading and altitude.