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CH07_Anderson 7/25/01 9:00 AM Page 188

188 CHAPTER SEVEN

Fig. 7.9. Pathfinder, a maximum endurance, solar-powered airplane. (Photo
courtesy of NASA.)

In maximizing the time aloft, the speed is not the concern. What is
important is the rate at which fuel is burned. The maximum endurance
in the air for any fuel-carrying airplane is just at the speed of minimum
fuel consumption. For a piston/propeller airplane, the propulsive power
is almost directly proportional to the engine power. The engine
The speed at which the pilot power, and thus the fuel consumption, is just proportional to the
should fly a propeller-driven required power for flight. So the speed at which the pilot should
airplane for maximum endurance fly a propeller-driven airplane for maximum endurance is at the
is at the minimum of the power minimum of the power curve.
curve. Things are different for jet-powered airplanes. As
discussed in Chapter 5, “Airplane Propulsion,” the fuel
consumption of the engine is dependent on engine power, not
propulsive power. The engine’s propulsive efficiency increases with
speed and the propulsive power increases with speed, while the
thrust of the engine remains constant. This means that the airplane
gets more propulsive power for the same fuel flow as the speed
increases. Or put another way, for a given propulsive power the fuel
flow can be reduced as the airplane speeds up. This relation to speed
means that the minimum fuel flow for a jet is not at minimum
required power, as it is for a propeller-driven airplane, but at
minimum required thrust, i.e., minimum drag. So a jet pilot should
fly at the minimum drag speed for maximum endurance. Since
minimum drag is a higher speed than minimum power, an airplane