Page 202 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 189

Airplane Performance 189

with a jet engine will cover more ground flying at maximum The speed at which the pilot
endurance than a propeller-driven airplane. should fly a jet airplane for
Maximum endurance means maximum time in the air. This maximum endurance is at the
is useful for the very few that have a need to stay in the air for minimum in the drag curve.
a long time. But a more interesting cruise condition is
maximum range.

Maximum Range

What is the speed for the maximum range on a tank of fuel? The best
range for an airplane is different from the maximum endurance. At
maximum endurance the airplane is traveling quite slowly.
Now we want to cover as much ground as possible on the
The world record for a nonstop,
available fuel. As with endurance, the conditions for maximum
nonrefueled flight is 24,987 mi
range are different for a propeller-driven airplane and jet.
(40,204 km), set by the specially
For a propeller-driven airplane, the propulsive power is
designed Voyager in 1986 while
roughly independent of speed. For the maximum range one
flying around the world.
does not want to find the minimum in power, as for
endurance, but the minimum in power divided by speed. Power is
equivalent to fuel consumption and speed is equivalent to distance
traveled. So power divided by speed is equivalent to gallons per mile.
But power divided by speed is just drag. So, for the maximum range a
propeller-driven airplane flies at the speed for minimum drag.
For the maximum range a
This speed is higher than the maximum endurance speed.
propeller-driven airplane flies at
Now let’s consider a jet. Recall that a jet flew at the
the speed for minimum drag.
minimum drag speed for maximum endurance. This was
because there was a speed term associated with the efficiency
of the engine. So the argument goes as before, and we get the result
that for maximum range a jet-powered airplane flies at the speed for
the minimum drag divided by speed.
The maximum endurance and range for a propeller-powered
For the jet we get the result
airplane are at the speeds for minimum power and drag. For a
that for maximum range a jet-
jet the maximum endurance and range are at the speeds for
powered airplane flies at the
minimum drag (power/speed) and minimum drag/speed.
speed for the minimum drag
We know what speed to fly for maximum range. But the
divided by speed.
load of the airplane decreases with time. So how does an