Page 52 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 39

How Airplanes Fly 39

120
The Effect of Altitude on Power
100

80 Total Power
Power 60 @ 3000 ft
@ 12,000 ft

40

20

0
0 20 40 60 80 100 120 140

Speed

Fig. 2.14. Total power required for flight at two altitudes.

10 percent increase in induced power. An airplane flying on the
backside of the power curve would require more power and fly with a
greater angle of attack when going to a higher altitude.
The situation is the opposite for the parasitic power. A reduction in
air density translates to a reduction in the number of collisions
with the air, and thus there is a reduction in the parasitic power.
Before 1900, Langley’s law, found
An airplane at cruise speed where parasitic power dominates
experimentally by Samuel
finds it more economical to fly at a higher altitude. Usually
Langley, said that total power
flying at a higher altitude does not translate into flying at a
required for flight decreases with
higher speed because nonturbocharged engines experience a
speed. It has been shown that
reduction in power that is similar to the reduction in
all of his experiments were
atmospheric pressure. That is, if the atmospheric pressure is 65
performed on the backside of
percent that of sea level, the maximum power of the engine is
the power curve.
also approximately 65 percent of its sea-level performance.
The Effect of Load on Induced Power

Now let us examine the effect of load on induced power. First,
remember that the induced power associated with flight is propor-