Page 50 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 37

How Airplanes Fly 37

100 Power vs. Speed

80 Power
Total
Power 60 Induced
Parasitic

40

20

0
0 20 40 60 80 100 120 140
Speed
Fig. 2.13. The power required for flight.

vertical velocity of the downwash has been halved. Thus, the induced
power has been halved. From this we can see that the induced power
varies as 1/speed for a constant load. The induced power is shown as
a function of speed by the dotted line in Figure 2.13. This shows that
the more slowly the airplane flies the greater the power requirement
to maintain lift. As the airplane slows in flight, more and more power
must be added until finally the airplane is flying at full power
with the nose high in the air. What is happening is that as the
The induced power varies as
airplane’s speed is reduced, more and more energy must be
1/speed for a constant load.
given to less and less air to provide the necessary lift.

Parasitic Power
Parasitic power is associated with the energy lost by the airplane to
collisions with the air. It is proportional to the average energy that the
airplane transfers to an air molecule on colliding times the rate of col-
lisions. As with the energy given to the bullet above, the energy lost to
the air molecules is proportional to the airplane’s speed squared. The
rate of collisions is simply proportional to the speed of the airplane.
The faster the airplane goes the higher the rate of collisions. So we