Page 54 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 41

How Airplanes Fly 41

Drag

So far, we have discussed power at length with only brief references to
the topic of drag. With an understanding of power we are in a position
to understand drag, which is part of the pilot’s culture and vocabulary.
Drag is a force that resists the motion of the airplane. Clearly, a low-
drag airplane will fly faster than a high-drag airplane. It will also
require less power to fly the same speed as the high-drag airplane. So,
what is the relationship between power, drag, and speed?
Power is the rate at which work is done. In mathematical terms it is
also a force times a velocity. Drag is a force and is simply equal to
power/speed. We already know the dependence of induced and
parasitic powers on speed. By dividing power by speed, we have the
dependence of drag on speed. Since induced power varies as 1/speed,
induced drag varies as 1/speed squared. Parasitic power varies as
speed cubed, so parasitic drag varies as speed squared. Figure 2.16
shows the dependence of induced, parasitic, and total drag on the
speed of the airplane.

16
Drag vs. Speed
14
12
Drag
10 Total
Induced
Drag 8 Parasitic
6

4

2

0
20 40 60 80 100 120 140

Speed
Fig. 2.16. Drag as a function of speed.