Page 169 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 156

156 CHAPTER SIX

Seismic recorders in California flight so difficult. The design of supersonic aircraft must
record earth movement resulting account for the power required overcoming wave drag, and
from the shock waves of the airplanes that fly supersonically are optimized to reduce it.
Space Shuttle as it approaches Wave drag is more complicated than induced drag and
NASA Dryden (Edwards AFB) for parasitic drag. Unlike the latter two contributions to drag,
landing. wave drag is not a simple function of speed but is a
complicated function of Mach number. For example, doubling
the Mach number, and thus the speed, the power needed to overcome
wave drag may increase by less than a factor of 3, whereas the
parasitic power would increase by a factor of 8. In this case the wave
drag has increased by less than 50 percent. Remember power is drag
times speed.
So why does the wave drag decrease so slowly? The drag caused by
oblique shock waves depends on the angle the shock wave makes
with the direction of the airplane’s travel. The more perpendicular the
shock wave the greater the drag. As the Mach number increases, the
angle of the shock wave decreases. This is illustrated in Figure 6.5.
Thus there is a counteracting effect of increasing Mach number. The
shock strength increases with Mach number, but because the
Can a person in free fall reach a shock angle is smaller the wave drag does not increase very
supersonic speed? At 10,000 ft a fast. Since the angle of the shock is a function of the body
person would have to be angle, supersonic aircraft have very sharp noses and leading
traveling at 750 mi/h (1230 edges so that the oblique shock wave is produced at as small
km/h). A person in a dive would an angle as possible.
have difficulty reaching even 500 An additional consideration of wave drag is that it is a
mi/h (820 km/h). function of the air density. The greater the air density the
higher the wave drag and thus the greater the power required

Shock Wave Shock Wave

Speed Speed

Lower Mach Number Higher Mach Number
Fig. 6.5. As the Mach number increases, the angle of the shock wave
decreases.