Page 167 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 154

154 CHAPTER SIX

Supersonic Subsonic
Mach > 1 Mach < 1

Normal Shock Wave
Fig. 6.3. Density and Mach number change
across a normal shock wave.

Air is turned

Air is still
supersonic (M>1)

Fig. 6.4. An oblique shock wave.

wave. As with a normal shock wave, the air density increases and the
air’s velocity decreases across an oblique shock wave. But the changes
are not enough for the air to become subsonic, as in a normal shock.
Therefore, the air behind an oblique shock wave remains supersonic
relative to the aircraft. There are rare exceptions to this rule.
All supersonic objects create shock waves. Normal shock waves
cause a higher change in density than oblique shock waves. Therefore,
supersonic aircraft are designed to avoid producing normal shock
waves, since the greater the change in density across a shock wave the