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High-Speed Flight 153

frame of the wing) and can only communicate a very short A Concorde “lapped” a Boeing
distance directly above a point on a wing. This will have a 747 on June 17, 1974. The
profound effect on the behavior of air flowing over a wing at Concorde left Boston and flew to
transonic or supersonic speeds. Paris and back in less time than
There is an additional element in supersonic aerodynamics it took a Boeing 747 to fly one
that does not exist in subsonic aerodynamics. This is the trip from Paris to Boston.
formation of shock waves.

Shock Waves

Compression in air can happen over such a small dis-
tance that it forms a shock front or shock wave. The
dimensions on which the air density changes are so
small that the change is essentially instantaneous. In
supersonic flight a shock wave occurs when air must
suddenly change speed and/or direction. Figure 6.2
shows a picture of shock waves on a Space Shuttle Fig. 6.2. Shock wave on a model of the
model in a supersonic wind tunnel. Space Shuttle.
There are two types of shock waves of interest
involved with flight: normal (meaning perpendicular) shock waves
and oblique (at an angle) shock waves. Normal shock waves are
perpendicular to the direction of flight and are seen primarily on the
surface of transonic wings or in pipes. They are caused by an abrupt
change in density and pressure. Figure 6.3 shows what happens
across a normal shock wave. Before the shock wave, the air is
traveling at greater than Mach 1. Behind a normal shock wave, the air
is subsonic and the air’s density has increased.
Oblique shock waves are formed at an angle with respect to
Shock waves also occur on a
the oncoming air and occur when supersonic air must be
galactic scale. A moving galaxy,
turned. Because a supersonic airplane is traveling so fast, the
measuring thousands of light-years
air has no chance to move out of the way as it does in
in diameter, can have a shock
subsonic speeds. Therefore, the moment the air hits the
wave that is a few light-years
leading edge of the wing it must turn. The air turns almost
thick. Because of the difference in
instantaneously and forms the oblique shock wave. However,
scale, its behavior is identical to a
the shock wave forms at a given angle depending on the angle
shock wave on a wing.
it must be turned. Figure 6.4 illustrates an oblique shock