Page 170 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 157

High-Speed Flight 157

to overcome it. Thus, aircraft do not fly supersonically at low
altitudes.

Transonic Flight

Commercial transports fly in the Mach 0.8 to 0.86 range, just below
the speed of sound. This speed is not chosen arbitrarily. It is based on
the presence of wave drag. But, if the airplane is flying at a speed less
than the speed of sound, how can there be wave drag?
A wing diverts air down. In bending the air down, it creates lower
pressure on the upper surface of the wing, which causes the air to
accelerate. This topic has been covered in Chapter 2 and will not be
repeated here. However, at speeds approaching the speed of sound the
air that is accelerated over the top of the wing becomes locally
supersonic.
When air flows over the top of a subsonic wing, it accelerates to
the point of greatest curvature of the air. At this point the pressure is
the lowest and the speed of the air is greatest. From that point to the
trailing edge of the wing the airspeed decreases and the pressure
increases in order to match the pressure of the air at the trailing edge.
This is the trailing-edge condition.
The picture is quite different for the air flowing over the top of a
transonic wing. The air accelerates as before, but by the time it
reaches the point of maximum curvature it is traveling at greater
than Mach 1. As the air continues to bend, because it is traveling
faster than the communication speed of air, it is not able to
effectively pull air down from above. Thus the density is
substantially reduced, causing the pressure to continue to go down
and the velocity to increase. This situation leaves the wing with the
problem of how to meet the trailing-edge condition. The
solution is the formation of a normal shock wave as shown
Northrop designed a high-speed
in Figure 6.6. At this shock wave the pressure and density
flying wing, the XP-79, which
increase abruptly and the velocity of the air goes below
was a flying ram. Its objective
Mach 1. After the shock wave the air can slow down further
was to slice off the tail of the
and the pressure continue to increase to meet the trailing-
opponent with its leading edge.
edge condition.