Page 172 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 159

High-Speed Flight 159

is that the force caused by the pressure difference across the shock
wave is higher than the force of the wave drag. Thus, the shock wave
will move forward on the wing. As it does, the pressure difference
decreases until the wave moves to a place on the wing where the force
from the pressure difference just equals the force due to wave drag. If
the airplane were now to increase its speed the force due to wave drag
would increase and the shock wave would move toward the trailing
edge. At some speed the normal shock wave will in fact reach the
trailing edge.
Look again at the picture of the fighter flying at transonic speed in
Figure 6.1. In the region where the air is supersonic with increasing
speed, the pressure, density, and temperature are decreasing. At a
point before the normal shock wave the air has cooled enough to
cause condensation, producing the cone of fog above and below the
wing. The backside of the cone is a flat surface. This is the location of
the normal shock wave where the pressure and temperature increase
and the condensation disappears. One may ask why there is a normal
shock wave on the bottom of the wing. The fighter has almost
symmetric wings and since the angle of attack is so small at
transonic speeds, there is a reduction in pressure and
The difference in density across
acceleration of the air on both the top and the bottom of the
the shock wave on the top of a
wing. It is just that the acceleration and reduction in pressure
wing can sometimes affect
are not as great on the bottom. Commercial jets that fly
sunlight so that one can actually
transonic speeds are designed so that the normal shock wave
see the shock wave.
forms only on the top of the wing.
In transonic (and supersonic) flight, the velocity of the air over
the wing continues to increase until the normal shock wave is
reached. Because of this, the center of lift is farther back on the wing
than in subsonic flight. For a typical wing in subsonic flight the
center of lift is about one-fourth chord length back from the leading
edge of the wing. That means that the wing produces 50 percent of
the lift by that point. The moment the wing becomes transonic the
center of lift moves farther back. As the speed increases further, the
center of lift continues to move back. At very high-speed flight, the
center of lift can move as far as to a half chord length back from the
leading edge.