Page 173 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 160

160 CHAPTER SIX

In the early attempts at breaking the sound barrier, the jump
in the center of lift caused the wing to pitch down.This
reduced the acceleration of the air over the wing causing it to
become subsonic again.The center of lift would then jump
forward on the wing and the wing would pitch up, becoming
transonic.This process would repeat itself rapidly until the
wings broke off. The X-1 made it through this transition
because of a better wing design that made it strong enough to
enter supersonic flight.

As the airplane flies faster the shock wave gets stronger. The result is
a rapid increase in wave drag, and thus more power is required. The
rapid rise in drag, and thus increase in the power required, as one
approaches Mach 1 is called the Mach 1 drag rise. The design of the wing
and body of the airplane is optimized to operate at the “elbow” (or sharp
increase) of the power as a function of Mach number, shown in Figure
6.8. Note that the power has a local peak at Mach 1 and then
drops off before continuing to rise again. Above Mach 1 an
Before the oil embargo of 1973,
oblique shock wave occurs at the leading edge of the wing. As
the Boeing 747 was designed to
operate above Mach 0.9. stated before, as the speed increases above Mach 1 the oblique
However, the fuel costs forced a shock wave bends back at a decreasing angle and thus the
lower operating speed. One increase in wave drag power grows more slowly than the
design change in the 747-400 increase in parasitic power. Some early supersonic airplanes had
model was to reoptimize the to enter a dive to transition from subsonic to supersonic flight.
wing to fly at Mach 0.85. Only after reaching supersonic flight could the airplane level off
and maintain supersonic flight with the available power.

Wing Sweep

In Chapter 3 wing sweep was discussed. In transonic and supersonic
flight a swept wing is necessary to reduce wave drag. In 1935, a gath-
ering of top aeronautical scientists from around the world gathered in
Rome and showed results of high-speed analysis and wind-tunnel
experiments. One German result went largely unnoticed. This result
was that a swept wing reduced drag at high speeds.