At sufficiently high airspeeds (which are still subsonic) the local fluid velocity (on the upper surface of the wing, say) may become supersonic, and a treatment in terms of incompressible flow theory becomes inadequate. Supersonic flow always results in the production of a shock wave , which is a sharp discontinuity in the pressure and density of the fluid. A shock requires a considerable amount of energy to produce, which results in a greatly increased drag as the airplane approaches the speed of sound--this is known as wave drag wave drag. To minimize the wave drag the wings on most high speed commercial jets are swept backwards. As sketched in Fig. 4.7, sweeping the wings reduces the component of the velocity which is perpendicular to the wing, therefore delaying the onset of shock production on the surfaces of the wing, and reducing the wave drag.
Figure 4.7: The swept-wing configuration.
As the airspeed is increased still further the wave drag continues to increase, until the speed of sound is reached (the ``sound barrier''); beyond this point the drag drops somewhat. Once the speed of sound is exceeded the entire plane sets up a conical shock wave; the angle of the apex is
where c is the sound speed, v is the airspeed of the plane, and Ma is
the Mach number (see Fig 7.15 in
Melissinos).
The wings on supersonic aircraft are also swept, so that they remain inside
the Mach cone in order to avoid various aerodynamic instabilities.
