We begin our discussion with a brief survey of the behavior of flows past airfoils; when a conventional symmetric airfoil accelerates from subsonic speed to supersonic speed the flow pattern usually develops in the manner shown in Figure 11. As the flight speed of the airfoil reaches the critical speed, the local flow speed equals the local sound speed. Beyond the critical speed, a supersonic region appears on the airfoil which is usually terminated by a nearly normal shock through which the flow speed jumps from super sonic to subsonic. With a further increase in the flight speed, the shock moves aft and the size of the supersonic region and the shock strength both increase. If the pressure jump through the shock is sufficiently large, separation of the boundary layer occurs. This shock induced separation starts when the local Mach number, the ratio of local flow and sound speeds, just upstream of the shock is about to When the boundary layer downstream of the shock separates.