Suppose a cylindrical wing (constant chord, incidence, etc.) is placed in an airstream at an angle of yaw — i.e., it is swept back. Now, even if the local speed of the air on the upper surface of the wing becomes supersonic, a shock wave cannot form there because it would have to be a sweptback shock — swept at the same angle as the wing — i.e., it would be an oblique shock. Such an oblique shock cannot form until the velocity component normal to it becomes supersonic.