What is a Shock Wave?
When an airplane travels less than the speed of sound, the air ahead of it actually begins to flow out of the way before the plane reaches it. The pressure waves created by the airplane passing through the air end up being smooth and gradual.
But as an airplane reaches the speed of sound and catches up to its own pressure waves, the air ahead of it receives no warning of the plane’s approach. The airplane plows through the air, creating a shock wave. As air flows through the shock wave, its pressure, density, and temperature all increase—sharply and abruptly.
Lift and Drag at Supersonic Speeds
Air acts much differently at supersonic speeds than it does at subsonic speeds.
When an aircraft approaches the speed of sound, the airflow over the wing reaches supersonic speed before the airplane itself does, and a shock wave forms on the wing. The airflow behind the shock wave breaks up into a turbulent wake, increasing drag.
When the airplane exceeds the speed of sound, a shock wave forms just ahead of the wing's leading edge. The shock wave that formed on the wing is now at the trailing edge.
When the wing is tilted upward, a shock wave forms below its leading edge, and an expansion wave forms above its leading edge. The higher pressure behind the shock wave and lower pressure behind the expansion wave result in a single force that pushes the wing up and back.
The upward part of this force is lift; the backward part of this force is drag.