How the Airflow Changes as It Goes Over the Piper Archer

The flow of air changes with the Mach number, and part of that is the compression. We can treat air as incompressible below about point Mach 3 because the density changes are less than about 10%. But now you're traveling much faster than Mach 3. If you put your metaphorical finger over a compressible supersonic flow of air coming out of a hose, the air comes out slower, not faster. The same thing happens at supersonic speeds. It's just suddenly there and it has to adjust. So that's what a shockwave is.