High-speed flight

In high-speed flight, the assumptions of incompressibility of the air used in low-speed aerodynamics no longer apply.

In subsonic aerodynamics, the theory of lift is based upon the forces generated on a body and a moving gas (air) in which it is immersed.

At airspeeds below about 260 kn (480 km/h; 130 m/s; 300 mph), air can be considered incompressible in regards to an aircraft, in that, at a fixed altitude, its density remains nearly constant while its pressure varies.

This accelerated air can, and does, reach supersonic speeds, even though the airplane itself may be flying at a subsonic airspeed (Mach number < 1.0).

When flow velocities reach sonic speeds at some locations on an airplane (such as the area of maximum camber on the wing), further acceleration will result in the onset of compressibility effects such as shock wave formation, drag increase, buffeting, stability, and control difficulties.