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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 in which it is immersed. At airspeeds below about 260 kn , 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. Under this assumption, air acts the same as water and is classified as a fluid.
Subsonic aerodynamic theoryalso assumes the effects of viscosity are negligible,and classifies air as an ideal fluid, conforming tothe principles of ideal-fluid aerodynamics such ascontinuity, Bernoulli's principle, and circulation.In reality, air is compressible and viscous. While theeffects of these properties are negligible at lowspeeds, compressibility effects in particular becomeincreasingly important as airspeed increases.
Compressibility isof paramount importance at speeds approaching thespeed of sound. In these transonic speed ranges, compressibilitycauses a change in the density of the air aroundan airplane.
During flight, a wing produces lift by acceleratingthe airflow over the upper surface. This acceleratedair can, and does, reach supersonic speeds, even though theairplane itself may be flying at a subsonic airspeed. At someextreme angles of attack, in some airplanes, thespeed of the air over the top surface of the wing maybe double the airplane's airspeed. It is, therefore, entirelypossible to have both supersonic and subsonic airflowson an airplane at the same time. When flowvelocities reach sonic speeds at some locations on anairplane , further acceleration will result in theonset of compressibility effects such as shock waveformation, drag increase, buffeting, stability, andcontrol difficulties. Subsonic flow principles areinvalid at all speeds above this point.