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 knots, air can be considered incompressible, 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 (the propertyof a fluid that tends to prevent motion of one partof the fluid with respect to another) 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 (and to a lesser extent viscosity) isof paramount importance at speeds approaching the
speed 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 (Mach number < 1.0). 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 airflowon an airplane at the same time. When flowvelocities reach sonic speeds at some location on anairplane (such as the area of maximum camber onthe wing), further acceleration will result in theonset of compressibility effects such as shock waveformation, drag increase, buffeting, stability, and
control difficulties. Subsonic flow principles areinvalid at all speeds above this point.

ource

cite book
title = Pilot’s Handbook of Aeronautical Knowledge
year = 2003
publisher = U.S. Federal Aviation Administration
location = U.S. Government Printing Office, Washington D.C.
id = FAA-8083-25
pages = p. 3-35
url=http://www.faa.gov/library/manuals/aviation/pilot_handbook/

USGovernment|sourceURL= [http://www.faa.gov/library/manuals/aviation/ Pilot's Handbook of Aeronautical Knowledge]