Aerodynamics
Supersonic Flight
Flight at speeds greater than that of sound are supersonic. Near a Mach number of one, some portions of the flow are at speeds below that of sound, while other portions move faster than sound. The range of speeds from Mach number 0.8 to 1.2 is known as transonic. Flight at Mach numbers greater than five is hypersonic.
The compressibility of air becomes an important aerodynamic factor at these high speeds. The reason for this is that sound waves are transmitted through the successive compression and expansion of air. The compression due to a sound wave from a supersonic aircraft does not have a chance to get away before the next compression begins. This pile up of compression creates a shock wave, which is an abrupt change in pressure, density, and temperature. The shock wave causes a steep increase in the drag and loss of stability of the aircraft. Drag due to the shock wave is known as wave drag. The familiar "sonic boom" is heard when the shock wave touches the surface of Earth.
Temperature effects also become important at transonic speeds. At hypersonic speeds above a Mach number of five, the heat causes nitrogen and oxygen molecules in the air to break up into atoms and form new compounds by chemical reactions. This changes the behavior of the air and the simple laws relating pressure, density, and temperature become invalid.
The need to overcome the effects of shock waves has been a formidable problem. Swept-back wings have helped to reduce the effects of shock. The supersonic Concorde that cruises at Mach 2 and several military airplanes have delta or triangular wings. The supercritical airfoil designed by Richard Whitcomb of the NASA Langley Laboratory has made air flow around the wing much smoother and has greatly improved both the lift and drag at transonic speeds. It has only a slight curvature at the top and a thin trailing edge. The proposed hypersonic aerospace plane is expected to fly partly in air and partly in space and to travel from Washington to Tokyo within two hours. The challenge for aerodynamicists is to control the flight of the aircraft so that it does not burn up like a meteor as it enters the atmosphere at several times the speed of sound.
Resources
Books
Anderson, John D. Jr. Introduction to Flight. New York: Mc-Graw-Hill, 1989.
Craig, Gale. Introduction to Aerodynamics. New York: Regenerative Press, 2003.
Leishman, J. Gordon. Principles of Helicopter Aerodynamics. Cambridge: Cambridge University Press, 2003.
Smith, H. C. The Illustrated Guide to Aerodynamics. Blue Ridge Summit, PA: Tab Books, 1992.
Wegener, Peter P. What Makes Airplanes Fly? New York: Springer-Verlag, 1991.
Periodicals
Hucho, Wolf-Heinrich. "Aerodynamics of Road Vehicles." Annual Review of Fluid Mechanics (1993): 485.
Vuillermoz, P. "Importance of Turbulence for Space Launchers." Journal of Turbulence 3, no. 1 (2002): 56.
Wesson, John. "On the Eve of the 2002 World Cup, John Wesson Examines the Aerodynamics of a Football and Explains how the Ball Can Bend as It Travels Through the Air." Physics World 15, no.5 (2002): 41-46.
Sreela Datta
Additional topics
Science EncyclopediaScience & Philosophy: Adrenoceptor (adrenoreceptor; adrenergic receptor) to AmbientAerodynamics - Skin Friction And Pressure Drag, Airfoil, Supersonic Flight - Basic air flow principles