study of gases in motion. As the principal application of aerodynamics is the design of aircraft, air is the gas with which the science is most concerned. Although aerodynamics is primarily concerned with flight, its principles are also used in designing automobile and train bodies for minimum drag and in computing wind stresses on bridges, buildings, smokestacks, trees, and other structures. It is also used in charting flows of pollutants in the atmosphere and in determining frictional effects in gas ducts. The wind tunnel
is one of the aerodynamicist's basic experimental tools; however in recent years, it has been supplanted by the simulation of aerodynamic forces during the computer-aided design
of aircraft and automobiles.
The Basic Forces of Thrust, Drag, and Lift
There are three basic forces to be considered in aerodynamics: thrust, which moves an airplane forward; drag, which holds it back; and lift, which keeps it airborne. Lift is generally explained by three theories: Bernoulli's principle
, the Coanda effect
, and Newton's third law of motion
. Bernoulli's principle states that the pressure of a moving gas decreases as its velocity increases. When air flows over a wing having a curved upper surface and a flat lower surface, the flow is faster across the curved surface than across the plane one; thus a greater pressure is exerted in the upward direction. This principle, however, does not fully explain flight; for example, it does not explain how an airplane can fly upside down. Scientists have begun suggesting that the Coanda effect is at least partially responsible for how planes fly. Regardless of the shape of a plane's wing, the Coanda effect, in which moving air is attracted to and flows along the surface of the wing, and the tilt of the wing, called the angle of attack, cause the air to flow downward as it leaves the wing. The greater the angle of attack, the greater the downward flow. In obedience to Newton's third law of motion, which requires an equal and opposite reaction, the airplane is deflected upward. At the same time, a force that retards the forward motion of the aircraft is developed by diverting air in this way and is known as drag due to lift. Another kind of drag is caused by the slowing of air very near to the aircraft's surface; this can be reduced by making the surface area of the craft as small as possible. At low speeds (below Mach
.7) the ratio between lift and drag decreases with gains in speed; accordingly, aerodynamic development for many years stressed increases in thrust over real reductions in drag.
Creation of Shock Waves
Above speeds of Mach .7 the air flowing over the wing accelerates above the speed of sound, causing a shock wave
(also known as a sonic boom
) as the airplane compresses air molecules faster than they can move away from the airplane. The danger of this shock wave is its effect on control surfaces and fragile wing members, and for many years it was thought to represent a near-solid barrier to faster flight. The problems associated with this shock wave were ultimately conquered through the use of swept-back wings and the moving of critical control surfaces out of the wave's direct path. Chuck Yeager
, in 1947, was the first to fly at sustained supersonic speed. Other troublesome phenomena associated with supersonic flight are the shock waves that build up at engine air intakes, and the much larger wave that trails after the craft.
Effect of Hypersonic Speeds
Recently, intense research has gone into the development of planes that can fly at hypersonic speeds, approximately five times or more than the speed of sound. At these speeds the properties of air change radically; there is a rapid increase in temperature associated with the air flowing at such speeds along a plane's surface. The U.S. Air Force is working to develop an aircraft that could travel at 13,000 mph (21,000 kph), a speed that would generate temperatures greater than 3,500℉ (2,000℃).
See A. M. Kuethe and C. Y. Chow, Foundations of Aerodynamics (5th ed. 1997); D. Anderson and S. Eberhardt, Understanding Flight (2001); G. Craig, Introduction to Aerodynamics (2003); D. Bloor, The Enigma of the Aerofoil: Rival Theories in Aerodynamics, 1909–1930 (2011).
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