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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.