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Aerodynamics, branch of physics dealing with the motion of air and other gases and their flow around a body in motion, used particularly in the development of the airplane and other aircraft. Aerodynamic forces depend on the body's size, shape, and velocity and on the density, compressibility, viscosity, temperature, and pressure of the gas. At low velocities flow around the body is streamlined and causes low drag; at higher velocities turbulence occurs, with fluctuating eddies, and drag is much greater. Additional drag is created by friction. Pressure impulses radiate at the speed of sound ahead of the moving body; at supersonic velocities these impulses pile up, producing a shock wave—the “sonic boom.” In airplane design all of these factors must be considered. In normal cruising flight the lift provided by the wings must equal the aircraft's weight; the forward thrust of the engine must balance the forces of drag. Lift occurs because the wing's upper surface is more convex, and therefore longer, than the lower surface, creating a difference in air speed and thus pressure, according to Bernoulli's principle.

See also: Wind tunnel.

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