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Photo of a subsonic aircraft
 with some of its characteristics

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. Exactly how the air re-acts to the aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

For aircraft speeds which are very much less than the speed of sound, the aircraft is said to be subsonic. Typical speeds for subsonic aircraft are less than 250 mph, and the Mach number M is much less than one, M << 1 . For subsonic aircraft, we can neglect compressibility effects and the air density remains nearly constant.

The first powered aircraft to explore this regime was the Wright Brothers' 1903 flyer. Modern general aviation and commuter airliners continue to fly in this speed regime. At such low speeds, propellers provide a very fuel efficient propulsion system. On the slide we show a C-130 cargo aircraft which is powered by four turboprop engines. The wings of subsonic aircraft are typically rectangular in planform and made of light weight aluminum, although the Wrights used wood and cloth in their wing construction.

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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 13 2021

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