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Photo of a subsonic airliner
 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 near the speed of sound, the aircraft is said to be transonic. Typical speeds for transonic aircraft are greater than 250 mph but less than 760 mph, and the Mach number M is nearly equal to one, M ~= 1 . While the aircraft itself may be traveling less than the speed of sound, the air going around the aircraft exceeds the speed of sound at some locations on the aircraft. In the regions where the local airspeed is near or greater than the speed of sound, we encounter compressibility effects and the air density may vary because of local shock waves, expansions, or flow choking.

The first powered aircraft to explore this regime were the high performance fighters of World War II. These aircraft seemed to encounter a sound barrier at which drag was increasing faster than thrust. There was speculation in the mid-1940's that manned flight was not possible at speeds faster than the speed of sound, even though the muzzle velocity of rifle bullets is supersonic. Of course, the flight of the X-1A in 1947 proved that people could fly faster than sound and, until the recent retirement of the Concorde, any person with enough money can fly supersonic. As mentioned above, even though modern airliners typically fly at about M = .85, the flow over the wings is transonic or supersonic. Drag increases dramatically as an aircraft approaches Mach 1, so airliners use high thrust gas turbine propulsion systems. On the slide we show a DC-8 airliner which is powered by four turbofan engines. The wings of airliners are typically swept in planform to reduce the transonic drag. For Mach numbers less than 2.0, the frictional heating of the airframe is low enough that light weight aluminum is used for the structure.


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

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