This page is intended for college, high school, or middle school students.
For younger students, a simpler explanation of the information on this page is
available on the
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
in honor of Ernst Mach, a late 19th century physicist who studied gas
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
remains nearly constant.
The first powered aircraft to explore this regime was the
Modern general aviation and commuter airliners continue to fly
in this speed regime. At such low speeds,
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
of subsonic aircraft are typically
and made of light weight aluminum, although the Wrights
used wood and cloth in their wing construction.
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