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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