The Bell X-1 "Glamorous Glennis" was the first aircraft to
achieve mach speed. It did so on October 14th 1947 with Capt.
Charles "Chuck" Yeager at the controls. It reached 807.2 mph (mach
1.06).
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The mach number is the speed, compared to the speed of sound in
air. Thus, mach 1 is the speed of sound, mach 2 is twice (2 times)
the speed of sound, mach 3 is 3 times the speed of sound, etc.
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There is no plural for the noun 'mach'; the different levels of
mach are usually shown by a numeral for various speeds, such as
mach 1 or mach 2.