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usage of Mach number (flight related)



In common parlance, one hears things like "The Concorde flies (used
to fly) at Mach 1.5," Or "the F-16 can do Mach 2," or "the Space
Shuttle reaches Mach 25 at some point during its reentry," and
descriptions such as these. (I don't know if those are the actual
numbers or not; it's beside the point of my question).

The question is: since Mach number refers to the local speed of
sound, it varies with altitude. So is there a hidden assumption that
all Mach numbers referring to flying speed are (supposed to be)
quoted with respect to the speed of sound at a reference temperature?
Or are the numbers always quoted as local, making it difficult to
calculate for a layperson just what airspeed the craft is traveling
at?

[For the sake of this discussion, I'm following the ideal gas
approximation of the sqrt(T) dependence of the speed of sound - I
don't know that there are significant corrections for real air, or
that it would matter, given the variability of temperature at a given
altitude - I only know that a pilot would indeed be interested to
keep tabs on his *local* Mach number].

While searching for information on this, I came across

http://www.physlink.com/Education/AskExperts/ae563.cfm

which strikes me as being pretty poorly worded on several levels.


Stefan Jeglinski