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Re: usage of Mach number (flight related)



At 07:31 PM 10/30/2004, Stefan J., you wrote:
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.

///

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?

//

Stefan Jeglinski

The speed of sound is sensibly constant at a given temperature,
so an indicator showing what proportion of THAT speed is being flown,
would indicate a scaled version of true air speed, more or less.
This would have its uses, but the Machmeter has other objectives.

For sub-sonic transports, there is an interest in knowing the distance
from both stall speed, and limiting Mach number. The two approach
each other at height. Some aircraft exhibit "Mach tuck" at speed.

For trans sonic aircraft, there is interest in knowing the local
speed at which the transition occurs.
So to answer the question directly: the mach number indicated is the
ratio of the aircraft's speed to the local speed of sound, though this
speed is slower where the air is cooler.



Brian Whatcott Altus OK Eureka!