Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: usage of Mach number (flight related)



Stefan Jeglinski wrote:

The question is: since Mach number refers to the local speed of
sound, it varies with altitude.

Yes.

> 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?

No.

Or are the numbers always quoted as local,

Local.

> making it difficult to
calculate for a layperson just what airspeed the craft is traveling
at?

a) The typical aircraft has two gauges: one indicates airspeed,
while the other indicates Mach number.

b) It isn't thaaat tricky to convert from one to the other, given
the temperature.

In the "standard atmosphere" the temperature is 15C at sea level
and declines by very nearly 2C per thousand feet until you reach
the tropopause at 36,000 feet and -56.5C. Then in the stratosphere
the temperature is constant, independent of height, -56.5C.

The real atmosphere doesn't differ too much from the standard
atmosphere. Useful reference for this (and many other things):
"The Aviation Formulary"
http://williams.best.vwh.net/avform.htm

c) There are good reasons for knowing both the airspeed and
the Mach number. There are some things that happen at a given
CAS (calibrated airspeed) and there are other things that
happen at a given 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.

Well, it's poorly worded in some places, and dead wrong in
other places. Not worth salvaging.