PPRuNe Forums - Machmeter works how?

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andre.dossantos1985

22nd Jan 2018 13:05

Machmeter works how?

Hello guys,

i have a question regarding the machmeter.

How does it change if we descend during an inversion with constant CAS? I think it should increase because the temperature is decreasing as well because of the inversion... right?

thanks for help ;)

dook	22nd Jan 2018 20:08

Incorrect.

The machmeter measures the ratio of dynamic pressure and static pressure.

It is not dependent upon air temperature.

Therefore in your scenario the reading will decrease as normal.

AerocatS2A

22nd Jan 2018 22:32

The speed of sound is dependent on temperature though, so wouldn’t a drop in temp cause an increase in Mach for a given TAS as the local speed of sound has decreased?

AerocatS2A

22nd Jan 2018 22:50

Never mind, I did some research and see that temperature cancels out in the equations and Mach number can be expressed with regards to pressure only.

gearlever

22nd Jan 2018 22:51

MN = TAS / speed of sound

Both TAS AND speed of sound are functions of square root of absolute temperature K.

So the effect of temperature to MN is nullified.

dook	23rd Jan 2018 11:08

Quote:

Both TAS AND speed of sound are functions of square root of absolute temperature K.

Correct with the second part at least.

andre.dossantos1985

23rd Jan 2018 14:02

okay thanks a lot.

Pugilistic Animus

13th Feb 2018 18:46

https://www.pprune.org/flight-testing/489290-machmeter.html

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