Originally Posted by
selfin
There is no Mach number in an incompressible flow because the speed of sound is infinite. Neither Mach meters nor airspeed indicators need to be "corrected for compressibility" so it does not necessarily follow that determining Mach number by a quantity such as `(Pt - Ps) / Ps` will result in an error. The Mach number can be related to the normalised pressure rise `(Pt - Ps) / Ps` using a simple algebraic expression without needing to adopt an incompressible flow assumption. The applicable technical standard for Mach meters
Nothing prevents you from calculating a mach number, from speed and temperature, even in an incompressible flow.
Nothing would prevent you from building a machmeter which would would exactly compute the mach from the following formula :
TAS=sqrt(2(Pt-Ps)/rho)
LSS=sqrt(gamma R T)
Rho=Ps/RT
TAS/LSS = sqrt(2(Pt-Ps)/Ps Gamma)
If you measure a higher Pt due to compressibility then yes you're going to need to correct it.
If you measure a CAS of 280kt at FL330, you're probably gonna want to correct something if you want to know your EAS.
In facts, you have to correct for compressibility
if you use this above mentionned formula, simply because the formula is not valid for compressible flow.
For compressible flow, where a machmeter becomes useful, there are better suited formulas that you could use, and then you wouldn't need to correct for compressibility, because you avoided the need for it by choosing a better formula.
Then, I could agree that the answer marked as correct was actually wrong, i'm not in the head of the question's writers...
I was simply trying to find an explanation regarding this, without being able to tell whether I would answer 480 or 450 should this question appear on my official test (can we trust aviation exam or this other question bank when they tell us a surprising correct answer like this ?)
And from what you wrote after, I know you'll understand my message because you've written yourself the "better suited formula" I was talking about
Regarding the little technical details of real macheters, I really have no idea wheter early mach meters were able to mechanically reproduce this sort of complicated formula as written in wikipedia :
https://wikimedia.org/api/rest_v1/me...782ff6024667fa
To begin with I don't understand how you could divide a number by another with a system of mechanical gears and hands, so I don't see how you would elevate this ratio to the power 2/7
It is however very easy for modern ADC to use formulas as complicated as required thanks to IT.