Originally Posted by PBL
Shevell is somewhat misleading in that he implicitly suggests that Mach number versus AS correction is dependent on altitude, whereas of course it is dependent solely on temperature.
Originally Posted by Rivet gun
I agree that Shevell is somewhat misleading, but not nescessarily for the reason you suggest.
M = a.TAS and a=sqrt(gamma.R.T) so M = A.sqrt(T).TAS where A is constant. So M/TAS = A.sqrt(T).
I hope you are not disagreeing!
Originally Posted by Rivet gun
Shevell correctly implies that the relation between CAS and EAS depends on pressure altitude and Mach number.
First, pressure
altitude??? Why make things complicated? Why not just freestream (=static) pressure?
EAS is an algebraic function of (stagnant pressure - freestream pressure), calculated by the pitot-static system, and speed-of-sound (usually denoted a). CAS is the same algebraic function, but where a is replaced by the constant a_s, the speed of sound at sea level in the ISA, so an algebraic function of (stagnant pressure - freestream pressure), measured by the pitot-staic system, alone.
As Anderson says,
Originally Posted by Anderson, p181
The static temperature in the air surrounding the airplane is difficult to measure. Therefore all high-speed (but subsonic) airspeed indicators are calibrated....assuming that [the speed of sound] is equal to the standard sea-level value a_s = 340.3 m/s.
Originally Posted by Rivet gun
The relation between Mach number and TAS depends on static temperature
And the relation between EAS and CAS also, as noted by Anderson and, derivatively, myself.
Originally Posted by Rivet gun
The relation between Mach number and EAS depends on statc pressure (i.e pressure altitude)
"i.e. pressure altitude"?? You prefer to think in terms of pressure altitude rather than in terms of static pressure? Every static port will disagree with you!
I would say that the relation between Mach number and EAS is as follows. TAS = M.a EAS = sqrt(rho/rho_s).TAS = sqrt(rho/rho_s).M.a = A.sqrt(rho).M.a where A is constant = B.sqrt(rho).M.sqrt(T) where B is constant and since p=rho.T.const, this is = C.M.sqrt(p). I would then say that the relation between Mach number and EAS is proportional to sqrt(p). More informative, no?
Originally Posted by Rivet gun
The relation between TAS and EAS depends on density
Specifically, proportional to sqrt(rho).
PBL