To add to the debate, I remember the relationships between the speeds via the following:
IAS = Indicated Airspeed
CAS = Calibrated Airspeed = IAS +/- Instrument & Position Error
EAS = CAS - Compressibility Correction
TAS = EAS * Density Correction
EAS is defined as:
V_e = V_0*\sqrt(\rho/\rho_s)
where
V_0 = TAS
\rho = ambient density
\rho_s = sea level density
As one may infer from
Old Smokey's post the compressibility correction (typically known as the
F factor) that one may apply to
CAS to acquire
EAS may be thought of as a function of two variables, namely,
CAS and
altitude.
The derivation is relatively simple and starts from the isentropic flow relationship for pressure:
p_0/p = (1+(\gamma-1)/2*M^2)^(\gamma/(\gamma-1))
If we solve for TAS, V, in terms of the difference between p_0 and p, and substitute for M, bearing in mind we know that:
M = V/(\sqrt(\gamma*p)/\rho))
we find that:
V = a complicated expression
which may be subsequently re-arranged, using the fomula for
EAS, to give V_e.
The unknowns in this equation are the static pressure and the difference between the static and total pressure, which is exactly what a pitot-static system measures.
As
Old Smokey alludes to, it is standard practise to calibrate the instrument at sea level, whereupon, the complicated expression above is evaluated in terms of sea-level standard pressure. The speed given by such a calculation is the
CAS.
It is now clear that, to evaluate the
EAS at other altitudes, a correction must be made, and that this correction is the ratio between the equation for V above, evaluated using the actual pressures at the altitude you are interested in and sea level values:
V_e=
F*V_cal
where
V_cal =
CAS
F = Compressibility Correction factor
I certainly agree that in the days of EADI's and ADC's, whereupon I suppose that at some stage during the path from sensing airspeed to displaying it, a pressure transducer is involved, it would not be hard to apply the relevant correction to the signal.
Maybe there are certification issues involved here involving the nature of standby instrumentation?
If one retains traditional standby instruments, one would have to remember, in the event of a failure of all primary instruments that one was working with CAS again rather than EAS.
But, of course, we seem to work with CAS successfully at the moment...
