The red line on this graph shows qc/q, the ratio between impact pressure and dynamic pressure (as defined in NACA Report 837) as a function of Mach number. Also shown is cL/cL0, the ratio of lift coefficient at a Mach number to liftcoefficient at M=0 for a range of AoA's of a large transport airplane.
Thanks, that's interesting. I suppose if you re defined CL as lift/(qc*S) the variation of CL with Mach number would be less than it is using the conventional definition of CL as lift/(q*S).
But I think this is not an arguement for CAS because qc/q does not equal CAS/EAS. qc/q = 1 + M^2/4 + M^4/40 (+ futher binomial terms if you want). However CAS/EAS = 1 at sea level for all subsonic speeds. CAS is a function of qc, but it is a fucntion designed to reduce qc to TAS (and EAS) at sea level. So if we really wanted to place qc center stage how would we calibrate the airspeed indicator? I'm not sure.
And why do very high altitude aircraft have EAS indicators? Does anybody know what the Virgin spaceship uses?
I suppose if we broadened the debate we could ask why we display airspeed on the PFD at all. Perhaps we could display some other paremeter for aircraft control such as manoeuver margin or angle of attack, but given the ability of modern EFIS to display stall speed, manoeuver margin etc on the speed tape such a change would be hard to justify.
Given then, that we are going to use "airspeed" as a control parameter we need to choose an airspeed which is closely related to the aerodynamic forces. But pilots also need to know TAS and Mach number. Therefore it makes sense to choose an airspeed for contol which also has a simple mathematical relationship with TAS and Mach number. I think EAS best fits these criteria.