High Altitude Minimum Airspeed

[CliveL]

PBL

Maybe because it does?

Sorry PBL - Rivet-gun is correct - it doesn't.
CAS relates to EAS; the speed of sound expressed in EAS is independent of temperature as discussed on another thread. Ergo Mach Number as a ratio between EAS/CAS and the speed of sound is also independent of temperature.

Selfin

What I do not understand is why an incompressible solution is used when dealing with ‘speeds’ above Mach 0.3. The idea of basing any calculations on an incompressible formula in the transonic regime is simply baffling to me.

I see where you are coming from, but the answer is probably historic. There is an ENORMOUS amount of data kicking about which has been calculated using dynamic pressure as usually defined and to shift now would make that useless. Presumably if one changed to your 'impact pressure' as a basis for reducing measured data one would hope to eliminate compressibiity effects from the coefficients i.e. they might then be independent of Mach No, but looking at the other curves in Hazelnuts 39's chart I don't think that would be so, so to paraphrase his remark re change from CAS to EAS in an earlier posting - what would be the point of changing now to impact pressure rather than dynamic pressure?

Hazelnuts 39

Your graphs of Cl/Clo look a lot like those of a large transport aircraft with which I am very familiar, and seeing your location is France (mostly) I am wondering if we have worked together????

[CliveL]

Rivet Gun

So in EAS terms there was a about a 21% difference in the position of the yellow band between FL80 and FL 380.

In CAS terms the difference was about 27%

Like you I refreshed my memory on the magnitude of the effect. In my case I took the data from my 'bible' - Dick Shevell's Fundamentals of Flight.

For subsonic aircraft the differences aren't too large - at 250 kts CAS/FL350 for example the difference is 12 kts, but if you have a supersonic aircraft and high altitude it becomes more important - at M 2.0/FL500 it is about 80 kts. If anyone is sufficiently interested Shevell gives his source as USAF Series T.O. 1F-5a-1 Flight Manual

[HazelNuts39]

Clive;

It is possible we have met sometime. In my working life I had frequent contacts with Toulouse, but never worked there. France became my part-time home only after I retired.

regards,
HN39

[Rivet gun]

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.