Compressibility error vs compressibility effects..

[CatchThePigeon]

As I understand it, EAS compensates for "compressibility error", but this "compressibility error" is a very different thing that the compressibilty effects an aircraft at transonic speed will be subject to, right?

I found a good thread here about using mach v EAS above the transition altitude which seemed to say that the "compressibility error" that EAS compensates for is in fact the delta between static pressure at sea level and whatever altitude you are measuring your speed at to get EAS.


As an aside to the above, I understand EAS is useful for predicting how an aircraft might behave at various speeds at altitude, but can it be used to predict trans/supersonic handling? i.e. will an aircraft at e.g. 380 or so KCAS at sea level behave the same as the same aircraft at mach.98 and 30,000 feet? It seems to me that one will be experiencing compressibilty effects while the other won't.

Apologies if this seems like really basic stuff, I'm just trying to confirm my basic understanding.

[Oktas8]

Until someone more knowledgeable comes along...

The transonic region is the range of speeds from the speed at which the air flow first becomes supersonic over one part of the aircraft, to the speed at which all parts of the aircraft are bathed in supersonic flow. You know when air flow becomes supersonic at a particular location, because a shock wave forms. Since these shock waves form as a function of the speed of sound, people specify Mach number in this context. If there is no possibility of shocks forming (e.g. well below the transonic regime), there is little point in considering Mach number.

A shock wave is an extreme demonstration of air being compressible, but air is actually compressing and decompressing over a much wider range of speeds, without forming shocks.

Air is compressible at all times, but it's only significantly so above about 250kts TAS. So if for example you're going to measure the stall speed of an airliner at all altitudes, you will specify EAS for simplicity. It saves having to graph the change in stall speed (CAS) as altitude increases. If you specified it in terms of Mach number, you'd still have to graph the change in Mach as altitude increases. So there is no point in using Mach or CAS for a number that is essentially a constant in terms of EAS.

If you're cruising at maximum speed & altitude in an airliner, the potential for shock waves limits your performance envelope much more strongly than EAS limits performance. Hence airliners at altitude use Mach. Horses for courses as they say.

Hope that helps.

[CatchThePigeon]

Yep, that certainly helps a bit. But what I'm mainly interested in is to what degree EAS could be used for calculating aerodynamic loads in trans/supersonic v subsonic regimes.

I appreciate the convenience of showing stall speed in EAS, but if I said, for example that an aircaft at 400kts and sea level would handle the same and be subject to the same stresses as an aircraft at 400EAS and 40k (i.e. around mach 1.2) would I be correct? (I don't think I would be, but I want to get this confirmed!)

It seems to me that EAS is useful at subsonic speeds to calculate loads, express various limitations etc, but if you are considering an EAS that is subsonic for one part of the flight envelope and supersonic for another, it's usefulness is somewhat limited, as you are dealing with quite different flight regimes.

Thanks again.

[Oktas8]

would I be correct

No.

Aerodynamic behaviour depends on EAS (dynamic pressure), Mach Number and Reynolds Number (scale effect). Change any one of those significantly, and the answers change significantly too.

An example: at low subsonic speeds, when air decelerates it increases in static pressure. Bernoulli, and so on. However at supersonic speeds, as air decelerates the static pressure increases. So the loads are very different.

[CatchThePigeon]

Ta, I'm never likely to fly a supersonic aircraft in practise, but it's nice to have my basic assumptiosn confirmed in case the chance ever crops up ;-)

If you have the patience and inclination, back to compressibility error.

The pitot is reading dynamic pressure, but as you go faster, compression affects the dynamic pressure experienced in the pitot, and this has to be factored into the reading.

Or more technically, Bernoulli's equation assumes an incompressible flow, but as air is in fact compressible using the Bernoulli equation to deduce airspeed requires that compressibility be taken into account.

Is that even close to an accurate description of compressibility error?

[Oktas8]

OK, happy to help. I must say though that I'm no expert either - just a pilot who has spent some time dabbling in principles of flight. Since the likes of John Tullamarine and others of his calibre haven't commented yet, I'll keep going.

Bernoulli's equation assumes an incompressible flow, but as air is in fact compressible using the Bernoulli equation to deduce airspeed requires that compressibility be taken into account. ... Is that even close to an accurate description of compressibility error?

Yes.

Some minor corrections though (no pun intended...)

A pitot tube measures total pressure, not dynamic pressure. The freestream air has dynamic pressure and static pressure. As the air is brought to rest inside the pitot tube, Bernoulli tells us that 100% of the dynamic pressure is perfectly converted to static pressure, which the sensors then measure. This is also called total pressure or stagnation pressure or pitot pressure.

As you've suggested air is actually compressed in this process, which Bernoulli's description cannot account for. Thus the pitot tube measures a higher impact pressure than you would get by adding dynamic + static pressures.

The ASI over-reads at high TAS or high Mach Number.