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# What is the use of calibrated airspeed / what speed creates flutter

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# What is the use of calibrated airspeed / what speed creates flutter

31st May 2015, 08:19

Join Date: Oct 2010
Location: France
Posts: 272
What is the use of calibrated airspeed / what speed creates flutter

I'm not working on ATPL theory, I've passed that several years ago.
As I'm a CRI now I think it's a good idea to check my knowledge.
I'm just considering subsonic flight here.
This is below what I understood, would anyone tell me if I got it right?

1a TAS is the true airspeed. I don't think TAS is of any use for the pilot. When we use TAS, it's because we want to know our ground speed. But TAS in itself will not help to fly the aircraft.
1b Nevertheless at high speed the TAS will cause the aircraft to flutter.
For small aircrafts the VNE/VNO are in IAS, meaning that at VNO, I'm closer to the risk of flutter at high altitude because my TAS is higher.

2 To fly the aircraft, the only useful speed is the EAS, as it's directly linked to the dynamic pressure, which is directly linked with lift and drag.

3a I don't see any use of the CAS. I think it has been invented only because it's easier to mesure than EAS, and that CAS is very close to EAS in the area of the flight enveloppe (low altitude low speed) where manoeuvres are required. But an ASI which would provide a best approximate of EAS instead of CAS would be better fit for purpose.
3b Is it a certification requirement that the IAS be the best approximate of the CAS, or is it permitted to design the IAS so that it indicates the best approximate of the EAS?
1st Jun 2015, 14:32

Join Date: Feb 2000
Location: UK
Posts: 13,555
Originally Posted by 172510
I'm not working on ATPL theory, I've passed that several years ago.
As I'm a CRI now I think it's a good idea to check my knowledge.
I'm just considering subsonic flight here.
This is below what I understood, would anyone tell me if I got it right?

1a TAS is the true airspeed. I don't think TAS is of any use for the pilot. When we use TAS, it's because we want to know our ground speed. But TAS in itself will not help to fly the aircraft.
Agreed. The progression, not as well understood as it should be, is:-

IAS --> (system error corrections) --> CAS --> (compressibility corrections) --> EAS --> (density corrections) --> TAS --> (wind corrections) --> groundspeed.

1b Nevertheless at high speed the TAS will cause the aircraft to flutter.
Not quite. Flutter *may* occur, and the speed at which it occurs is defined in TAS, and most commonly it's at high speed. I've seen flutter under 100kn, and in theory at least increasing speed through that - if stuff hasn't fallen off on the way through, may damp the flutter back out.

For small aircrafts the VNE/VNO are in IAS, meaning that at VNO, I'm closer to the risk of flutter at high altitude because my TAS is higher.
For all aircraft Vne/Vno are declared in IAS. Both are set by a safety margin below Vdf - the flight test diving speed, and a further margin below Vd - the design speed limit. There are numerous reasons why Vdf may be declared. Ones I've dealt with have included canopy buckling, flutter, aileron reversal, longitudinal stability flattening or reversal. Flutter's the only one of those likely to be defined in TAS, most will be declared in EAS from an analytical viewpoint.

Mne/Mno are a separate issue, but defined in similar ways. Clearly you fly to the lower of the two, so at low altitude Vne is the critical factor, and at high altitude, Mne takes over.

2 To fly the aircraft, the only useful speed is the EAS, as it's directly linked to the dynamic pressure, which is directly linked with lift and drag.
Nope, the only useful speed is IAS, because that's the one the dials read in.

3a I don't see any use of the CAS. I think it has been invented only because it's easier to mesure than EAS, and that CAS is very close to EAS in the area of the flight enveloppe (low altitude low speed) where manoeuvres are required.
It wasn't invented, it's an intermediate step that you need in calculating these things at any time.

But yes, for all reasonable purposes below about 0.6M and FL100, EAS=CAS.

But an ASI which would provide a best approximate of EAS instead of CAS would be better fit for purpose.
CAS is only useful directly when it approximates to EAS, but we do need it when doing performance calculations at the high level of flight testing, certification and design.

3b Is it a certification requirement that the IAS be the best approximate of the CAS, or is it permitted to design the IAS so that it indicates the best approximate of the EAS?
Most civil airworthiness standards require that IAS is within 3% or 5 knots: whichever is larger of CAS from 1.3Vs to Vne.

There's no standard preventing an EAS calculator in the cockpit - indeed some ASIs are designed to do exactly that, and some FMS work out TAS from IAS, via EAS - although I suspect that many of those fail to allow for PECs (IAS to CAS corrections) in the transition, so there will be errors from that source in the TAS value determined, as well as any other errors that creep in en-route.

G
1st Jun 2015, 15:08

Join Date: Oct 2010
Location: France
Posts: 272
I thank you very much, yet it's not yet completely clear.
Imagine a world where certification rules do not exist, and where it's cheap and easy to have CAS, EAS, TAS indicator without any error, in that world, you built your aircraft.
What need could you possibly have of a CAS indicator?
My understanding is that a CAS ASI is useless, as long as I have an EAS indicator which is better fit for purpose.
Am I right?

As nowadays, with ADCs, it seems to me that it's easy to design an ASI that can provide an indication within 3%/5kt of EAS, why still bothering with CAS in the certification process?
Why did the regulator chose to use the calibrated airspeed instead of the equivalent airspeed in the IAS requirement?

(b) Each airspeed system must be calibrated in flight to determine the system error. The system error, including position error, but excluding the airspeed indicator instrument calibration error, may not exceed 3% of the calibrated airspeed or 9.3 km/h (5 knots), whichever is greater, throughout the following speed ranges:
(1) 1·3 VS1 to VMO/MMO or VNE, whichever is appropriate with flaps retracted.
(2) 1·3 VS1 to VFE with flaps extended.

Last edited by 172510; 1st Jun 2015 at 16:29.
2nd Jun 2015, 22:17

Join Date: Feb 2000
Location: UK
Posts: 13,555
I don't know the answer to your question, I can offer an educated guess.

I suspect that when those words were drafted - probably in the 1940s or 1950s, the difference between CAS and EAS was trivial and not considered as important by anybody.

Over the 1950s and 1960s aeroplane performance became greater, and engineers - and eventually pilots - were required to start including compressibility corrections and allowing for the difference between the two. However, I don't think that the regulations ever caught up.

But, realistically, does it actually matter? We will always need a quantity called IAS: that which is indicated in the cockpit, and that's what aeroplanes will be flown to. TAS will always be important for navigation calculations, and some aerodynamics. EAS will always matter to engineers, but never really matter to pilots other than as an intermediate quantity.

So, is there really an issue in having CAS - which just ensures that at each step from IAS to G/S there's a single set of corrections, rather than two corrections.

G
3rd Jun 2015, 11:40

Join Date: Feb 2007
Location: UK
Posts: 315
"...I've seen flutter under 100kn..."

I know one test pilot who had a microlight break-up around him from flutter at around 40 knots. Luckily, it was still close enough to the ground that he survived. I know the story is true, he has shown me the video.
4th Jun 2015, 08:23

Join Date: Oct 2010
Location: France
Posts: 272
I think that the confusion comes from the fact that the CAS is corrected for compressibility, but the correction is only valid at sea level, which makes the CAS a very strange animal.
The seal level only correction has probably been invented because it's the only compressibility correction you can make when only Pt-Ps is available.
My guess is that:
In the old days, there was a speed I could name HAS, historical air speed, which was defined as
HAS = sqrt (2 *(Pt-Ps)/ rho0)
HAS is very close to CAS and EAS at low speed.
Then the need to make corrections for high speed arose.
There is no way you can calculate EAS from HAS only.
So the CAS was invented. The CAS takes into account compressibility, but at sea level only. CAS can be calculated from HAS only, without the necessity to know separately Pt or Ps. CAS is closer to EAS than HAS is.

By the way is there any official or customary name for what I called HAS?
4th Jun 2015, 16:36

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Yes - IAS. Or at least it is if there are negligible gauge errors.

I don't know what you do, but my reference correction chart for CAS to EAS has both altitude and Mach number on it. (Gratton, Initial Airworthiness, p43.).

G
6th Jun 2015, 10:03

Join Date: May 1999
Location: Bristol, England
Age: 60
Posts: 1,469
I have fundamentally the same question as 172510. We are told CAS is the displayed airspeed on most EFIS platforms, certainly Boeing and Airbus. Very few display EAS, but why not? It would seem that IAS=EAS is more use than IAS=CAS.
6th Jun 2015, 10:37

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Location: UK
Posts: 594
I think that the issue here is that to display EAS requires an ADC. There are two inherent problems here:

1. It is an additional expense for light aircraft, gliders, microlights etc that is, frankly, unnecessary.

2. If the ADC fails, there will need to be a reversionary IAS mode that will display an airspeed that is CAS (unless there are significant static source pressure errors). If operating at high altitude this will mean that airframe limits will have to be promulgated as both CAS and EAS and all 'V' speeds will also have to be promulgated with respect to both airspeeds and, for high altitude airfields, will be different. The potential safety implications for using the wrong set of speeds far outweighs the advantages of having EAS rather than CAS. Until the reliability of an EAS system equals that of a CAS system then I think that we must stick with CAS.

I have been flying for many years over a fairly large flight envelope and cannot think of one instance where having EAS rather than CAS displayed would have helped me.
6th Jun 2015, 13:30
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I'm confused here.

For the pilot, the gauge displays a number of some description which, for any given stage of flight, configuration, etc., needs to be kept between a minimum and maximum value via pilot intervention. A good pilot manages to keep the number somewhere near a desired value at any given time.

Whether the actual number is IAS, CAS, apples or oranges, is rather moot.

Different matter if we are talking engineering stuff, of course.
6th Jun 2015, 16:18

Join Date: May 1999
Location: Bristol, England
Age: 60
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Well yes, and following that argument TAS could be used as the displayed speed on EFIS. But the TAS of critical speeds changes with air density so that wouldn't be a good choice. I think the argument is that the speed that has the same critical values with changes in mach number and density is EAS, not CAS or TAS, so, if the ADC can calculate it, why not use that?

LOMCEVAK's second point may be the conclusive answer, that the standby instruments should display the same speed as the main instruments. I can see the sense of that. I have been told that one type of modern jet does display EAS, was it the Phenom?

PS, Ghengis, CAS to TAS is two corrections, not one, compressibility and density. Picky I know.

Last edited by Alex Whittingham; 6th Jun 2015 at 16:25. Reason: to add postscript
6th Jun 2015, 18:15

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IAS --> (system error corrections) --> CAS --> (compressibility corrections) --> EAS --> (density corrections) --> TAS --> (wind corrections) --> groundspeed.
Earlier post.

G

Last edited by Genghis the Engineer; 6th Jun 2015 at 18:59.
6th Jun 2015, 20:50

Join Date: Oct 2010
Location: France
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IAS --> (system error corrections) --> CAS --> (compressibility corrections) --> EAS --> (density corrections) --> TAS --> (wind corrections) --> groundspeed.

You need EAS, not TAS.
EAS is complicated to calculate because you need to know separately Ps and Pt

So you define CAS,being as close as possible to EAS, that can be calculated with Pt-Ps , without having to know separately Pt and Ps.

CAS is easier to calculate. You only need to know Pt-Ps, you do not need Pt and Ps separately
Note that, contrary to a common belief, CAS does take into account compressibility, but at sea level only. Yet CAS does not take into account density.
Without compressibility, at low speed, CAS would be what I call HAS above
$HAS = \sqrt{2(Pt-Ps)\over \rho_0}$
You can check that HAS is a first order approximation of CAS for slow speed.

Then you define IAS as being as close as possible to CAS (the regulation says 3% 5kt), because your pitot and static are not perfectly aligned or perpendicular to the flow, and because the flow is disturbed by the plane.

Genghis said above that IAS is what I call HAS but I don't understand why it should be.

Last edited by 172510; 6th Jun 2015 at 21:02.
6th Jun 2015, 22:56

Join Date: Feb 2000
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We use IAS to fly by.

CAS is an intermediate step.

EAS is used for structural calculations.

TAS is used for aerodynamic calculations.

G/S is used for navigational calculations.

If there were no system or gauge errors IAS = CAS. Ultimately IAS is at one end, G/S is at the other - you can correct in either direcction as required.

Your term HAS is IAS, because it's a first go at CAS from measured Pt and Ps. CAS is IAS (your HAS) with the system errors calibrated out. In most cases, we act as if IAS=CAS; not strictly true, but so long as the certification process ensured that operating speeds are correct in IAS, close enough.

I'm with Lomcevac here - EAS is essential for engineering calculations, but I really can't see why it's of any value to a pilot operating an aeroplane.

G
7th Jun 2015, 11:04

Do a Hover - it avoids G

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I'm with you Genghis
7th Jun 2015, 15:50

Join Date: Oct 2010
Location: France
Posts: 272
I agree that EAS is of no practical use. But in that case CAS is of no practical use either. HAS is a largely sufficient concept. And IAS with 3%/5kt of HAS is sufficient to fly an aircraft.
Why would you invent CAS as HAS is sufficient? That was my initial question. English is not my first language, so maybe that's why it has not been clear so far that that was actually my only question.
EAS is the theoretically ideal speed to indicate to the pilot.
HAS is sufficient for practical purposes.
What is the use of CAS?

Last edited by 172510; 11th Jun 2015 at 19:02.
8th Jun 2015, 16:48

Join Date: Mar 2000
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Gents, I am but an ordinary (now retired) ex military airline TC.
In writing an article about 'speeds' for my last Airline Training Department, one of my very clever ex-military TP (Edwards trained) penned the following summary for me which you may or may not agree with - I found it helpful:

We can’t see AOA, compressibility or boundary layer characteristics in the cockpit, but the wing can feel them. So the designers give us a pressure gauge and decide what banana units it should read. By clever maths….they make it read CAS (kts) because that’s what gives us the most useful info at slow speeds and near the ground. By fluke, it also works quite well throughout most of the envelope – keeping us from stalling and overspeeding. But it doesn’t show Reynolds and compressibility….so they decided they should electronically add warnings of stall and overspeeds usually by lookup tables cross referenced with air data inputs and fed into the cockpit speed tape, audio etc……The SR71 uses KEAS because the CAS indicator becomes virtually useless at high Mach numbers.”

Cheers,
mcdhu
8th Jun 2015, 17:52

Join Date: Oct 2010
Location: France
Posts: 272
CAS (kts) because that’s what gives us the most useful info at slow speeds and near the ground.

At low speed and near the ground, what I call HAS is sufficient.
So the question remains open. Why CAS?
8th Jun 2015, 21:04

Join Date: Feb 2000
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Because it is a useful intermediate step between IAS and everything else.

G
27th Jun 2015, 21:24

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Location: Hampshire
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A difficult subject!

Not an easy subject, and one that often creates confusion. I trained at ETPS and have subsequently worked on a number of projects concerning air data systems and one concerning flutter, so hopefully I can add some clarity!

Firstly, consider dynamic pressure (Pd). This is calculated from either 0.5 x actual density x TAS, or 0.5 x ISA sea level density x EAS. Either version gives you the same dynamic pressure. This is why EAS is an Equivalent Air Speed; for a given dynamic pressure it is the air speed equivalent to TAS if the density is assumed to be ISA sea level density instead of the actual air density the aircraft is experiencing.

The reason for using EAS is that TAS is hard to determine because it needs a knowledge of the actual air density, which is impossible to measure directly in flight. By fixing density at the ISA sea level value EAS thus becomes a simple and very useful proxy for dynamic pressure, which pilots need to know for all sorts of reasons including takeoff performance, stall avoidance, minimum control speed, minimum drag speed, gear and flap limits, etc.

A key benefit of EAS is that because it links a speed value to a dynamic pressure value regardless of actual density, important speeds such as stall, flap and gear limits, etc, don't change with height or OAT until Mach effects start to come in to play. If your airspeed limits were expressed in TAS you would need to recalculate them whenever the air density changed, which would not be particularly fun, helpful or safe!

Unfortunately in flight there is no easy way to measure EAS accurately because a pitot-static probe measures total pressure (Pt) and static pressure (Ps) in order to determine impact pressure (qc = Pt - Ps), which is not quite the same as dynamic pressure. Dynamic pressure assumes the air does not compress and change density in the pitot tube as it comes to a halt, whereas in reality at higher speeds the air does compress, meaning the measured impact pressure is greater than the free-stream dynamic pressure. In short, measuring EAS is not easy!

So, before the days of air data computers your ASI gave you an indicated air speed (IAS). If your ASI was accurate and the pitot static probe feeding it happened to measure an accurate representation of free-stream static and total pressure (i.e. the position error or pressure errors were very small) then at speeds below about 0.3M (onset of compressibility) your IAS would closely approximate EAS. Sadly it is not at all easy to position pitot-static probes in such a way that they measure free-stream Pt and Ps accurately, and errors in Ps in particular are hard to eliminate entirely, unless one uses a very long nose boom or trailing static in order to move the static pressure port outside the influence of the aircraft's static pressure field. Any error in measured Ps causes an error in qc and thus errors in IAS. During flight test these errors are measured and then compensated for (by means of static source error correction data in the air data computer), thereby converting IAS to CAS. CAS will be a much better approximation to EAS than IAS would be, which is why CAS is so important. If an aircraft has an air data computer the PFD will almost certainly display CAS, not IAS, although some aircraft display EAS.

So, to summarise, IAS is an approximation of EAS but includes pressure errors and does not correct for compressibility effects either, so it is only of use on slow speed, low altitude aircraft that fly well below Mach 0.3. CAS is a better approximation to EAS because the pressure errors are virtually eliminated over most of the flight envelope but compressibility effects are still present, so at higher speeds CAS starts to over-read significantly. However, as a tool for avoiding unsafe air speeds (high or low) CAS is perfectly adequate for most aircraft, particularly for subsonic flight.

This brings us back to TAS. In an Air Data Computer this is often back-calculated from Mach Number and Total Air Temperature, which means there is no need to calculate EAS as an intermediate step. Mach number is calculated directly from Ps and qc, OAT is calculated from TAT and Mach, then OAT is used to calculate the free stream speed of sound, from which TAS is calculated using Mach. TAS is needed for navigation and also for derivation of the current wind vector and drift angle, provided one has a means of measuring GS and TRK (eg GPS, IRS, Doppler radar, etc).

For design engineers and flight test engineers TAS is needed to calculate climb and turn performance, because TAS is a geometric speed, i.e. it represents an actual distance travelled in an actual period of time. In contrast, IAS, CAS and EAS are just proxies for dynamic pressure and are not actual speeds as such.

Lastly, flutter. EAS is used for defining and assessing flutter because it is a useful proxy for dynamic pressure regardless of altitude and OAT, provided Mach effects are not an issue. Once Mach effects start to influence flutter behaviour then flutter limits are usually given as both an EAS and Mach limit, so at lower altitudes the EAS limit takes precedence and at higher altitudes the Mach limit takes precedence. The cross-over altitude depends on the actual EAS and Mach values the flutter data is based on. However, the structural damping of flutter is influenced by air density and hence altitude. Therefore TAS can also be used to define speeds where flutter can be a problem. But for flight test purposes EAS is easier to use.

I hope this step-by-step but perhaps somewhat long-winded explanation is helpful!

Regards, WF

Last edited by WeekendFlyer; 27th Jun 2015 at 21:39.