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Disparity in IAS on gauge and P3D value


Ray Proudfoot

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Ray Proudfoot

I'm currently at FL500 enroute from GCTS to SBGL. The IAS gauge shows 520kts but if I press Shift+Z the IAS shown there is 493kts. That value is confirmed by monitoring offset 02BC via FSUIPC.

I don't understand why there's such a difference. A bug perhaps? If you want to check it yourself and have a paid version of FSUIPC select the Logging tab, enter 02BC and S32 in the spaces shown. Select FS Title Bar to display the value. You will need to divide that value by 128 to get the iAS.

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Ray Proudfoot

I searched for "IAS inaccurate", "IAS wrong", "CAS inaccurate" and "CAS wrong". Found nothing.

For such a superbly modelled aircraft this seems such a basic error. Why not just use the value available in FSX/P3D?

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Andrew Wilson

The new Concorde uses the correct CAS calculation :)

The value P3D produces is slightly in-accurate Ray. We correct this for subsonic flight - and now supersonic flight also. 

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Ray Proudfoot
1 minute ago, Andrew Wilson said:

The new Concorde uses the correct CAS calculation :)

Thanks Andrew. That's good news on two fronts. A corrected CAS (or IAS to us lesser mortals) and the fact that work is continuing on Concorde after recent events made me think it might not see the light of day.

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Fraser Gale
1 hour ago, Andrew Wilson said:

The new Concorde uses the correct CAS calculation :)

The value P3D produces is slightly in-accurate Ray. We correct this for subsonic flight - and now supersonic flight also. 

I'm glad you said this @Andrew Wilson cause I didn't want to...

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Fabrice Estienne

Hi Ray,

This is natural.

Actually FSX and its successor P3D provide an unconventional CAS computation.

I asked the fix of LM (P3D forum / P3D feature request / CAS (IAS) computation versus Mach):

"The anemometric model of P3D is the inheritor of the one of FSX, in turn the one of FS9. This model involves a serious defect as regards the CAS (IAS) versus Mach computation.
Some add-on publishers have fixed the issue into their gauges (FSLabs for A320-X, PMDG for the B777 and B737) by means of internal derivation of the IAS from the Mach.

........ after a long debate ........

I wish LM had the ability to fix this defect and I remain aware of the difficult update of the simulation core that affects the air speed computation nonetheless."
 

Now, Andrew Wilson (I guess he is the writer of the code) has featured a Calibrated Air Speed (CAS) derivation to be true to life (nearly).

Most probably he picks the Airspeed Indicated simulation variable (that you see when yous press Shift+Z ) then he applies its technique.

For the record  M2.00 at FL500 is equivalent to 532 kt of CAS, not to 520 kt of CAS.

This is why for my own personal use I wrote a true air data computer to stand in for those of FSX + P3D.

Cheers

Fabrice

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Andrew Wilson

Hi Fabrice,

 

With kind assistance from Hervé Sors, the new Concorde features correct CAS for the whole flight envelope of the aircraft.

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Ray Proudfoot

Hi Fabrice,

Whilst P3D looks great compared to FSX and its predecessors the same inherent faults are still there. It's good to see the IAS/CAS bug is being addressed by Andrew.

Stars appearing with the sun still above the horizon still grates. I suppose they'll fix it one day.

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Fabrice Estienne

Hi Andrew,

I mailed Lefteris  Kalamars on Friday 27 may 2016 (you were reading me in copy):

"the unique solution consists of the derivation of CAS from the Mach into the gauges.

I don't know if you are well up on the Hervé SORS's methods. He has deeply studied the CAS issues as regards the FS and P3D and he introduces an Advanced Aeronautical Calculator (Adacalc) as freeware with along a library Adacalc.dll that contains masses on speed function of witch the CAS_From_Mach function.

I attest that Adacalc is a high-precision utility and I guess the library Adacalc.dll is alike..."

Now, it's well-chosen you found an agreement.

From my part I chose to go through a patch into the FSX / P3D core that puts out that's what I call IAS (I can display the real IAS when pressing Shift+Z) .

Hence I can fly any add-on with the real CAS and I do'nt need to modify any specific code of gauge.

Andrew, I hope you were planning to rewrite the code of the Concorde ADC to provide a genuine Max Climb law + Max Cruise law with along a VMO to be true the life which varies according the gross weight !

 

Hi Ray,

I am not very hopeful about LM. They fix up the peripheral code as regards the video effects and performance, but apparently today they have not the ability to modify the core programing logic. That's quite understandable because some modules inherited from Microsoft ESP platform are complete tips.

The pilot uses IAS because he is mortal but the aeronautical engineer (+ pilot) very often says CAS because most probably he doesn't fly enough...

The Indicated Speed (IAS) is the airspeed that is shown on the airspeed indicator.

The Calibrated Airspeed (CAS) is the airspeed corrected for the position (incidence or slip of the aircraft) or installation error.

The Equivalent Speed (EAS) shall not be used since this is a legacy definition in the old days (< 1950) when the anemometers did not comply with the Saint Venant Formula.

Nevertheless EAS is sometimes used by the student pilost because they use obsolete flight computers + teaching methods that lead to lack of understanding of the different air speeds.

Today all anemometers, ADC and ADR use the Saint Venant Formula and the Rayleigh Formula as regards the supersonic flights.

The Mach is the sound airspeed according the static temperature of the atmosphere.

Mach 1 at - 56.5 °C = 573.5 kt

Mach 2 at - 56.5 °C  = 1147 kt

Hope this will help you.

Regards

Fabrice

 

 

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Ray Proudfoot

Hi Fabrice,

Your knowledge of aeronautics far exceeds mine. Even though you've explained CAS I'm afraid it goes right over my head. I always thought the primary speed gauge in FSL Concorde measured IAS since that is a variable in FSX/P3D. CAS would appear to be an external variable only used by specialist aircraft such as Concorde.

If the speed shown on Concorde's IAS gauge matches that shown either in the FSUIPC offset or Shift+Z that will keep me happy. :)

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Andrew Wilson

Andrew, I hope you were planning to rewrite the code of the Concorde ADC to provide a genuine Max Climb law + Max Cruise law with along a VMO to be true the life which varies according the gross weight !


Already done :)
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Fabrice Estienne

Hi Ray,

The primary speed variable is always the Mach number supplied by the Pitot probe (Mach 1 at see level = 661 kt of CAS).

From the Mach the ADC computes the IAS - named CAS  when no incidence neither slip correction or neither installation correction are applied - which is the speed related to the see level that shall not cross the VMO (Velocity Max Operating) a structural limitation of the aircraft.

Then the ADC derives the static air temperature from a temperature probe that measures the total air temperature (static temp increased by the Mach number).

Finally The ADC derives the TAS from the Mach number according the static air temperature.

It's as easy as ABC except the formulas originated from the thermodynamic.

Thanks to the job accomplished by Andrew, we will flight a great Concorde simulation to be true to life.

Nevertheless I would like to point out to you that the FSLabs Concorde owns its internal derivation of IAS (CAS) which varies from the IAS computed by FSX / P3D (displayed when pressing Shift+Z) since the computation is made into the proprietary gauges and therefore doesn't impact the FSX /P3D simulation variables.

But it's of no importance, the main think is that we are able to flight this iconic aircraft with her unusual flight laws and I admit that's just where the strength and difficulty of the job lies.

Regards

Fabrice

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Ray Proudfoot

Thanks Fabrice. That has helped clear the fog. So the ADC is fundamental to calculating the various speeds and everything comes from the Mach speed.

It sounds like even when the new version is released the IAS (CAS) displayed on the speed gauge will still vary slightly from the Shift+Z value. In that case I’ll not bring up Shift+Z.

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Fraser Gale
2 hours ago, Fabrice Estienne said:

Hi Ray,

The primary speed variable is always the Mach number supplied by the Pitot probe (Mach 1 at see level = 661 kt of CAS).

From the Mach the ADC computes the IAS - named CAS  when no incidence neither slip correction or neither installation correction are applied - which is the speed related to the see level that shall not cross the VMO (Velocity Max Operating) a structural limitation of the aircraft.

 

Is this specifically related to the Concorde ADC or to ADCs in general? 

It's just I have a schematic here for the Concorde ADC that shows all the inputs and outputs and according to it, the static pressure and total pressure go into a differential pressure transducer, the output of which goes direct to an airspeed computer that through a servo amplifier gives Vc (CAS).  The Mach computer is completely separate within the ADC, so CAS is not calculated from Mach but directly from the pitot/static inputs (with alpha corrections amongst others). 

Frazz

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Fraser Gale
5 hours ago, Attilio Cardone said:

Wow. Where I can find this scheme for ADC?

In the engineering training manuals although they are difficult to come by these days. Keep an eye on eBay!

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Fabrice Estienne

Hi Ray + Frazer + Attilio,

The Captain's and First Officer's ASI in their normal mode of operation are supplied with their respective ADC, hence they display the CAS speed worked out by these ADC (Crouset type 52).

In their standby mode of operation these ASI are supplied with Pitot and static pressures from the nose probe and an independent electrical supply as automatic control. In others words they operate as a pneumatic anemometer calibrated according the Saint Venant or Rayleigh formulas.

"Everything comes from the Mach speed" as Ray so rightly said.

Frazer, I guess you have a set-up diagram of the ADC as teaching material (which one ?). To be accurate, the CAS is derived from a conventional Mach (Mc), the one the aircraft had at see level, and the TAS from the Mach of flight (‘Mach de vol’) commonly M. The magical equations are

CAS = a0 x Mc

TAS = a x M

a0 = 661 kt the sound speed at 15 °C (see level ISA)

a = the sound speed at the flight level = 38.96695 kt  x  square root (Ts °K)

The ADC doesn’t compute the Saint Venant formula or the Rayleigh formula every time it has to deliver the CAS, MACH or TAS, but this device has in memory all the necessary values of pressure ratio for the aircraft speed envelop. These values are computed by the manufacturer according the above-mentioned formulas.

Every aircraft has its own VMO section according the flight envelop and gross weight, therefore the ADC contains a dedicated logic to deliver the VMO at every configuration of the flight.

I don’t produce the formulas here since it’s an intricate matter unless you want to demonstrate them yourself because that is necessary for a proper understanding of the anemometric techniques.

Cheers,

Fabrice

 

ADC_de_Concorde.thumb.PNG.9f528b6fc4a3a56aecb05de726746d52.PNG

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Fraser Gale

I have several diagrams including one the same as yours (except in English rather than French) but one of them shows the actual process schematic of inside the ADC, whereas the diagram you show here is the inputs and outputs.  The diagram I have shows the logic (albeit electro-mechanical) to be a computer deriving indicated airspeed from Pt and Ps with alpha correction etc to make the output calibrated airspeed - there is no mention of using Mach number to calculate CAS or TAS.  The Mach number calculation is separate.

Frazz

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Fabrice Estienne

Hi Fraser,

Could you make public these diagrams ?

The CAS is always derived from the conventional Mach which is in turn is computed from the pressure ratio according the see level pressure (1013.25 hPa).

This is not specifically related to the Concorde ADC.

This is because the Mach is drawn from the Saint Venant or the Rayleigh formulas.

All anemometers systems as pneumatic ASI from 1950, even more so ADC and ADR are calibrated from the Saint Venant or Rayleigh formulas.

CAS is an aeronautical factor while the Mach is a major component of the aerodynamics science.

At the risk of repeating myself we obtain two Mach:  the Conventional Mach (Mc) and the Mach of flight (M).

Mc --> CAS

M with along the OAT --> TAS

This is trivial.

With subsidiarity these documents (you have well in hand) as teaching materials either servicing are compiled by people who are not often specialist in the subject.

Hope this point will help you.

Good night.

Fabrice

 

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Fraser Gale
2 hours ago, Fabrice Estienne said:

Hi Fraser,

Could you make public these diagrams ?

The CAS is always derived from the conventional Mach which is in turn is computed from the pressure ratio according the see level pressure (1013.25 hPa).

This is not specifically related to the Concorde ADC.

 

 

Is it possible that back in the early 1970s when Concorde's ADCs were finalised that they worked in a slightly different way compared to other aircraft? 

I am not disputing that what you say isn't correct for aircraft in general and it might be the case for Concorde as you seem to be very definite about it, it's just your theory doesn't match the information I have.  Do you have diagrams or information on the internal workings of Concorde's ADC specifically?  

I also know what you mean regarding simplified diagrams for training purposes but it would have made no difference to join the Mach computer box to the Pt and Ps inputs followed by the airspeed computer box as opposed to the way I have already stated it is.

I can't make the document public at the moment because my scanner isn't working and I only have the engineering manuals in hard copy. I'm also a little hesitant to upload documents as I've had experience of people passing them on without asking. 

When I have time, I will also look at the maintenance manual and the wiring diagrams to see if they can show any more detail - I'm not sure when this will be as I'm a little busy at the moment. 

Frazz

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