[INVESTIGATING]Airspeed indicator shows different speed to HUD speed readout

[Sharpe_95]

[OP edited on account of replaying the track and noticing that the issue does not appear to be linked just to extreme altitude - could a MOD please update the title to remove 'at extreme altitude'?]

 

I cant imagine this is very important (unless it speaks to a slight/imperceptible error at lower altitudes).

 

I noticed today when flying the F16 at extreme altitudes there was a noticeable mismatch between the speed readout in the HUD and the airspeed/mach indicator.

 

I have a track (but its necessarily long so I have not uploaded it), but I also took a screen shot (attached) to show the mismatch. I set the DED to show the INS page so that the G/S could be seen for reference. My AoA is not completely stupid (about 6-deg, so only about 2-3-deg off normal). I would add that clearly this is not a mismatch between the CAS and TAS.

 

Examples from the track (taken while sim paused to be sure of readouts, G/S taken from the INS page of the DED, some of the airspeed indicator readouts are approximate as it can be hard to get an exact figure):

12:15:24. A16 GS=492. HUD=388. Airspeed indicator=400. F2=391. (Afterburner applied c.15-degree climb).

12:15:46. A20 GS=501. HUD=361. Airspeed indicator=373. F2=363. (No afterburner, c.2-degree climb).

12.17.04. A25. GS=501. HUD=332. Airspeed indicator=347. F2=333. (No afterburner, c.2-degree climb).

12.18.56. A30. GS=605. HUD=367. Airspeed indicator=390. F2=367. (No afterburner, c.1-degree climb).

12.21.15. A45. GS=695. HUD=297. Airspeed indicator=339. F2=296. (Afterburner, c.1-degree climb).

12.26.25. A55. GS=787. HUD=256. Airspeed indicator=300. F2=254. (Afterburner, c.0-degree climb).

 

-S

Edited May 10, 2020 by Sharpe_95

[mvsgas]

I thought Calibrated Airspeed (CAS on the HUD) is a different thing from Indicated Airspeed (IAS on the Airspeed/Mach indicator)

 

The HUD will show CAS, True Airspeed (TAS) and Ground Airspeed (GND SPD) which are all different than IAS.

 

Also in the USAF F-16C circa 2007 (AFAIK)

When the landing gear handle is down, CAS will be displayed on the HUD regardless of airspeed or the position of the velocity switch. In DGFT, CAS is auto selected.

[Sharpe_95]

I thought Calibrated Airspeed (CAS on the HUD) is a different thing from Indicated Airspeed (IAS on the Airspeed/Mach indicator)

 

The HUD will show CAS, True Airspeed (TAS) and Ground Airspeed (GND SPD) which are all different than IAS.

 

Also in the USAF F-16C circa 2007 (AFAIK)

 

My fault, you are quite right the HUD shows CAS i have modified my OP to avoid confusion.

 

However, the error is still the same, AFAIK the airspeed/mach indicator is showing CAS as well? If so then there is still an error given the mismatch in image in the OP.

 

-S

[Frederf]

The round dial is physically driven by the pitot-static system and not a digital value supplied by CADC like HUD.

 

However it's surprising that the IAS is that much higher than CAS at such mild conditions like 55 kft M1.4.

 

What was the "F2" airspeed under these conditions?

[Sharpe_95]

The round dial is physically driven by the pitot-static system and not a digital value supplied by CADC like HUD.

 

However it's surprising that the IAS is that much higher than CAS at such mild conditions like 55 kft M1.4.

 

What was the "F2" airspeed under these conditions?

 

Based on Frederf's suggestion I have subsequently reviewed the track, updated the OP and sent BIGNEWY a track file.

 

-S

Edited May 10, 2020 by Sharpe_95

[BIGNEWY]

Sadly the replay did not play correctly for me, taxi did not go well.

 

If possible please show the issue with the shortest track possible, if an air-start shows the issue please do that.

 

thanks

[Sharpe_95]

Sadly the replay did not play correctly for me, taxi did not go well.

 

If possible please show the issue with the shortest track possible, if an air-start shows the issue please do that.

 

thanks

 

^will do Newy will make a new track presently.

 

-S

[Sharpe_95]

Sadly the replay did not play correctly for me, taxi did not go well.

 

If possible please show the issue with the shortest track possible, if an air-start shows the issue please do that.

 

thanks

 

PSA 3 files. The first two are perhaps a little longer than you might be hoping for - but they specifically cover the whole issue in a range of regimes in case they are useful to you/the dev team. The third is exactly what you requested.

 

Tape#1 is from runway at burner all the way up. From this tape it would appear the mismatch is to do with altitude. Note that AoA is tested (including in a dive) and the mismatch still occurs.

 

Tape#2 is from an air start at A15 typically only using full mill power and allowing speed to increase/decrease at a range of altitudes (A15, A25, A36). From this track it seems the issue is not linked to AoA, might be linked to altitude and/or may be linked to faster speeds (mismatch is less at lower altitudes & speeds, worse at higher altitudes & speeds).

 

Tape#3 is an air start at A34 using full gate to pick up speed to highlight the mismatch, then using alt hold with thrust all the way back and boards out to show how the mismatch becomes closer as the speed decreases.

 

I hope this is more helpful Newy?

 

-S

PS: For some reason the air start seems to have some alignment issue/red tape on the HSI - I can confirm this issue happens even with a fully aligned aircraft with no red tape on the HSI.

Gnd Spd Error#1.trk

Gnd Spd Error#2.trk

Gnd Spd Error#3.trk

Edited May 11, 2020 by Sharpe_95

[Dee-Jay]

[OP edited on account of replaying the track and noticing that the issue does not appear to be linked just to extreme altitude - could a MOD please update the title to remove 'at extreme altitude'?]

 

I cant imagine this is very important (unless it speaks to a slight/imperceptible error at lower altitudes).

 

I noticed today when flying the F16 at extreme altitudes there was a noticeable mismatch between the speed readout in the HUD and the airspeed/mach indicator.

 

I have a track (but its necessarily long so I have not uploaded it), but I also took a screen shot (attached) to show the mismatch. I set the DED to show the INS page so that the G/S could be seen for reference. My AoA is not completely stupid (about 6-deg, so only about 2-3-deg off normal). I would add that clearly this is not a mismatch between the CAS and TAS.

 

Examples from the track (taken while sim paused to be sure of readouts, G/S taken from the INS page of the DED, some of the airspeed indicator readouts are approximate as it can be hard to get an exact figure):

12:15:24. A16 GS=492. HUD=388. Airspeed indicator=400. F2=391. (Afterburner applied c.15-degree climb).

12:15:46. A20 GS=501. HUD=361. Airspeed indicator=373. F2=363. (No afterburner, c.2-degree climb).

12.17.04. A25. GS=501. HUD=332. Airspeed indicator=347. F2=333. (No afterburner, c.2-degree climb).

12.18.56. A30. GS=605. HUD=367. Airspeed indicator=390. F2=367. (No afterburner, c.1-degree climb).

12.21.15. A45. GS=695. HUD=297. Airspeed indicator=339. F2=296. (Afterburner, c.1-degree climb).

12.26.25. A55. GS=787. HUD=256. Airspeed indicator=300. F2=254. (Afterburner, c.0-degree climb).

 

-S

 

 

Are the pressure / temperature (Delta ISA) taken in account? http://www.hochwarth.com/misc/AviationCalculator.html

 

[Sharpe_95]

Are the pressure / temperature (Delta ISA) taken in account? http://www.hochwarth.com/misc/AviationCalculator.html

 

 

Dee-jay,

I would not know for sure, it is entirely possible hence why I have put it to the Dev team. But the range of error coupled with the range of attitudes in which it is present seems to indicate something more ingrained in the code?

 

I am frequently wrong however and know close to the square route of SFA about this kind of stuff - I am a real physics moron (in fact you could just delete the physics bit and the sentence would still be accurate).

 

I just assumed that the airspeed indicator is what fed the HUD when the HUD is set to readout CAS (if not where does it draw its data from)? As I see it we have 4 sources of speed information: Sim F2 mode, INS ground speed, Airspeed Indicator and HUD airspeed (which can be measured in CAS/TAS/G/S). In my ignorance I just assumed that the HUD speed when set to CAS is the same as the airspeed indicator readout (which I also assumed was CAS)?

 

-S

Edited May 11, 2020 by Sharpe_95

[Dee-Jay]

Hi!

 

But the range of error coupled with the range of attitudes in which it is present seems to indicate something more ingrained in the code?

 

Agree.

 

 

I just assumed that the airspeed indicator is what fed the HUD when the HUD is set to readout CAS ... In my ignorance I just assumed that the HUD speed when set to CAS is the same as the airspeed indicator readout (which I also assumed was CAS)?

 

Nope. HUD is CAS, airspeed indicator is IAS.

I didn't found the conversion tables in real manufacturer documentations.

 

Regards.

[Frederf]

-1-1, at 50,000' the maximum difference between IAS and CAS is ~5 knots. This deserves some careful comparison under ISA conditions at the published altitudes looking as TAS, IAS, CAS, IMN, and Mach

[BIGNEWY]

I will ask the dev team, if I find anything out I will let you know.

 

thanks

[Sharpe_95]

I will ask the dev team, if I find anything out I will let you know.

 

thanks

 

Thanks Newy :thumbup: