Warbird compasses wrong by +10 or more degrees

[Nealius]

I noticed something weird with the compass repeaters in some of the warbirds and checked all of them (except Dora and I-16) out. I found that they have a +10 degree discrepancy from both the info bar heading and the heading indicated by the gyrocompass or any other cockpit compass. 

 

Method: 

1. Hot start aircraft on ramp at High Halden (magnetic declination +0.69)

2. Run engines up to 1,100 or 1,200rpm for vacuum pressure and electric power to give proper compass readings

3. Cross-check compasses with info bar (I did not check the big Brit compass in the Mossie or Spit)

 

Results:

Aircraft	Infobar heading	Directional Gyro	Compass (Magnetic)
Bf 109 K4	011	NA	022
Fw 190 A8	012	NA	025
P-47D-40	013	014	013
P-51D-25	012	014	024
Spitfire	012	013/014	NA
Mosquito	012	013/014	022

 

The P-47 is the only one with correct compass readings. The directional gyros in the aircraft that have them are okay, but the magnetic compasses/compass repeaters (manuals are inconsistent in naming) are off by 10 or more degrees. In the Kurfurst and Anton particularly you will have to subtract 10~13 degrees to find your actual heading becuase you don't have a directional gyro. Even if the infobar heading is True and not Magnetic, the decllination of Kent is less than one degree, which appears to be corroborated on the directional gyros. 

Compass.trk

Edited September 24, 2021 by Nealius

[DD_Fenrir]

Historical Declination Viewer (noaa.gov)

 

Shows in Europe in 1944 that declination was 10° West.

Edited September 24, 2021 by DD_Fenrir

[Nealius]

2 hours ago, DD_Fenrir said:

Historical Declination Viewer (noaa.gov)

 

Shows in Europe in 1944 that declination was 10° West.

 

 

Mission date is 2011. Assuming the 1944 declination is hard coded into the Channel map, why doesn't the P-47 show 10 degrees of variance, and why are the gyro compasses not affected? 

[Quadg]

You normally set a gyro compass to true north and not magnetic north so you don't need to add/subtract magnetic declination when working with the map and compass.

This is why a gyro compass is preferred to a magnetic one. That and the fact a gyro compass is more stable to changes. There is a slight delay in a magnetic compass stabilising to a new direction. The down side is a gyro compass can wander off accuracy over time, with lots of changes to direction. It will lose true north.

Why you normally have to set it every time you start the aircraft.

The p-47 magnetic compass looks like the one with the error.

[Nealius]

39 minutes ago, Quadg said:

The p-47 magnetic compass looks like the one with the error.

 

Partially. If I set my mission date to 2011 it should not be using 1944 magnetic declination. That might be a Channel issue. 

Edited September 24, 2021 by Nealius

[Art-J]

What happens on Caucasus map? That one should be non-bugged enough in this aspect and, off top of my head, it should have about 6 deg declination on southeastern airbases, right?

[Quadg]

I doubt they have the declination changes stored for the last 80 years.

unlike moon phases the changes in declination are random. you cannot use a simple equation to work it out. for changes in date.

 

[Nealius]

34 minutes ago, Art-J said:

What happens on Caucasus map? That one should be non-bugged enough in this aspect and, off top of my head, it should have about 6 deg declination on southeastern airbases, right?

 

Same test procedure with the P-51 at Batumi, infobar heading 215.

 

2011: Magnetic compass indicates 202 or 203 (-12 or -13 degrees); NOAA indicates declination of -6 degrees

1944: Magnetic compass indicates 205 (-10 degrees); NOAA indicates declination of of between -4 and -5 degrees.

 

It appears on Caucasus that magnetic declination changes with year, but the P-51's Magnetic compass is showing nearly twice the declination value.

Edited September 24, 2021 by Nealius

[Buzzer1977]

52 minutes ago, Quadg said:

I doubt they have the declination changes stored for the last 80 years.

unlike moon phases the changes in declination are random. you cannot use a simple equation to work it out. for changes in date.

 

Actually you can ... The World Magnetic Model algorithm is updated every 5 years and provides a annual update of the model data for current declination predictions.
https://www.ngdc.noaa.gov/geomag/WMM/

Historical magnetic data can be computed with the International Geomagnetic Reference Field IRGF-13. That's not exact as the WMM but sufficient for a simulator like a DCS
 https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

[Quadg]

13 minutes ago, Buzzer1977 said:

Actually you can ... The World Magnetic Model algorithm is updated every 5 years and provides a annual update of the model data for current declination predictions.
https://www.ngdc.noaa.gov/geomag/WMM/

Historical magnetic data can be computed with the International Geomagnetic Reference Field IRGF-13. That's not exact as the WMM but sufficient for a simulator like a DCS
 https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

 

 I did say "simple equation" rather than a mathematical model

plus the channel does not have a modern version so the date isn't just the only thing you need to change to make it modern.

 The magnetic declination is just as wrong as everything else.. 

 

[Buzzer1977]

3 hours ago, Quadg said:

 

 I did say "simple equation" rather than a mathematical model

plus the channel does not have a modern version so the date isn't just the only thing you need to change to make it modern.

 The magnetic declination is just as wrong as everything else.. 

 

Well, if you take a look into the IRGF reference implementation source code it's not that complicated. https://www.ngdc.noaa.gov/IAGA/vmod/geomag70_linux.tar.gz

[Frederf]

10 hours ago, Quadg said:

I doubt they have the declination changes stored for the last 80 years.

unlike moon phases the changes in declination are random. you cannot use a simple equation to work it out. for changes in date.

 

They do have a fit function for variation in DCS. I dunno how sophisticated it is. Probably helps to do it down to 5 or even 1 year data points.

 

A-10C2 magvar 12:00 Jan 1 by 25 year chunk, Anapa:

1900 1.4

1925 2.4

1950 3.4

1975 4.6

2000 5.4

2025 7.8

2050 10.0

2075 12.3

2100 14.5

 

 

[=475FG= Dawger]

12 hours ago, Quadg said:

You normally set a gyro compass to true north and not magnetic north so you don't need to add/subtract magnetic declination when working with the map and compass.

This is why a gyro compass is preferred to a magnetic one. That and the fact a gyro compass is more stable to changes. There is a slight delay in a magnetic compass stabilising to a new direction. The down side is a gyro compass can wander off accuracy over time, with lots of changes to direction. It will lose true north.

Why you normally have to set it every time you start the aircraft.

The p-47 magnetic compass looks like the one with the error.

Did you just make that up?

 

That isn't how it is done.

[Nealius]

Which is all besides the point of:

 

Why are the warbirds showing 2x declination at Caucasus?

 

Interestingly at Batumi, the Jug's magnetic compass shows proper declination, and the gyrocompass is +16 degrees off. 

Edited September 25, 2021 by Nealius

[kablamoman]

42 minutes ago, Nealius said:

Interestingly at Batumi, the Jug's magnetic compass shows proper declination, and the gyrocompass is +16 degrees off. 

 

It's just a DG, not an actual compass. You have to manually match it to the magnetic compass.

[Nealius]

3 minutes ago, kablamoman said:

 

It's just a DG, not an actual compass. You have to manually match it to the magnetic compass.

 

That has never been necessary on the Channel map. They both indicate same heading on mission start. 

 

I have tried matching the gyro on the P-51 to its magnetic compass, but it never holds. 

Edited September 25, 2021 by Nealius

[kablamoman]

Both are directional gyros -- just dumb, they need to be matched to the magnetic compass, and they will fall out of sync with time or sporty maneuvering.

 

This is how they work and it's completely normal. I think it's just a coincidence if it happens to match the compass on mission start (I think it may depend on orientation of the aircraft on mission start, or maybe something in the mission settings).

Edited September 25, 2021 by kablamoman

[Nealius]

That does not match my experience in the 50+ hours I've put on the Jug in the Channel map. Every single time at multiple airfields they are both synced,and remain synced for the duration of the mission. On the other hand, in the Mustang (probably another 20+ hours), if I manually set the gyro to match the compass, it will be out of sync as soon as I taxi to the runway. 

[kablamoman]

Don't know what else to tell you, bud. Pretty sure that's what they are and how they work.

 

They were probably powered by vacuum suction, and so their ability to stay synced may depend on the vacuum pump which would be in turn powered by the engine -- so taxi may give enough suction to keep it set on one plane but not the other with a low power setting.

 

One could also be electrically powered (don't think this is likely).

 

Probably could also depend on how they've simulated the DG or the suction pressure in the game. One could just be coded differently than the other. Who knows.

Edited September 25, 2021 by kablamoman

[Nealius]

I think the P-51's magnetic compass is electric. The same compass is in the Mossie and that requires battery+inverter+compass switch. The magnetic compass in the Jug is suction, but the gyro I'm not sure. 

[razo+r]

A magnetic compass is usually only "driven" by magnets. 

 

Directional gyros usually have a vacuum pump and gyros. 

 

I am starting to think you confuse the usually adjustable DGs with the compass. 

Edited September 25, 2021 by razo+r

[Nealius]

4 minutes ago, razo+r said:

I am starting to think you confuse the usually adjustable DGs with the compass. 

 

In the Jug that's certainly possible, as the magnetic compass and DG are the same rotary style. In the Mustang and Mosquito I am not confusing them as the magnetic compass is the dial type with a moving needle. The needle is caged in the 12 o'clock position until electric power is supplied in the case of the Mustang and Mosquito. 

Edited September 25, 2021 by Nealius

[kablamoman]

That's a repeater compass (it's receiving a signal from a master compass that you don't really see), same idea as the ones in the axis planes. Yeah, the gauge itself (needle) is probably powered electrically.

 

The compass you see in the P47 is a plain old whiskey compass. Just a compass card floating in fluid.

 

Both aircraft also have Directional Gyros to go along with these that you have to manually set and resync with the repeater compass (p51) or the whiskey compass (p47) from time to time -- these DGs are what get powered by suction spinning the rotors inside them.

Edited September 25, 2021 by kablamoman

[Nealius]

8 minutes ago, kablamoman said:

That's a repeater compass (it's receiving a signal from a master compass that you don't really see), same idea as the ones in the axis planes.

 

Then something's not right with the master compass, then. After setting the master compass on the Mosquito it still does not show correct variation at Batumi. Is there any way to adjust the master compass on the Mustang or the Axis planes? They're showing incorrect declination as well. 

 

I also recently was told the Channel map does not yet have declination modeled at all, and yet the compasses are 10+ degrees off.......None of it is making any sense.

[kablamoman]

3 minutes ago, Nealius said:

Then something's not right with the master compass, then. After setting the master compass on the Mosquito it still does not show correct variation at Batumi. Is there any way to adjust the master compass on the Mustang or the Axis planes? They're showing incorrect declination as well. 

 

In real life maintenance crew could perform a compass swing to fine tune it and also provide a written record with corrections, but I suspect they may just need to tweak some variables for the map in question if it's not giving proper readings (or maybe there is a bug in the code somewhere).

 

As far as its practical use goes in the sim goes, as long as it's pointing in the right general direction, that works fine as you navigate primarily using landmarks primarily anyway (in conjunction with a slightly inaccurate compass).

 

If you're trying to do extended navigation with dead reckoning over cloud cover, well... Good luck!

 

Hopefully they are able to fix it.