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Magnetic compass incorrect declination


Nealius

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Batumi, 2016, magnetic declination should be -6°. 

 

Control Test

A-10C. Infobar heading (true) 306° compass and HUD reading 300° as expected.

 

Fw190 Test (1200rpm to ensure suction and electric power)

Infobar heading 305°, expected compass reading 299°. Actual compass reading 280° which is 19° more than correct declination.

Compass-Fw190A8.png

Compass-Fw190A8.trk


Edited by Nealius
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17 hours ago, Nealius said:

Batumi, 2016, magnetic declination should be -6°. 

 

Control Test

A-10C. Infobar heading (true) 306° compass and HUD reading 300° as expected.

 

Fw190 Test (1200rpm to ensure suction and electric power)

Infobar heading 305°, expected compass reading 299°. Actual compass reading 280° which is 19° more than correct declination.

Compass-Fw190A8.png

Compass-Fw190A8.trk 68.36 kB · 3 downloads

 

This could be from a phenomenon called "magnetic deviation" or a variance caused by local electrical magnetic fields from the aircraft itself (generators/alternators, electrical equipment, ect). In modern aircraft we have a compass deviation card which is filled in by mechanics when they test for magnetic deviation errors caused by the specific equipment installed in the aircraft. No idea if ED has modeled anything like this though.


Edited by Hawkeye91
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On 9/25/2021 at 4:28 AM, Nealius said:

Actual compass reading 280° which is 19° more than correct declination.

 

In DCS the aircraft needs to be level for the compass to read accurately (not sure what is being modelled).

 

Tested using the Dora but the Anton is likely the same i.e. screen shots show the same "wrong" compass headings at Senaki in Chuck's Guide.

 

See this post for additional detail

 

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2 hours ago, Veteran66 said:

Historical Magnetic Declination:

 

I don't recall DCS's exact magnetic declination model but it uses declination tables, using the A-10C's MFD position page allows the in game MagVar for a particular place/time to be read to 1 decimal place.

 

Although @Nealius gets the sign of the declination wrong

 

• Magnetic = True - MagVar i.e. 299°M = 305°T - ( +6°E)

 

his post is accurate in what is observed in the DCS Anton.

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On 9/25/2021 at 10:11 PM, Hawkeye91 said:

This could be from a phenomenon called "magnetic deviation" or a variance caused by local electrical magnetic fields from the aircraft itself (generators/alternators, electrical equipment, ect).

 

The Anton in common with most Axis aircraft uses a mother / daughter compass system, with the mother/master compass positioned in the fuselage away from sources of electrical/magnetic interference.

 

The Mutterkompass can be a magnetic or gyro compass with electrical signals used to drive one or several remote daughter readouts.

 

This is the best source I've found so far for it's operation

 

https://sites.ph9.com/RemcoCaspers617/upload/editor/files/Patin remote compass.pdf

 

I'm not sure it's modelled correctly in DCS as, AFAIK the Mutterkompass is gimbaled and will remain in the horizontal plane up to a max. 25° but in DCS shows a large error while stationary on the runway (how would a pilot check the compass heading before takeoff ?).

 

Focke Wulf Fw-190 A-8, Mutterkompass (Mother_Master Compass).png


Edited by Ramsay
Add pictures of the Mutterkompass

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  • 4 weeks later...
On 9/27/2021 at 7:03 PM, Ramsay said:

I'm not sure it's modelled correctly in DCS as, AFAIK the Mutterkompass is gimbaled and will remain in the horizontal plane up to a max. 25° but in DCS shows a large error while stationary on the runway (how would a pilot check the compass heading before takeoff ?)

Does this 25° limit apply to bank as well? I find the daughter compass freezes during a level turn with 10° of bank, making it impossible to roll out on a specified heading. 

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6 hours ago, Nealius said:

Does this 25° limit apply to bank as well? I find the daughter compass freezes during a level turn with 10° of bank, making it impossible to roll out on a specified heading.

In a turn or under acceleration, all bets are off as AFAIK the Master compass is still susceptible to Magnetic Dip but better "damped" i.e.

When making a turn the compass may lead or lag the turn 

• For middle Latitudes (not near the equator or poles), the rule of thumb for the under/over shoot is 15° + 1/2 Latitude i.e. for Batumi (~40°N), the understeer might be approx 35°, so you might turn to a heading of 155° before stopping your turn and waiting for the compass heading to "catch up" and read 120°M for the RWY13 approach.

Not sure how much (if any) of this is modelled in DCS.

 


Edited by Ramsay

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7 hours ago, Ramsay said:

When making a turn the compass may lead or lag the turn 

The problem here is that it neither lags nor leads. It stops completely, meaning you can't apply that under/overshoot rule. 

Let's say that a 35° overshoot is valid here. I make a left turn from 090 to 270. I should anticpate to roll out when the compass reads 305, and wait for it to catch up to show 270, correct?

So I start my turn. The compass may move to 070, then it stops. As I continue my turn it's still pegged at 070. I come around and know for a fact I'm heading west because of the setting sun, but my compass is still indicating 070, or some other random value it might have shifted to and pegged on like 350 at some point in the turn. The only indicator I have that I've arrived on a near 270 heading is the setting sun, so I roll out, see where the compass goes, then endlessly waffle left or right to get the heading I actually wanted.  

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I've been looking at where the magnetic compass figures come from in the sim for a while now, and think I'm getting close now, and this morning I've managed to recreate the compass readings from the warbirds on the ground using my own calculations (well, only tested it for the FW190-D9 and A8 so far, but for those two at least; still need to test it out on the others).

Going back to the opening post, I'm now pretty sure the reason the compass (or the repeater) gives confusing readings is because of the magnetic inclination, ie. the dip angle of the magnetic field towards the ground.  This is a significant angle, more than 60 degrees for the Caucasus, and is what affects the compass on the ground since the nose-up pitch of the aircraft means that (unless pointing directly magnetic north or south) there's a component of the field pulling the needle further east or west.

As Ramsay says, if the compass is gimballed so that it stays horizontal this shouldn't happen for small angles, but from my experiments it seems like this isn't modelled - I get a good agreement with the actual readings if I assume the compass is fixed in the plane of the aircraft.

I've only been testing the figures on the ground - it's too difficult to take exact readings while flying unless I can find a way of getting the actual compass reading logged - so there may be further lag effects due to movement as well, but I haven't noticed any lag in the changes qualitatively while flying so I suspect not.

Here's a quick description of my method:

- I get the magnetic declination (variation) and inclination figures from one of the files under Data/MagVar/TabularData in the DCS Saved Games folder.  These files are created at mission start if one does not already exist for the month and year of the mission, cover the whole globe and contain declination and incliniation at each whole-number latitude and longitude.  I've written a small Lua library which calculates the figures for a particular decimal lat and long by linear interpolation and read them in scripts, which I can release if people are interested.

The declination can also be got from the functions in Export.lua - there's a function LoGetMagneticYaw which returns the heading adjusted by the declination - but I haven't found any other way of getting at the inclination figures.

- From the declination and inclination, I make up a unit vector in the field direction in DCS world units (ie. x = map north, y = up, z = map east).

- I then transform this vector by three rotations:

  First by the heading.  After this rotation, the angle of the x-z component of the vector is the 'MagneticYaw' figure mentioned above.

  Next it is rotated by the pitch angle, then finally by the bank angle.

  This leaves me with a unit vector for the magnetic field relative to the aircraft coordinates (x = fore, y = up relative to aircraft, z = starboard).

- An arctan of the resulting x and -z coordinates should give the compass reading (ignoring the deviation due to the aircraft, which is modelled, but I won't go into in this post to avoid it getting even longer).

To illustrate this, I've attached a couple of spreadsheets which you can look at if you like.  One is a test I did using an FW-190D9 at Mozdok, where I made notes of the compass readings when parked in the middle of the runway at all angles (in 15 degree increments).  I had a telemetry script logging a few values, and using these values along with my method outlined above get a result very close to the readings I made in the cockpit - within +/-1.5 degrees, which is about as accurately as I can read the compass.

The second spreadsheet is one I've just done doing the same thing but with just one entry, for the same details as Nealius's opening post, ie. Batumi, 2016 (June), takeoff from parking hot, parking bay 10, which gives an infobar heading of 305, so expected magnetic if the aircraft was level would be 299.5, but after adjusting for pitch and bank the calculated reading is 279.1 - pretty close to the 280 degrees shown.

EDIT: I just tried attaching the spreadsheets but it didn't seem to work - the first just said 'uploading' and the second 'queued', and no change after a while even though only small files.  I'll try again in fresh post shortly (bit of a forum newbie I'm afraid).

 


Edited by JBDCS
Fixed some typos
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