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Gyrocompass alignment


carrollhead
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I think that the gyrocompass aligns itself to a different reference compared to other aircraft in DCS:World. For example, the Easterly runway at Kutaisi has a magnetic heading of 068, but the Mig seems to show about 072-073.

 

Is it aligning to true North rather than magnetic, or am I doing something daft?:)

 

I'm not sure if it should be showing true, but this does have an effect on precision approaches.

 

(Variation is about 5E according to RW documents, so my "True North" idea seems to hold water)

 

Sorry if this has already been reported, I couldn't see anything though!


Edited by carrollhead
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Did you adjust it?

From the manual (page 54):

To adjust it, press and hold the NPP course magnetic correction button (CM95); you will see that the compass circle is adjusting.

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Is it aligning to true North rather than magnetic....

 

Yes it's aligning to True North, typical of Russian aircraft. You're doing nothing wrong :)

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I do not think that is correct, for two reasons:

 

1) There is no way for the systems to know where true north is at. For that, you either have to have an INS which can detect true north from the rotation of the earth during initial alignment on the ground.

 

That, the -21 doesn't have.

 

Failing that, you can derive true north from magnetic north through knowing the local magnetic declination. This can either be done through a navigation system with a computer and a database (which we don't have) or through manually inputting the declination. The latter method seems to have been employed in some incarnations of the -21, where there was a 'magnetic declination setter' in the lower center panel. The documentation I can understand (i e the English manual) is somewhat sparse as to what the various systems do. The procedures suggest that it is used only for the landing system, but either way, we don't have it.

 

2) The game (module) manual mentions several times that what we are getting is magnetic heading. :)

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My assumtion based on Vipers comment was that it would simply be setup as a constant offset based on the area of operations. Since the -21 can't fly all that far it would probably be an acceptable compromise.

 

However, until I read that I was under the impression that pretty much all aviation used magnetic north as the reference (the exception being flights near the poles where true becomes the reference).

 

Pressing the calibration button should reference the gyro to a magnetic sensor - a more automated version of correcting for gyro drift in light aircraft.

 

Anyway - 5-6 degrees is enough to screw up a precision approach so it would be interesting to hear what the intended behaiviour of the instrument actually is. :)

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Magnetic is indeed the reference almost universally used, and it is a slaved gyro.

 

The range of the MiG-21 is quite a few degrees worth of magnetic declination change.

 

5° off corresponds to a mere 30 km/h crosswind component - 16 knots - at 340 km/h, so if that will throw off your precision approaches I think you have a problem. Remember that we're not talking the wind at the threshold but the wind during the approach here.

 

Cheers,

/Fred

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OK, maybe I was exaggerating just a little bit using the phrase "screw up" :), it doesn't stop me from actually landing in bad weather.

 

It does however introduce an extra error, and one I'd like to able to account for. It's not impossible over the next few years that we'll be flyng DCS planes in different global locations, where the variation is a more significant factor. (about 15E for the Vegas area when we get NTTR for example)

 

Even at the values on the current map it has an effect when using RSBN radials to define a position, or work out a heading to steer over larger distances. One of the fun things (for me anyway:book:) is to practice older navigation stuff and still try to get to a target area/airport. Although I agree you have to accept the limitations of hardware and allow for errors, not knowing exactly what the instruments are trying to show you makes that more difficult.

 

(Some googling seems to indicate that RSBN should be able to determine azimuth to an accuracy of between 0.2-1 degrees, although some of the sites I looked at were in German so I'm not sure if I'm reading it right!)

 

 

I think magnetic should be the correct reference, but I'm aware that Viper knows his stuff, so I'm not sure. If he is correct, then why do some of the FC3 planes have a deviation card mounted in their cockpits? (it's only eye candy I know - but I guess they're there because the cockpits they're modelled on probably had them)

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That is a mindblowing question in DCS, because we have a flat Earth here.

In MiG-21, when you are pushing NPP course magnetic correction button (CM95)" it adjusts according to its magnetic compass receiver signal (magnetic North). So if you take the conventional magnetic compass (taking into account it's compass deviation, remember dev. cards) and compare its indication with NPP, they will be the same. But only in REAL LIFE.:)

And if you take off from the airfield and land at another, you will see that your NPP is lying to you. That happens, because "NPP" didn't updated your mag. north, and it was working as a gyroscope. So you were flying along a kind of a Gyroscopic Magnetic Course. So that's why it's important to align such systems periodically, especially if you are flying far. But this is not about DCS.

In DCS, it is considered to be a true North in all maps (ME, F10), and most of airplanes. Others, i.e. A-10C have magnetic North as a reference direction.

In MiG-21 by aligning NPP you make your reference direction DCSTrue North :)

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Slaved gyros will continually adjust to magnetic north, as long as they are in slaved mode. No reason to suspect the MiG-21 to be any different here.

 

Also, @carrolhead, in approach plates headings in general and the final approach course will be in magnetic. In other words, using true is what would introduce an error. If you had a fleet using true as primary reference, you'd have to create a specific set of plates.

 

Cheers,

/Fred

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Did you find any problem with the needle that point to ground radio stations?

It was working fine (but inverted) before the patch.

 

Since the patch it dosent seem to work. Even when flying over the station. RSBN is selected and in the right channel, distance to the station works but the needle is stuck at the 45 angle.

 

This happened in MP, to me and the 7 members of my squadron who where flying the Mig.

I saw this bug yesterday flying in the 104th server.

We tested it with Sochi Adler and Kutaisi.

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Slaved gyros will continually adjust to magnetic north, as long as they are in slaved mode. No reason to suspect the MiG-21 to be any different here.

 

Also, @carrolhead, in approach plates headings in general and the final approach course will be in magnetic. In other words, using true is what would introduce an error. If you had a fleet using true as primary reference, you'd have to create a specific set of plates.

 

Cheers,

/Fred

Don't know exactly how it is on MiG, but some more complicated aircrafts (military mostly), and even Mi-8 have switcher MK-GPK (magnetic compass - gyrocompass). So you can choose either to fly an orthodromy, or conventionallly and always correcting your heading.

It seems to me that MiG-21 does not have slaved mode, because there is that correcting button, and MiG is not such a complicated aircraft.

 

RSBN azimuthal beacon, btw can be directed to true north. And Su-24 flying over my house use true N (I have friends there).


Edited by GUMAR

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Also, @carrolhead, in approach plates headings in general and the final approach course will be in magnetic. In other words, using true is what would introduce an error. If you had a fleet using true as primary reference, you'd have to create a specific set of plates.

 

Cheers,

/Fred

 

I know this - hence wanting to know the correct behaviour of the instrument. As I said in post #1 I think it should be magnetic. Viper knows his stuff, so I can't just write off his "Russians use True" comment as wrong. It just need clarification from the Leatherneck guys as to what the thing is actually trying to show us:smilewink:

 

Andy

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