Inverted Compass ??

This discussion makes me think of this video :D

And so to navigate in DCS, you can use the Mission Editor/F10 map to get the True heading, then apply the magnetic variation to get the desired Magnetic heading in the cockpit.

Okaaay ! here was my mistake, i understand now ! Tank you.

So i just have to make an adaptation of my jeppsens with that trick and all will be correct !

Thank you again for your explanations and the time you spent for me ! :)

This discussion makes me think of this video

So i just have to make an adaptation of my jeppsens with that trick and all will be correct !

I believe you should have a collection of DCS-specific charts in the \\Doc folder of the install. Use those.

Thank you again for your explanations and the time you spent for me !

From the devs:

 

"After further investigation we have found that it a normal indication when the aicraft is not level (which is the case for P-51 when it is staying on the runway). The reason of such behaviour is in the direction of the total magnetic vector relative to the aircraft - 5-7 degrees West and about 60 degrees down."

 

If you're not sure what that's about, as I wasn't initially:

http://magician.ucsd.edu/essentials/WebBookse11.html

 

Attaching screenshot showing correct magnetic indication when the aircraft is airborne and more level.

 

[ATTACH]79809[/ATTACH]

 

How's that for realism? :)

So what I'm understanding from this is, "Yes, the remote compass actually does change with the aircraft orientation, and the flight manual is just plain wrong."

Am I correct?

I believe you should have a collection of DCS-specific charts in the \\Doc folder of the install. Use those.

Hooooo thanks !! i've just found it ! i was only looking in the Windows7 start menu for the pdf manuals, i didn't know there was something like this in DCS folders ! thank you very much ! :thumbup:

Edit : to Hijack : sorry, i can read English (with difficulties) but earing it is just too much for me :cry: i don't really understand what they say in your video, witch is very frustrating as i'm a guitarist :detective:

So what I'm understanding from this is, "Yes, the remote compass actually does change with the aircraft orientation, and the flight manual is just plain wrong."

 

Am I correct?

No, the manual is correct, though maybe not as clear as it could be. The magnetic compass, although experiencing some fluctuations while the aircraft is maneuvered, will continue to provide accurate readings once the maneuvering stops and the aircraft is returned to a level attitude. The directional gyro on the other hand, suffers from increasing drift ("precession") errors as a result of maneuvers and will quickly begin to provide incorrect readings unless manually reset by the pilot. However the problem is that this second part about gyro drift isn't working (i.e. modeled) in DCS Mustang at the moment, so the gyro continues to provide accurate readings throughout the flight.

Hooooo thanks !! i've just found it ! i was only looking in the Windows7 start menu for the pdf manuals, i didn't know there was something like this in DCS folders ! thank you very much ! :thumbup:

 

Edit : to Hijack : sorry, i can read English (with difficulties) but earing it is just too much for me :cry: i don't really understand what they say in your video, witch is very frustrating as i'm a guitarist :detective:

Hey, Lib, here is the same scene in French (though longer - the scene is at 1:20)

No, the manual is correct, though maybe not as clear as it could be. The magnetic compass, although experiencing some fluctuations while the aircraft is maneuvered, will continue to provide accurate readings once the maneuvering stops and the aircraft is returned to a level attitude. The directional gyro on the other hand, suffers from increasing drift ("precession") errors as a result of maneuvers and will quickly begin to provide incorrect readings unless manually reset by the pilot. However the problem is that this second part about gyro drift isn't working (i.e. modeled) in DCS Mustang at the moment, so the gyro continues to provide accurate readings throughout the flight.

But the flight manual says,

This type of compass doesn’t float around and fluctuate when the aircraft is maneuvered.

But the flight manual says,

We've been down this road several times. :) I don't recall hearing an answer on it but I had the same inquiry. Doesn't match the advertised behavior of the remote compass per the flight manual.

The magnetic compass, although experiencing some fluctuations while the aircraft is maneuvered, will continue to provide accurate readings once the maneuvering stops and the aircraft is returned to a level attitude.

You can't compare it to the gyro compass, as said in #31 the gyro compass would not read perfect headings if it where modelled completely (the disadvantage of the gyro compass is not modelled).

Also notice that in the area we fly there is a magnetic variation of about 5-degrees also the aircraft itself produces a magnetic field that will influence readings. Dunno if that part is modelled in P-51D Mustang but in Black-Shark it definitly is.

I always though the purpose of a remote compass was to keep the compass away from metal masses (e.g. engine) or electrical systems of the aircraft.

Example: see p. 126 of the Ka-50 manual; there is a chart for its compass deviation, due to the aircraft's own magnetic field. I guess if fitted with a remote compass, there would be no need for such a chart.

As such, I'm not suprised the remote compass experiences some fluctuations while the aircraft is maneuvered. But I'm suprised at how the manual describes its purpose.

Cheers

Az'

OK, my sentence was too "absolute".

For instance here: http://www.theairlinepilots.com/forum/viewtopic.php?t=707

The main reason for usually mounting the detector unit of a remote indicating compass in the wingtip of an aeroplane is to reduce the amount of deviation caused by aircraft magnetism and electrical circuits

In a remote indicating compass system the amount of deviation caused by aircraft magnetism and electrical circuits may be minimised by mounting the detector unit in the wingtip

"reduce" and "minimized" are the words, here.

++

Az'

Think a simple bug is being given overly complicated and logical excuses as to why its not working,The only aircraft I trust Flying IFR are the FC3 Models and the A10c,Everything else I have to fly VFR,

Using just the compass never got me where it should and Following Heading,and Runway markings, they seemoff while your landing, then when you sit flush on the runway, its off by 5+ degree's. which in real life would get you killed nightflying or due to weather. I think its something that will be fixed in 2.0 till then we gotta deal with VFR.

I'll retract what I said in my earlier post. If everything in DCS is given in true bearings, then the remote indicator compass reading makes sense. I thought the 13 degree variation in my example was high but as I got closer to an airport, it lessened to 6 degrees.

I guess the only discrepancy in DCS is that the runway headings are in true degrees, not magnetic like in real life. Although, in some countries like Canada, many airports use true heading because we are so far north and the variation between magnetic and true north is larger.

I know this is old but it seems not to be satisfactorily answered. Also mentioned here: http://forums.eagle.ru/showthread.php?p=1459434

The P-51D's compass appears to be a floating, flux-gate type remote indicating unit. Unlike a traditional compass such systems experience no acceleration effects nor do they oscillate about an output. There is no mass coupling inherent in the design which causes precession. Typical compass behavior deflections are not seen in DCS P-51.

However, just because the magnetic sensing mechanism doesn't suffer from acceleration errors doesn't mean it doesn't suffer from adverse attitude errors. It is potentially these different errors which DCS P-51 is demonstrating.

About magnetic field lines; they are 3D vectors which have a direction in azimuth (N-S-E-W) but also an inclination vertically. In fact in the Caucuses this dip is severe, at least 60 degrees from level. Magnetic north is more "down into the ground" than it is toward any place on the horizon. For navigation we are interested in the projection of this 3D vector onto the level plane (to find compass direction) and so is the design of the instrument. Coincidentally Nevada has almost the same dip.

So we take our P-51 flying and align our airplane wings level aligned with the magnetic field in azimuth. We're going north. (Actually we're opposite to the magnetic field since the North pole is by convention a south magnetic pole. Makes sense since north poles of the magnet in our hand is attracted to it after all). We can imagine the arrow of magnetic field line is diving directly through our left wingtip and the remote compass sensor in a south to north direction. It's dive angle is about 60 degrees. If this arrow could case a shadow and we shone a light from directly above the airplane down the arrow would cast a perfectly north-south shadow. This is what the instrument reads. So far so good.

Now we roll to the right 90 degrees on edge but through some expert flying skill manage to remain heading north. Again imagine this arrow entering the trailing edge of the wing (sky side) and exiting the front edge (dirt side). From the instrument's point of view this arrow is cutting across it at a 60 degree angle from left to right. The "shadow" of the arrow onto the instrument suggests our heading is not north to south even though in reality it is. This is the same shadow if we were flying 300 but wings level. Doing a rather slow roll while maintaining a northward course we see the remote compass value vary from 0 to +60 to 0 (inverted) to -60 back to 0 (level) when we do a very slow roll.

The directionality of the instrument is most stable in roll when the arrow is parallel with the roll axis. For example going north with -60 pitch or south with +30 pitch roll produces very little change in indication when banking. The opposite vectors north +30 pitch and south -60 pitch mean the "arrow is straight down on top of the instrument" with corresponding instability in reading.

Pitch stability is best when the arrow is parallel (or antiparallel) to the pitch axis (wingtip to wingtip). I can demonstrate this by flying west, rolling my right wingtip to point 60 degrees low and pitching up and down. The needle doesn't move. However if I point the bottom of the airplane at the arrow direction (west, 30 AOB left) small changes in pitch result in huge reading changes.

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The first thing you may think of when hearing about this behavior is that the whole remote sensor would work infinitely better if it maintained a level attitude (but matched heading) regardless of the attitude of the airplane. Put the whole thing on a self-leveling 2D gyroscope and your worries are over! And that's exactly what they did... on other airplanes. The B-17 put such a compass inside a gyroscopic mount and it was good up to 65 degrees tilt. The AN5730 remote indicating compass system in the P-51 made do with a less sophisticated system. They filled a ball with fluid and floated the magnet.

The question remains how effective this design is if at all in compensating for this tilt error. Section VII of this manual ( timetravelair.com/techlibrary/EO 20-25BA-3.pdf ) states: Given a field of 0.18 gauss horizontally and 0.54 gauss (Frederf's note: 3:1 or a dipping field of 71.5 degrees)...Tilt the transmitter at a 10-degree angle about any axis. The reading must not change more than 6 degrees. The float must rotate freely when the transmitter is rotated at a 20-degree angle. (Frederf's note: Section II 2.a.I. mentions this 20 degrees as well).

The 6 degree spec is telling. If you take a flat sundial with sun ray plunging at that angle along its meridian and tilt the sundial laterally 10 degrees you're going to get a change of shadow angle by 7.5 degrees which is more than 6. This leads me to think that at least within a 10-20 degree limit this instrument was supposed to compensate for the attitude of the installation (and thus airplane) to some degree.

In DCS P-51 tilts of 10 degrees produce changes of much more than 6 degrees even with efforts to keep the local acceleration vertical. I have doubts if the RC is in good working order in this module. Further evidence suggests that the system should be largely immune to small tilt (<20 degrees) like that ground swing checks make zero mention of the orientation (tail wheel) of the airplane which would be critical on a DCS P-51. It would be so hard to navigate by this thing if your particular AOA could mess up your compass reading by 10 degrees.