[RESOLVED] Tomcat heading drift - Page 2 - ED Forums
 


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Old 04-08-2019, 07:17 PM   #11
Super Grover
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It's quite complicated. There are a few reasons why your magnetic heading could have been that wrong. Did you take off from a carrier? If yes, then probably you got some initial error on the AHRS directional gyro -> error in the magnetic heading in SLAVED mode after launch. Even if you do nothing, it should return to normal after a while (much longer while), but you have to fly level and constant speed (and flying constant low speed in the F-14 is a challenge).
The magnetic field from the carrier decays really fast with distance from the ship, so I don't think it could cause that. Plus the magnetic heading is gyro stabilized (in SLAVED), so even when you do a really close fly-by, it shouldn't impact your magnetic heading.

Then, there's the magnetic flux valve (Magnetic Azimuth Detector - MAD) in the vertical tail. It's fixed. It feeds the AHRS with the relative direction of the magnetic North. When you roll or pitch, it becomes sensitive to the vertical component of Earth's magnetic field. This means - it becomes erroneous. That's why the directional gyro slaves to the MAD only in horizontal flight. At least in theory. In practice - you fly constant speed in a nose up attitude or with some hardly noticed bank for long enough, your directional gyro will slave to a slightly erroneous magnetic North.

That's why it's important to observe the HSD and the TID for the MV acronym (magnetic variation). It is displayed alternately with the IN or IM when the difference between the computed magnetic variation (MC) and the manual magnetic variation (VM) is greater than 5°.

If your directional gyro/synchro is shifted, it may result in erroneous interpretation of the TACAN direction on all instruments. You might have the carrier straight in front of your nose, but the BDHI will show the TACAN a few degrees to your left or right. However, the radial (bearing) reading would be correct; it would be just your magnetic heading wrong.

And there's much more...
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Old 04-08-2019, 08:02 PM   #12
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I made a simple carrier landing mission: i start in the air, carrier is oriented at 316 deg, so the runway is oriented at 307 deg and if we consider the 5 deg magnetic deviation, it means i have to intercept the 302 deg radial to line-up for landing. The wind speed is 0, the carrier is static.

Here are 2 images i took when pausing the simulation at 2.4 NM from the carrier (one image from the cockpit and one from external view, taken at the same moment in time); as you can see, the ILS is centered and the carrier is straight ahead, so i am perfectly lined-up, but the indicated heading in the cockpit is 290 and the one in the external view is 307 deg, so there is a 17deg difference and also the TACAN points around 15 deg to the right while the carrier is straight ahead.
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Old 04-09-2019, 06:06 AM   #13
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The magnetic flux valve alone is even simpler than a normal compass we know from the GA aircraft. In opposition to a normal compass, it is not balanced so will show the magnetic north only in horizontal flight. On the other hand, it is unaffected by acceleration errors because it has no moving parts.
It doesn't have to be balanced, because, in normal operations, it is gyro stabilized.
When in a level flight at high AoA (let's say 15 units), flying east or west, the magnetic flux valve behaves like a whiskey compass would behave when decelerating: it turns south - hence your observation. However, when flying north or south, it would turn much less or might even not turn at all. It is because in a nose up attitude, the flux valve is tilted back against the horizon, just like a whiskey compass when decelerating.
As I wrote above, the magnetic flux valve output isn't presented directly to the crew - in normal operations, it is gyro stabilized in the AHRS. When flying low AoA, it should show more or less the correct magnetic heading. When you decelerate and fly high AoA, it will slowly align with the erroneous magnetic heading from the magnetic flux valve.
In our F-14, we simulate the magnetic field declination and inclination, the magnetic flux valve, the AHRS gyro stabilization and the AHRS slaving rates. Hence, you should be able to experience all imperfections of the magnetic field sensing devices; and as we can see you are experiencing them . If you are curious and want to check what happens internally, you can switch to the backup modes which expose some of the raw measurements. You can switch the HUD and VDI to use the AHRS as the attitude source. Just switch the NAV MODE selector (number 12 here: http://www.heatblur.se/F-14Manual/co...on-display-tid ) to AHRS/AM. Then, if you want to present the magnetic flux valve output, select COMP on the compass panel (number 4 here: http://www.heatblur.se/F-14Manual/co...-control-panel ) - your HUD, VDI and BDHI will use the MAD output directly.


For those, who want to dig more, two links for a smooth take off:
- A few words on Earth's magnetic field, and magnetic dip:

(sidenote: the magnetic dip for the Caucasus is ~60° down)

- A short introduction to magnetic compass errors:
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Old 04-09-2019, 06:22 AM   #14
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The earths magnetic poles are currently moving very rapidly. Your problems will get worse every day.
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Old 04-09-2019, 07:13 AM   #15
Alex Voicu
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Thanks or the explanations Krzysztof, i will take a closer look later this evening.
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Old 04-09-2019, 10:06 AM   #16
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Thank you SuperGrover for such a detailed explanation. The level of detail in the Tomcat continues to amaze me and your knowledge of all this shows the brains behind what has made the F-14 the most incredible aircraft simulation.
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Old 04-09-2019, 10:18 AM   #17
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It's these extreme details that most players probably won't even notice, because they are mostly under the hood, that make a module really outstanding (at least for me)! I absolutly love these quirks and to work around them.
Really outstanding job HB!
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Old 04-09-2019, 02:49 PM   #18
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Quote:
Originally Posted by Super Grover View Post
When in a level flight at high AoA (let's say 15 units), flying east or west, the magnetic flux valve behaves like a whiskey compass would behave when decelerating: it turns south - hence your observation. However, when flying north or south, it would turn much less or might even not turn at all. It is because in a nose up attitude, the flux valve is tilted back against the horizon, just like a whiskey compass when decelerating.
This is indeed the case and incredibly accurate in the sim.

I changed my carrier heading from 090 to 360 and don't really see any heading drift at all. It's much easier now to line up on the downwind and achieve the right abeam distance.
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Old 04-14-2019, 02:19 PM   #19
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This morning I did a CASE III training with no visibility.

At catapult position I observed a compas drift due to carrier's magnetic field.

After carrier take off I flew straight on carrier's heading 090. It took about 10min and 40nm to have a correct compas heading (cross check with external view data).

Then I turned back to the carrier and at about 30nm from it, compas start to drift again.

So my CASE III pattern was a nightmare...
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Old 04-15-2019, 01:06 PM   #20
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As I explained in the previous posts, it's highly improbable that what you experienced was due to the carrier's magnetic field.


You don't have to wait 10 minutes to correct the compass readings after a catapult launch. You can fast slave the compass heading using the HDG push button on the COMPASS panel: number 5 here http://www.heatblur.se/F-14Manual/co...-control-panel
Press it with your LMB and hold until the SYNC indicator shows no deviation. It will work only when your wings are level, your speed is constant, and your vertical speed is zero.
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