AHRS - General understanding

[TomatoSalad83]

Hey guys,

 

there is a topic in the bug report section about it, but as it's not a bug I hope it's okay if I open a thread here.

 

 

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I am not quite sure if I understood the AHRS system and compass control system correctly. It would be awesome if you could review my thoughts on it and correct me if needed. :)

I wanna start with the sensors:

 

MAD - Magnetic Azimuth Detector

More or less a simple compass showing the magnetic north of the earth with all it's weakspots, such like vulnerable to magnectic anomalies, errenous values in extreme attitude and so on.

 

DG - Directional Gyro

This should be a spinning gyro which is due to it's cardan suspension and angular momentum mostly unaffected by any change of direction / attitude.

 

Unlike the gyrocompass the DG is not able to seek the north of the earth, therefore you need to turn the BDHI rose where the north is assumed (compared to the MAD).

In civil aviation the BDHI / DG has to be compared and calibrated every ~15min due to friction of the DG system and earth rotation.

 

 

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Okay I hope the basics are correct so far. Now I want to transfer it to the F14 Compass Control:

 

Mode Switch: COMP

The system reads the magnetic azimuth detector only. That mode is pretty easy to understand and well described in the manual.

 

Mode Switch: DG

The system reads only the DG reference. That means: The north has to be calibrated manually with the HDG knob and once set it may be affected by friction and earth rotation. (?)

 

Mode Switch: SLAVED

The system reads the DG reference which is automatically and continuously calibrated by the MAD readings. So no need to calibrate the DG manually. That process should be pretty slow and happens only in level flight to avoid errenous readings.

 

My question here is: The HDG button in SLAVED mode is used to synch as well. Is the only difference here that it happens much faster?

 

 

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This pretty much looks like a tutorial but due to my lack of english skills it was easier to formulate. It would be pretty awesome if some of you can give me a feedback if I understood the system correctly.

 

 

Thanks in advance

 

 

Julian :)

Edited October 16, 2019 by TomatoSalad83
The system is called AHRS not AHRD

[Spiceman]

Yes, it’s literally exactly like your average Cessna. In Slaved mode you’re flying the fancier model with the Garmin G1000 and the auto slaving DG/HSI. In DG mode you’re flying the 1980s 172 with the DG you have to slave every 15 minutes or so. In COMP mode, yep, the MAD is tied directly to the BDHI turning into not much more than a vertical wet compass.

 

In Slaved, pressing the HDG button bypasses some of the hysteresis functions and slaves the DG faster.

Edited October 16, 2019 by Spiceman

[Rabbisaur]

Some times when I fly F14 with Jestor, the artificial horizon will skew. I've never had a clue what was the cause and how to fix that. Can you also say something about that?

[Spiceman]

Hmm. I’ve never seen that in the sim. IRL it’d be a bad IMU and in the jet you’d revert to AHRS/AM and you’re now using the AHRS gyro for attitude reference and the NAV computer would be calculating your position from pitot static inputs (not very accurately).

[*Aquila*]

Some times when I fly F14 with Jestor, the artificial horizon will skew. I've never had a clue what was the cause and how to fix that. Can you also say something about that?

Are you sure you ask Jester to do a fine INS alignment when you start the plane up?

[Rabbisaur]

Yes, usually first flight is normal. Landed rearm refuel fly for another 30 minutes an then the ladders are 30 degrees off. Have to turn it off and fly with back up gauges some time or just VFR.

[TomatoSalad83]

@Spiceman Thank you very much for the information! :)

 

@Rabbisaur I have that problem too, usually when quickstarting on a carrier and/or landing. According to another topic there are many possible problems

• big magnetic anomalies for example a carrier
  
• wrong alignment so the DG has an initally wrong calibrated north
  
• in theory the MAD should only be asked for the north only in level flight but if you fly for too long in a small bank the DG will slave to a slightly wrong MAD input
  
• the MAD seems to be vulnerable to wrong values in high AOA
  

 

So I guess it might be the high AOA when performing a landing leading to a wrong MAD output which is slaving the DG into the wrong direction.

I personally turn the compass control to DG before landing (on a carrier). As I said my english is bad so I suggest you read the thread by yourself, maybe you can gain more information from it and even correct me if I understood something wrong :D

 

https://forums.eagle.ru/showthread.php?t=238390

 

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[draconus]

Some times when I fly F14 with Jestor, the artificial horizon will skew. I've never had a clue what was the cause and how to fix that. Can you also say something about that?

Have you over-G the INS gyros?

[QuiGon]

Thanks @TomatoSalad83 for this very interesting topic and thanks @Spiceman for the contribution! There are some interesting details in there! :thumbup: