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Confused by auto pilot ATT


imacken

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Manual p86 says:

In ATT 'If the roll angle is <7° the aircraft engage a course hold where the aircraft will level and maintain the present course. If the aircraft has drifted from the set course the autopilot will

send a signal to the rudder servo and steer the aircraft back onto the set course.'

Doesn't work for me. Aircraft drifts off course all the time, and never steers back on course.

If I set ATT when on course for a waypoint, the Viggen just drifts way off course all the time, and I have to be permanently making adjustments.

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Does anyone have this automatically steering the aircraft ‘back onto the set course’ working?

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No. It does not. But you can keep the AP engaged and do heading corrections using the stick - as well as turning using the trim input

 

Yes, that's true but the manual says it does!

I guess this is another bug as no one seems to have this working.

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I'm not using the ATT mode much for longer periods, so I can't say anything about it drifting off course, but I have noticed in ATT+HÖJD mode, that it will drift off sometimes (especially when there is an imbalanced loadout like KB pod on one side and U/22 pod on the other side).

 

This might be related to the issue: https://forums.eagle.ru/showthread.php?p=3358163#post3358163

 

I suspect the manual is wrong. I mean, levelling the plane and returning to course does not make sense in ATT. In HOJD it holds altitude, so levelling makes sense there (does it also return to course in that case?).

I think it means that it will level the bank angle, not the pitch angle.


Edited by QuiGon

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I suspect the manual is wrong. I mean, levelling the plane and returning to course does not make sense in ATT. In HOJD it holds altitude, so levelling makes sense there (does it also return to course in that case?).

Why doesn't it make sense in ATT? The only difference with adding HOJD is that it then holds altitude.

No, it doesn't return to course with HOJD+ATT. I can't actually see any way to make the Viggen hold course, despite what the manual says.

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I'm not using the ATT mode much for longer periods, so I can't say anything about it drifting off course, but I have noticed in ATT+HÖJD mode, that it will drift off sometimes (especially when there is an imbalanced loadout like KB pod on one side and U/22 pod on the other side).

 

This might be related to the issue: https://forums.eagle.ru/showthread.php?p=3358163#post3358163

Yes, I was a contributor to that thread, and it is another example of how we all speculate about an issue when a simple answer from the devs would resolve it. Ragnar made 2 contributions recently to that thread, that don't say anything useful!

Why do you think the left rudder not mapping to HOTAS may be related to this? Even though I can't put it on the trim hat, I have left/right rudder trim mapped to keys, and it still doesn't work for me.

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Why doesn't it make sense in ATT? The only difference with adding HOJD is that it then holds altitude.

Because leveling the plane changes your attitude. First the manual says:

 

The mode will maintain the pitch angle when it is engaged and will maintain roll angle if the angle is > 7° and <66°.

Ok, pitch is always maintained, roll angle is maintained conditionally. Then it says:

If the roll angle is <7° the aircraft engage a course hold where the aircraft will level and maintain the present course.

So, now pitch angle is not maintained? So to hold pitch you have to bank more than 7o? This is where it stops making sense to me. Attitude hold should, well, hold the attitude.

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So, now pitch angle is not maintained? So to hold pitch you have to bank more than 7o? This is where it stops making sense to me. Attitude hold should, well, hold the attitude.

Again: I think this just means it will level out in roll angle, not pitch angle.

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Because leveling the plane changes your attitude.

Yes, and ATT holds that change.

 

First the manual says:

 

 

Ok, pitch is always maintained, roll angle is maintained conditionally. Then it says:

 

So, now pitch angle is not maintained? So to hold pitch you have to bank more than 7o? This is where it stops making sense to me. Attitude hold should, well, hold the attitude.

It doesn't say the pitch angle isn't maintained. Perfectly possible to level out (in banking terms), hold a course and the pitch angle.

However, as we know the 'maintain the present course' doesn't work.

Also, the pitch is not maintained in ATT unless speed is held steady. Climb at a particular pitch angle with burners on, engage ATT, then drop rpm to say 75%. Then you will gradually see the pitch angle drop and eventually you will crash! I would expect that the AP should do whatever it needs to do to maintain the pitch angle.

The whole AP thing here needs a proper explanation from someone as it is clearly not working in a way that mere mortals can understand, nor as stated in the manual!

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Also, the pitch is not maintained in ATT unless speed is held steady. Climb at a particular pitch angle with burners on, engage ATT, then drop rpm to say 75%. Then you will gradually see the pitch angle drop and eventually you will crash! I would expect that the AP should do whatever it needs to do to maintain the pitch angle.

 

The whole AP thing here needs a proper explanation from someone as it is clearly not working in a way that mere mortals can understand, nor as stated in the manual!

I would say its quite natural, since the AP doesn't control the engine unless in "Automatiskt fart kontroll" during a landing (which is a different system anyhow what I know of). It's actually quite similar to the AP in the Shark, more like a stability aid and you need so stabilize before giving full authority to it. Imagine what would happen if it would do everything in its power to maintain the pitch if almost cutting power, you would quickly be brought into a stall. Better to let the pitch decrease and keep the wings producing lift and give the pilot time to correct his mistake. After all it's the pilot flying.

Keeping the engine power stable is not really an issue for the pilot in a fixed wing, basically setting the throttle to get wanted power and he can basically let go and use his left hand for other things without bothering. Not very tiresome in long flights either. Leting go of your stick for long periods isn't that good idea without engaging ATT. Holding it and trying to manually maintain attitude (while possibly review mission plans, navigate, operating radios, etc) for long flights on the other hand is something different. That's what the AP in the Viggen is for, helping out with the stick and rudder, not the throttle (and not to the level of pilot taking a nap)

 

For the other issues discussed here I agree it would be nice to get some consistent official explanations.

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I would say its quite natural, since the AP doesn't control the engine unless in "Automatiskt fart kontroll" during a landing (which is a different system anyhow what I know of). It's actually quite similar to the AP in the Shark, more like a stability aid and you need so stabilize before giving full authority to it. Imagine what would happen if it would do everything in its power to maintain the pitch if almost cutting power, you would quickly be brought into a stall. Better to let the pitch decrease and keep the wings producing lift and give the pilot time to correct his mistake. After all it's the pilot flying.

Keeping the engine power stable is not really an issue for the pilot in a fixed wing, basically setting the throttle to get wanted power and he can basically let go and use his left hand for other things without bothering. Not very tiresome in long flights either. Leting go of your stick for long periods isn't that good idea without engaging ATT. Holding it and trying to manually maintain attitude (while possibly review mission plans, navigate, operating radios, etc) for long flights on the other hand is something different. That's what the AP in the Viggen is for, helping out with the stick and rudder, not the throttle (and not to the level of pilot taking a nap)

 

For the other issues discussed here I agree it would be nice to get some consistent official explanations.

Hmm, the AFK works in 2 ways. In normal flight it manages the engine to maintain a speed of 550 kph, but if the landing gear is down, it manages the speed to main AOA of 12 degrees (I think).

I hear what you say, but the manual does state that ATT maintains pitch.

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Hmm, the AFK works in 2 ways. In normal flight it manages the engine to maintain a speed of 550 kph, but if the landing gear is down, it manages the speed to main AOA of 12 degrees (I think).

 

I hear what you say, but the manual does state that ATT maintains pitch.

Yes, but for what I understand the AFK is suposed to be used during approaches not cruising, and you are supposed to correct the attitude as the speed changes to 550, when it settle at 550 it's perfectly fine to engage ATT and expect it to hold the pitch, even if you just cruising. I might be wrong. In my opinion your example where the ATT failed to hold pitch is an example of going outside the operation envelope for the AP so to say, you are not supposed to change the power after ATT activation, as you said yourself it works if keeping the speed stable (easy task ones initial stabilization, no need for an AP channel). Same as in the Shark if you are familiar with that. The AP is there to help you when operating it properly, but also not put you in danger (like risking going into a stall) if operating it in a bad way (like reducing thrust after engagement).

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Yes, but for what I understand the AFK is suposed to be used during approaches not cruising, and you are supposed to correct the attitude as the speed changes to 550, when it settle at 550 it's perfectly fine to engage ATT and expect it to hold the pitch, even if you just cruising. I might be wrong. In my opinion your example where the ATT failed to hold pitch is an example of going outside the operation envelope for the AP so to say, you are not supposed to change the power after ATT activation, as you said yourself it works if keeping the speed stable (easy task ones initial stabilization, no need for an AP channel). Same as in the Shark if you are familiar with that. The AP is there to help you when operating it properly, but also not put you in danger (like risking going into a stall) if operating it in a bad way (like reducing thrust after engagement).

 

I wasn’t suggesting that my example of pitch not holding was practical, it was just to demonstrate that ATT does not hold pitch as stated in the manual.

Anyway, we are straying off the original topic, i.e. ATT does not revert to set course if the plane drifts. We all agree on that!

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I suspect the manual is wrong. I mean, levelling the plane and returning to course does not make sense in ATT. In HOJD it holds altitude, so levelling makes sense there (does it also return to course in that case?).

The manual matches what's in the flight manual for the real aircraft, which states (in my translation)

 

If the bank angle is <7° when the attitude hold mode is engaged, course hold is achieved and works as follows. When engaged, a signal proportional to the bank angle is output from the attitude gyro and used as a steering input for the outer elevon servos and the trim hydraulics. This results in control surface deflection that makes the aircraft roll level and continously works to keep it level.

 

Additionally, at the moment of engagement, the course synchronizer is locked to the current course angle, and a reference course is thus obtained. If the aircraft then veers from this course, the course synchronizer outputs a corrective signal (Δ-course), which is used as a steering input for the control surfaces, making the aircraft return to the reference course.

 

Course corrections for course disturbances caused by e.g. turbulence normally result in bank angles up to ±5°.

This course hold is not supposed to level the plane in pitch, though.

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The manual matches what's in the flight manual for the real aircraft, which states (in my translation)

 

 

This course hold is not supposed to level the plane in pitch, though.

Good one, thanks. Then I guess the implementation of the course hold is bugged.

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The manual matches what's in the flight manual for the real aircraft, which states (in my translation)

 

 

This course hold is not supposed to level the plane in pitch, though.

 

Thanks, that made it more clear! :thumbup:

 

I agree with Holton181, that it seems to be not working correctly in DCS now, as the Viggen does drift off course.

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Good one, thanks. Then I guess the implementation of the course hold is bugged.

Yep, as I said previously, I think we can all agree that this is not working as per (both) manuals!

Again I'll say it would be good to get some contributions from devs on this and other issues that have been raised recently.

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Just tested the course and pitch hold in HÖJD mode during a 300km flight over water. And I must say it does seem to work. I didn't test the <7°-return-to-course behavior thoroughly, but focused on leveling out and pitch. It held bank, pitch, course and altitude rock solid.

BUT! it is incredibly sensitive about getting your stick and pedals centered. Touching them an itsy bitsy tiny bit and you start to drift. Toggling HÖJD off and on stabilized the bird again, but obviously at a slightly different altitude and course. To make sure it didn't just work because I had really made an effort centering my controls, I disabled HÖJD while zoomed in on the flight path vector and observed that it did bank and pitch slowly. Engaged HÖJDand it stabilized. Disabling again and it started to bank and pitch exactly the same as before. Turned HÖJD off and on several times and observed the same behavior. Then for fun I adjusted my controls very slightly and finally succeeded to get it stabilized merely by the controls and only SPAK.

 

I did briefly test the <7°-return-to-course by moving either stick or pedals just a notch, but could not observe any stabilizing nor return to previous course, but then again I can't guarantee that I was within <7°.

I also in the beginning reduced power from first stage AB to just disabling it with HÖJD active, with no change in attitude or altitude.

 

 

Note!

I have a "full length" helicopter HOTAS (PFT Lynx) with no springs or detents, enough friction on all axes to let go in any position and with small angle, 12 bit Hall sensors (12 bit controller board) on all of them, effectively giving me more than 11bit real and noise free resolution on my stick. Due to the sensitivity of the axis in the Viggen, I assume low resolution controllers and/or noisy pots can make life pretty hard with these things.

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Just tested the course and pitch hold in HÖJD mode during a 300km flight over water. And I must say it does seem to work. I didn't test the <7°-return-to-course behavior thoroughly, but focused on leveling out and pitch. It held bank, pitch, course and altitude rock solid.

BUT! it is incredibly sensitive about getting your stick and pedals centered. Touching them an itsy bitsy tiny bit and you start to drift. Toggling HÖJD off and on stabilized the bird again, but obviously at a slightly different altitude and course. To make sure it didn't just work because I had really made an effort centering my controls, I disabled HÖJD while zoomed in on the flight path vector and observed that it did bank and pitch slowly. Engaged HÖJDand it stabilized. Disabling again and it started to bank and pitch exactly the same as before. Turned HÖJD off and on several times and observed the same behavior. Then for fun I adjusted my controls very slightly and finally succeeded to get it stabilized merely by the controls and only SPAK.

 

I did briefly test the <7°-return-to-course by moving either stick or pedals just a notch, but could not observe any stabilizing nor return to previous course, but then again I can't guarantee that I was within <7°.

I also in the beginning reduced power from first stage AB to just disabling it with HÖJD active, with no change in attitude or altitude.

Thanks for that, but to be honest, it's not telling us anything new.

This thread is about ATT and it not returning to course after some drift or adjustment as the manual says it should.

I think we are happy the way that all other aspects of AP work except for that.

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Thanks for that, but to be honest, it's not telling us anything new.

 

This thread is about ATT and it not returning to course after some drift or adjustment as the manual says it should.

 

I think we are happy the way that all other aspects of AP work except for that.

But you said your self the course is drifting, didn't you? From OP:

If I set ATT when on course for a waypoint, the Viggen just drifts way off course all the time, and I have to be permanently making adjustments.

In that respect ATT and HÖJD should act the same. They don't drift from course. My comment was about that part of your OP, not the returning to course (as I said I didn't get working either).

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