AP heading mode switches between magnetic and true

[admiki]

As title says, if I engage route mode and set course (in this case 000), it flies heading of 006 on the HSI. When I press and hold trim release and change heading, once released, AP will now go to heading of 000 on the HSI. Behaviour observed in both cold and hot helicopter.

AP heading.trk

[AeriaGloria]

20 hours ago, admiki said:

As title says, if I engage route mode and set course (in this case 000), it flies heading of 006 on the HSI. When I press and hold trim release and change heading, once released, AP will now go to heading of 000 on the HSI. Behaviour observed in both cold and hot helicopter.

AP heading.trk 232 kB · 1 download

Hey I watched the track and flew myself. Not sure what you were seeing. 
 

When you flew heading 006, the roll channel was giving left output trying to get you back to heading 000. You also had 2 degrees of left drift at the time so the deflection was small. 

After trimming and returning to 000, the roll channel Now went close to 0, as when the roll channel is neutral is when route mode is telling you “on desired heading per route mode.” I think you were overpowering it when you were at 006. After the trim press the stick was slightly deflected more towards the left than before pressing trim. 
 

In such a case; the manual recommends using the trim hat until roll channel is center. I have pictures but they are too big too send, but you can see it by seeing the roll channel deflection after pressing route and how the stick position changes after trim 

I think that’s caused by the autopilot only using proportional/integrative functions and not derivative. If it had derivative, it would know to give more deflection if it wasn’t on the selected heading. But with just proportional and/or integrative functions, it only knows to give output proportional to distance from selected heading/attitude (proportional), and to fight changes in angular rate over time (integrative) 

Edited October 13, 2023 by AeriaGloria

[admiki]

Hmmm, I made sure that both roll and drift are zero.

2 hours ago, AeriaGloria said:

In such a case; the manual recommends using the trim hat until roll channel is center. I have pictures but they are too big too send, but you can see it by seeing the roll channel deflection after pressing route and how the stick position changes after trim 

Since my stick stays in place I don't like to use trim hat since it causes disagreement betwen physical and DCS stick, but I usualy just hold trim release until all is where it is supposed to be.

2 hours ago, AeriaGloria said:

I think that’s caused by the autopilot only using proportional/integrative functions and not derivative.

OK, this I did not know. That could explain it.

[AeriaGloria]

Yeah, it’s very weak. Sometimes I’ll just look at my left knee and move my stick in the direction the roll channel is pointing when in route mode to help it, or increase bank from the set 15 degree limit in route mode.

According to manual it’s expected to have a course deviation of up to 5 degrees with it, or Atleast it says it’s working “properly” if within that error. It would be amazing to ask a real pilot how well it worked in reality. I would assume that doppler would be causing it to snake around a bit as gusts and imperfections in doppler reception happen 

That being said, its deflection for every degree from selected course is surprisingly low. I just did a test, on a course of 000, I have to set a course to +/-7 degrees from my current course to have the roll channel deflect  100% in route mode. 

Perhaps that’s why it says in the manual to expect a possible +/-5 degree deviation, becuase within +/-6-7 degrees of set course, route mode causes the roll channel to to stop giving full deflection and come closer to neutral. 

2 hours ago, admiki said:

Hmmm, I made sure that both roll and drift are zero.

Since my stick stays in place I don't like to use trim hat since it causes disagreement betwen physical and DCS stick, but I usualy just hold trim release until all is where it is supposed to be.

OK, this I did not know. That could explain it.