Warrior & Arrow autopilot with DG not set correctly
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I'm not sure exactly how the autopilot works in heading mode when setting the heading on the DG so not sure if this is a bug or not.
Lets say my actual heading is 090 but my DG has drifted by 20 degrees so it is reading 070. If I set the heading bug to 090 (which would look like a 20 degree right turn on the DG) then activate the autopilot on heading mode the plane will continue straight (tracking an actual heading of 090 but showing 070 on the DG).
Is this what would happen in the real world or does the autopilot in heading mode work off the DG so if the DG is not set correctly then the autopilot won't fly the correct heading?
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@nickd27 In theory the autopilot could have its own heading source, but I think in the Arrow the DG is used as a reference. At least, this is what I have understood from the manual:
https://befcstl.org/wp-content/uploads/2017/10/piper-AutoControl_IIIB_Autopilot_POH.pdf
If that is the case, then the only thing the autopilot tries to do in 'heading hold' mode is to put the heading bug at the top of the DG. I.e. it aligns the indicated heading with the heading bug. If the DG has drifted off, then the actual heading with deviate from the indicated heading.
Note that some DG automatically correct for drift using a flux gate compass. In the Arrow there is no knob to adjust the DG, so I think it should be driftless.
https://en.wikipedia.org/wiki/Fluxgate_compass
TL;DR; I don't think there is anything wrong in the behavior of the autopilot. However the fact that the DG is drifting seems odd.
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Btw, I just realized that I'm always using the HSI instead of the DI. The HSI, to my knowledge, does not have drift, but the DI probably can drift.
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Yes I probably should have pointed that out in my first post. The HSI is the default and doesn't drift as it has it's own heading source and I imagine most people use that (since it is default + has more functionality) but I use the DG since that's what I have when I fly IRL. It does drift, and the autopilot functionality does not seem to work correctly when it drifts.
Thanks for your input. I am convinced now that this is most likely a bug so I'll report it
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I use DG exclusively as well, and I haven't really seen any issues with the autopilot following the heading bug in heading mode. But I haven't paid that close attention since SU5, and where I've been flying the winds have been very light, so it may be an issue if I get more of a crosswind. And the heading does drift a bit, I use the whisky crosscheck and the heading set to make sure it's on the correct heading before using heading mode.
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@nickd27 I re-read your original question. Initially I understand the phrase "activate the autopilot on heading mode the plane will continue straight" as pertaining to the expected behavior. But do I understand correctly that this is would you observed? This would mean that the autopilot is based on actual heading in stead of indicated heading. That would certainly be a bug.
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I use the whisky crosscheck and the heading set to make sure it's on the correct heading before using heading mode.
Yeah if you are keeping the DG aligned with the whiskey compass you wouldn't see this problem
This would mean that the autopilot is based on actual heading in stead of indicated heading
Yep that is correct. That is the issue I am reporting
I will record a video to show the error more clearly tonight and post it here
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Timestamps and descriptions of what I am doing are in the video description. I hope this helps to explain the issue
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@nickd27 So if you zero the DG with the whisky, then the AP follows the DG, but basically, the AP will follow the aircraft's actual heading, regardless of the DG. While it's handy to have that, it's probably not an accurate representation of the AP.
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Yeah that's right. It is handy that the autopilot works even if the gyro has drifted but I want realism so if I forget to check the gyro for an hour I want to end up 20 miles off to the side of my desired track wondering why there isn't an airport underneath me! :P
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@katchaplin
This is controlled by a 'hdg_ref_var' value in the [AUTOPILOT] section of the systems.cfg. The default value is '0' which means the magnetic heading is used by the autopilot. Setting it to '1' means the gyro heading is used. Unfortunately there is currently no way to update that value from the gauge code so we can't change it based on whether the HSI or DI has been enabled, so we have left that value out of the cfg meaning it defaults to '0' (magnetic), as most users prefer the HSI. For now you'll have to manually add 'hdg_ref_var=1' to the cfg if you prefer the DI.
