Sudden downward pitch when disengaging autopilot

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Clorix
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Sudden downward pitch when disengaging autopilot

Post by Clorix »

I've noticed that when I disengage the autopilot, there's a sudden, somewhat jarring, downward pitch movement which causes me to have to react quickly with a nose-up command to get it back on track. This is most noticeable on approach. Is this normal?

I would have thought the aircraft to be nicely trimmed out.

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Nick - A2A
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Re: Sudden downward pitch when disengaging autopilot

Post by Nick - A2A »

Hello Clorix,

Are you using a hardware trim axis of any sort in the Bonanza? If so bear, in mind the simulator can't control the physical position of this axis (unlike in the real aircraft which uses a trim servomotor) so you'll need to make sure you do your best to sync it with the trim wheel in the aircraft before you disconnect the autopilot.

Thanks,
Nick

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Clorix
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Re: Sudden downward pitch when disengaging autopilot

Post by Clorix »

No, I'm not using any trim axis assignments. My trim is set to my joystick's hat switch via the normal P3D assignments menu.

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Nick - A2A
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Re: Sudden downward pitch when disengaging autopilot

Post by Nick - A2A »

Could you give a little bit more info on a specific speed and configuration where you're seeing this happen? Bear in mind that if you disconnect the autopilot soon after making power or configuration changes, it may be still be trimming the aircraft. However, if the aircraft is stable in cruise or on approach, disconnecting the autopilot shouldn't require you to make any significant pitch inputs.

Thanks,
Nick

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Clorix
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Re: Sudden downward pitch when disengaging autopilot

Post by Clorix »

Here's a quick video I happened to capture the other day of the issue. In this instance, it isn't the worst I've seen it, although it's there. It's raining, but as you can see, it's a totally stable approach, no turbulence, no wind gusts. Only rain. It's otherwise completely smooth air. This also happens on a perfectly clear day, so I don't believe it to be weather related.

As you can see and hear, there are no power changes, no configuration changes at all. I'm neither accelerating or decelerating. Just stable. And then I disconnect the AP and the plane immediately starts pitching down. My joystick is calibrated just fine. It isn't overly bothering me at all, and as far as I know it could just be a normal Accu-Sim thing going on that I'm not privy to.

Link to video: https://www.youtube.com/watch?v=xVdtc0J45yc

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Nick - A2A
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Re: Sudden downward pitch when disengaging autopilot

Post by Nick - A2A »

Thanks vey much for posting the video Clorix. However, without being able to see what the control column or the trim wheel is doing it's of somewhat limited troubleshooting use.

I haven't been able to reproduce similar behaviour with my installation though. Could you just confirm if the weather loaded is a default Prepar3D weather theme, or generated using an add-on?

Thanks,
Nick

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Clorix
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Re: Sudden downward pitch when disengaging autopilot

Post by Clorix »

It's generated via Active Sky for P3D v4. I'll do another quick circuit or something and see if I can get it to happen again, this time with the controls visible.

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Nick - A2A
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Re: Sudden downward pitch when disengaging autopilot

Post by Nick - A2A »

Okay, thanks - that'd be great. Might be worth seeing if the same effect happens with default weather too. Active Sky is somewhat known for effects caused by its updraft and downdraft simulation.

Cheers,
Nick

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Clorix
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Re: Sudden downward pitch when disengaging autopilot

Post by Clorix »

I've made three more videos which will hopefully give you an idea of what I mean. It doesn't happen every time I disengage the AP which is why I'm unsure if it's the plane (maybe) or just me (probably). I apologize for the longer videos this time. I thought it might help in some way.

Straight and Level (ActiveSky): https://www.youtube.com/watch?v=6u2YlbIDWiM
Straight and Level (Default Weather - Clear Skies theme): https://www.youtube.com/watch?v=1RvB4Uerzxc
On Approach: https://www.youtube.com/watch?v=ulurD3Oa4Hs

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some1 - A2A
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Re: Sudden downward pitch when disengaging autopilot

Post by some1 - A2A »

Hello

This is quite normal, there are two factors here. First is the realistic autopilot behaviour, second is the lack of FFB in our joysticks
• The Autopilot does not fully trim the aircraft all the time. The way the system works, one servo controls the elevator position directly, and only if it reaches certain limit, the autopilot activates trim servo to unload the elevator. You can observe that in our airplane, how the yoke moves by itself under autopilot control, and haw the trim wheel is activated from time to time. This is very different to what flightsimmers are used to with default FSX/P3D planes (and most 3rd party), because default FSX/P3D autopilot flies the airplane using elevator trim alone. Which is not very realistic, but easier to code.
• In the real aircraft, when you disengage the autopilot, you can feel the change of forces in the controls, but as long as you keep the yoke steady, you'll maintain the flight path. Our joystikcs without FFB don't move when autopilot controls the airplane, so they will get out of sync. That means when you disengage the autopilot, you may notice slight pitch up or pitch down effect, because your physical controller is not in the same position as the virtual yoke used to be on autopilot.

That being said, I'll double check if the trim logic nullifies elevator position correctly, maybe there is some room for improvement.
Michael Krawczyk

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