X-Plane, Pitch trim, and AutoPilot

[djrisc]

Hello,
So I've been running in to an issue that is driving me crazy in X-Plane. I am curious how others are resolving this, as it's most likely something I'm just doing wrong.

So when I take off, I'll adjust the trim accordingly. Then when I get to a spot where things have calmed down from the takeoff I'll set the AP for heading/altitude hold if I'm flying any sizable distance.

The issue comes in to play when I disengage the AP. The AP is using pitch trim to hold it's altitude. So when I disable the AP and touch the trim wheel at all, I'll get a violent shift as there is a delta from where AP set the trim, and where the trim wheel was last registered via the controller. (IE. obviously the AP isn't literally moving my hardware trim controller.) So while AP adjusts trim it's becoming more and more out of sync with my hardware trim controller, resulting in a violent change in pitch when disabling the AP.

This has caused me to spiral out of control three times now. lol How are others dealing with this delta that is created? Do you simply zero out the trim before setting the AP?

Thanks!

Brett

[arb65912]

Brett, I am not an expert but it would depend on what aircraft you are flying. I fly Carenado's BE33 and P28A on Pilot Edge. Both planes move the trim as autopilot needs it and when I disengage it, plane is flying pretty much the way before autopilot was disengaged. Cheers, AJ

[djrisc]

So I'm flying the Carenado Bonanza V35. Do you have a hardware pitch trim wheel?

I believe what's happening is this:

Hardware controller set pitch to say: 35 (using numbers for comparison purposes)
Set AP, and AP sets elevator pitch trim value to: 55

When AP is disabled the elevator pitch is still now set to 55, but when I even touch my hardware trim wheel that will will abruptly pop down to 35, causing a massive jolt in elevator pitch.

Brett

[arb65912]

Hmm, I think that might be the cause, I understand that you are using the real wheel for trim like in real plane, makes sense what you describe.
V35 should fly exactly like BE33. I have trim on the Saitek yoke so I do not experience that problem, sorry I could not help. Cheers, AJ

[Keith Smith]

If the AP is having to massively adjust pitch after you engage the system, it's a sign that you were pretty far out of trim before engaging it.

My guess is that you should either get the plane trimmed out before engaging (so that when you disengage, things will be in sync), OR, discontinue the use of the hardware trim and swap to using a hat switch to make RELATIVE trim changes (ie, trim up, trim down), rather than the ABSOLUTE trim positioning system that you have now.

[Mark Hargrove]

This is a common problem with FSX as well (to the point that there is an FSUIPC "fix" for it under Misc options). I don't know yet how to fix it with X-Plane.

Keith, the problem isn't that the plane is out of trim when you engage the autopilot -- it's that the hardware wheel doesn't track the AP trim as the flight progresses.

Say you've just departed and have trimmed nose-up for your climb. The hardware wheel is now set correctly for that particular attitude. You engage the AP for attitude and heading hold. All is good. Later, you press ALT to engage altitude hold. The AP trims for level flight, but the hardware trim wheel is still nose-up. Later, you start your descent by setting the AP for VS of -1000 FPM. The AP now trims nose-down and the hardware trim wheel is now WAY out of whack -- could be 20 degrees off from the AP trim now.

Now you disengage the AP to hand-fly and experience a very cool accelerated stall as the nose pitches almost instantly up 20 or more degrees. It's a wonderful experience. I highly recommend it at low altitude -- assuming your heart rate has suddenly dropped dangerously low level and you don't have any injectable adrenaline handy. It'll fix you right up.


Brett -- here's one fix: http://www.simkits.com/product.php?prodid=1798

-M.

[Keith Smith]

Mark,

I think you're right, which leads to...

discontinue the use of the hardware trim and swap to using a hat switch to make RELATIVE trim changes (ie, trim up, trim down), rather than the ABSOLUTE trim positioning system that you have now.

[Jeff N]

Brett, are you using the Saitek Cessna trim wheel? How do you like it other than the AP issue? I've been thinking about picking one up, and the AP issue won't affect me as I always hand fly.

[djrisc]

Jeff,
Yes, im using Saiteks Cessna trim wheel. I really like it! I actually solved the problem by just no longer using the AP.

Brett

[Jeff N]

Awesome, thanks Brett. I now know what I'll be spending my $50 Amazon xmas gift card on