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Garmin G3X, Trim and Autopilot

JDA_BTR

Well Known Member
After some troubleshooting I've made some changes to the auto-trim and wanted to report on it.

What I found was that with the recommended autopilot settings for my G3X system, when slowing down and entering an approach the system would not settle down on pitch/speed/glideslope. I would try to adjust power for a nice controlled descent and just couldn't get it to cooperate. Tried lots of things. Finally arrived at the theory that the auto-trim was overcorrecting; as in the autopilot would want more trim, move it too fast, and then end up having to come back, then overcorrecting the other way, etc. I don't know this for sure because the system doesn't report it happening in any way.

So I flew some approaches with the auto trim off, manual trim by me. Little trim movements did a lot. The approaches were satisfactory.

Flew some with auto trim on - hard to manage. So then I decided to slow the auto trim way down. I put 100% trim speed at 65 knots, and 20% trim speed (I think that is the setting) at 100 kts. Anyway, you get the idea, I slowed it way down. The other thing I did was to make the % of trim speed at those speeds a lot slower. My thought was to give the auto system more time to incorporate the trim changes in the control loop feedback computation.

Now it flies on approaches just like it should. I'm going to tweak it some but I'm pretty happy as is currently after today's test on a gusty day. Troubleshooting this has been a little difficult because you can only change some of these settings with a G3X config mode reboot and I've not wanted to do that in the air. So it has been somewhat iterative between ground stops at home.

Happy to answer questions. I'm going to keep playing with it.
 
Yes I’ll copy it when at the hangar next and post it. I’ve chased this a while and my requests to Garmin for a troubleshooting guide or how to work thru a/p optimizations were referred back to “known good” RV14 settings. But I’m glad I’m not the only one. I’ll look forward to hearing if the changes help you too as validation.

This plane is super fast and even with 20 flaps down little changes make big velocity changes. It hates to go slow. But I was able to easily hand fly the needles on approach and with occasional manual trim. It was the auto trim that made it bonkers.
 
I wanted to add something else too. In level flight the system would give little bumps that was like a little push every now and then. That went away too with backing down the trim speed.
 
Yes, and that is what everyone including Garmin tells you to look at when it doesn't work right. It isn't sufficient in all cases. If you are trying to go 85-90 kts on an IFR approach with 20 degrees flaps then 100% trim speed is way too fast. That is what I've proven to myself. I'll post what I did to make it better soon.
 
Make sure the elevator doesn't have any drag, it should 'freefloat', stiff controls will usually make the AP twitchy.
 
Thanks !!

Same problem and had to just put up with it because everyone was saying the Vans set up was correct. (Must be pilot induced build or flying issue)
 
Elevator very free on mine. I admit the washers weren’t stacked right when it came back from the paint shop but corrected as soon as it was home.
 
Okay, between posts here and PM offline, I count 5 people besides me with similar problem. So there are plenty of us to make changes and check back with results. The thought is that the autopilot overtrims and hunts when the trim is too fast; no hard data to go by just trial and error. Before this I found it very hard to control with throttle changes; with this the throttle changes were small and easier to manage. The plane didn't get behind the game the way it would before.

Here is what I did in the config mode for Trim, on the "Trim Speed" page for configuration:

Airspeed Threshold
Fastest Movement: 60 kt
Slowest Movement: 100 kt

Roll Trim Speed
Fastest: 50%
Slowest 20%

Pitch Trim Speed
Fastest: 50%
Slowest: 20%

Yaw trim speed (can't matter, no yaw trim motor)

Trim airspeed source: Indicated AS (didn't play with this)
 
That's an interesting problem.... since adjusting my settings it is pretty good. I do have to manage power a bit for thermals and gusts but in general it is very controllable. The power available in the IO390 seems to make a very narrow window for setting speed down the glidepath. Tiny changes have big impact.
 
G3X trim and auto pilot

My 10 used to work great until the last firmware update
Thanks for your input. I’ll go out and recheck the settings.
 
I've been struggling with G3X auto-trim on my -7A too, (thread), so far no dice. I'll be trying some of the hints in this thread.
 
I continue to find stiff elevators and fat trailing edges being a major contributor to AP/trim problems.
 
Thanks for posting your findings.
As an aside, in my RV-6 I fly an ILS at 100kt minimum with no flap (above flap speed). It is much easier to fly the approach at a higher speed. We're not flying airliners and do not need to configure for landing before the approach. No problem with slowing down once visual at the bottom.
 
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