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G3X Touch Autopilot Autotrim .. Hunting

bkervaski

Hellloooooooo!
Testing
I've followed the Garmin G3X Touch Autopilot tuning guide. The AP works great and doesn't seem to work to hard to do its thing.

As a last step have enabled the autotrim.

Occasionally, the elevator autotrim hunts the elevator trim trying to capture an altitude, especially on initial activation of the AP. This results in the aircraft over and undershooting the altitude for a bit.

Seems like a gain thing but the AP is actually working quite well.

If I put the plane at the altitude prior to enabling the AP it doesn't hunt and can keep it there.

Is this normal? I'm new to an AP with automatic trim, no frame of reference.

Thanks for any advice!
 
I just fixed an RV14 with a similar complaint, turned out the elevator was so stiff that the AP was having trouble overcoming the friction and kept over and undershooting. If your elevator doesn't "free float" than that may be causing your issue.
 
Should be rock solid.

I think one would notice this quickly in flight but I would double check that your trim control signals are not reversed.

Also check the airspeed threshold and speed percentages. I think mine are still on the defaults which seem to work well with my airplane. Yours may be too fast.
 
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I just fixed an RV14 with a similar complaint, turned out the elevator was so stiff that the AP was having trouble overcoming the friction and kept over and undershooting. If your elevator doesn't "free float" than that may be causing your issue.

Elevator is very smooth and free. I would love to know what final numbers for the servos you had on this for comparison if it's not too much trouble.

I think one would notice this quickly in flight but I would double check that your trim control signals are not reversed.

Yea, did this already .. a bit scary when it starts pitching straight down :D
 
sometimes simple airplanes are not improved by a complex trim system. keep an eye on the trim to avoid a runaway condition.
 
Elevator is very smooth and free. I would love to know what final numbers for the servos you had on this for comparison if it's not too much trouble.



Yea, did this already .. a bit scary when it starts pitching straight down :D

Also check the airspeed threshold and speed percentages. I think mine are still on the defaults which seem to work well with my airplane. Yours may be too fast.
 
numbers

Bill,
These are the settings the I have in my -7. Pretty much solid but it may vary for you...

Roll Servo
Max Torque = 15%
Servo Gain = 0.40
Fine Adjust Amount = 1
Fine Adjust Time = 0.20

Pitch Servo
Max Torque = 30%
Servo Gain = 1.30
Fine Adjust Amount = 1
Fine Adjust Time = 0.20

Pitch Gain
Min Airspeed Limit = 65 kt (or as preferred)
Max Airspeed Limit = 180 kt (or as preferred)
Vertical Speed Gain = 1.00
Vertical Accel Gain = 1.00
Airspeed Gain = 1.00
Airspeed Accel Gain = 1.00

Electric Trim
Airspeed Thresholds = 80 kt / 150 kt
Roll Trim Motor Speed = 100% / 25%
Pitch Trim Motor Speed = 100% / 25%




I've also seen some posts with these numbers - You just have to figure out what's it doing wrong before making your adjustment.


Roll Torque 30%
Roll Servo Gain 0.65
Pitch Torque 60%
Pitch Servo Gain 2.15
Min Airspeed Limit 60 kt
Max Airspeed Limit 170 kt
Vertical Speed Gain = 1.00
Vertical Accel Gain = 1.00
Airspeed Gain = 1.00
Airspeed Accel Gain = 1.00
 
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bkervaski,

You should not see this behavior. When properly setup the autopilot should not hunt and should feel rock solid.

Please send us an email including your autopilot and autotrim settings as well as a flight data log of the autopilot hunting.

Thanks,
Levi Self

Team X Project Manager
 
I just fixed an RV14 with a similar complaint, turned out the elevator was so stiff that the AP was having trouble overcoming the friction and kept over and undershooting. If your elevator doesn't "free float" than that may be causing your issue.

Walt beat me too it....
My first question was going to be how free is the movement of the elevator?

Often times what people think is free and no friction (they are usually comparing to certified airplanes they have flown), is far from that.
 
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Here is my numbers for the A/P setting and I don't believe it hunts at all.


Roll Servo •Max Torque 50%
•Servo Gain 1.20

•Pitch Servo •Max Torque 50%
•Servo Gain 1.40

•Pitch Gain •Min Airspeed 65 kt
•Max Airspeed 190 kt
•Vertical Speed Gain 1.50
•Vertical Accel Gain 1.50
•Airspeed Gain 1.50
•Airspeed Accel Gain 1.50

•Trim •Airspeed Threshold 100 kt/150 kt
•Roll trim motor speed 100%/30%
•Pitch Trim motor speed 100%/40%
•Roll Auto Trim Min Speed 10%
•Pitch Auto Trim Min Speed 5%

For trim run time, I have set it to 6 seconds. Initially was two seconds and it was not nearly enough. I feel the elevator a lot heavier than RV7 but only in flight and specially in the higher speed. I have no idea if this is normal for the 14 and expected. It is free and smooth on the ground and feels like the 7 (on the ground)
For my roll, I have not set any limit time as it is not nearly as sensitive to adjust.

I have confirmed that my auto trim works.
 
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Often times what people think is free and no friction (they are usually comparing to certified airplanes they have flown), is far from that.

It's definitely free and no friction, I can pinky-push it up and down with no effort. Same positive and negative. Very balanced.

Compared to the Cherokee I was flying it's like magic and unicorns.

I'm thinking it's just something with my numbers. Going to take pics and send them to G3Xpert and see what they say.
 
I'm an autopilot and trim guy as my day job.

EDITED OUT what I initially said, I saw what handles the other guys have to tuning trim.

If they are running 100% and 25% duty cycle for their trims at those speeds and everything is fine, then maybe it's not the trim parameters.

The other thing would be if maybe the ALT CAP and/or servo gains are too high and they end up using more torque than you need. But you would feel that as it would be pretty sporty.
 
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bkervaski,

You should not see this behavior. When properly setup the autopilot should not hunt and should feel rock solid.

Please send us an email including your autopilot and autotrim settings as well as a flight data log of the autopilot hunting.

Thanks,
Levi Self

Team X Project Manager

Follow the instructions and set the speed for auto trim start low and adjust your % speed for the trim as speed increases.

I would not use settings from a 7 for the 14, nor use someone else settings - use the tuning procedure from Garmin.
 
RV 14-A

Thank you Walt for fixing this same problem on my RV14-A last month.

The bolt that connects the elevator push rod to the tail of the airplane was not perfectly adjusted and it was too tight. The pitch servo could not maintain the selected altitude until after the elevator was properly adjusted.

After adjusting this bolt the elevator moved with much less resistance and fixed the problem altitude hunting. I used the Van's recommended autopilot settings for the Garmin G3 touch. Contact Van's for those numbers.
 
Thank you Walt for fixing this same problem on my RV14-A last month.

The bolt that connects the elevator push rod to the tail of the airplane was not perfectly adjusted and it was too tight. The pitch servo could not maintain the selected altitude until after the elevator was properly adjusted.

After adjusting this bolt the elevator moved with much less resistance and fixed the problem altitude hunting. I used the Van's recommended autopilot settings for the Garmin G3 touch. Contact Van's for those numbers.

Now I am a bit puzzled by this, would you mind elaborating what adjustment is there with the bolt?
The AN3 bolt that connects the pushrod to the elevator horns go thru a bearing therefore should move freely. Unless there is an extra gaps between the horn and bearing in such way that pulls the horns out of its natural position, I can't see how/what other adjustments are there.

As I recall, mine took two washers (one on each side of the bearing) to align and also have proper shimming for the horn.
 
I used the Vans numbers and found the pitch corrections too aggressive and toned it down by a third and it's much better now. Not as smooth as the roll servo, but close and I think I can tweak it some more to get it just right.
 
It's definitely free and no friction, I can pinky-push it up and down with no effort. Same positive and negative. Very balanced.

How much weight did you end up using to balance your elevators after paint? As I understand it, they need to be balanced individually and not connected together.
 
If it's not too much trouble, for a sanity check and comparison, can some of you 14 guys post your G3X Touch AP settings?
 
Now I am a bit puzzled by this, would you mind elaborating what adjustment is there with the bolt?
The AN3 bolt that connects the pushrod to the elevator horns go thru a bearing therefore should move freely. Unless there is an extra gaps between the horn and bearing in such way that pulls the horns out of its natural position, I can't see how/what other adjustments are there.

As I recall, mine took two washers (one on each side of the bearing) to align and also have proper shimming for the horn.

Actually this was the age old problem of the elevator AN4 center support bearing bolt not being properly shimmed. Without the correct spacers installed, when the center bearing bolt is tightened it side loads the elev hinge bearings which then bind the elevators. On a correctly installed elev, all the bolts are tight and the elev continues to "free float".
 
Is the G3X autopilot compatible with the Van's provided elevator trim servo? Or do I need to buy a different servo if I want my autopilot to trim the plane?
 
Bill, did you get this corrected??

Bill Did you get this all tuned in??

It might not be the autopilot. There are three things that can contribute to this.
1. autopilot settings, already addressed with G3X instructions.
2. Trim speeds and ranges - the plane can climb and accelerate faster than the trim can adjust for.
3. Setting for low end of the auto trim control. You may have to set this as low as 50 kts.

Here's what happens. You set the autopilot to climb, then it goes into ALTS mode as it levels off and may hit ALT mode. As it accelerates, the trim must keep up. If it is too late (for the trim to catch up) the servo torque gets too high and can not hold well, then it will exceed the torque limit and suddenly nose dive.

You will have to study your data, if you did not record then go fly and reproduce the issue, it will reproduce if you set a fixed speed climb from say 1000 to 4000 ft at 100 kts. Or what ever you did before. I helped a 10 friend and this happened several times. It took an hour looking at the data to figure out what happened. It WILL tell you.
 
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