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  #1  
Old 12-19-2018, 08:07 AM
bkervaski's Avatar
bkervaski bkervaski is offline
 
Join Date: Mar 2017
Location: Birmingham, Alabama
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Default G3X Touch Autopilot Autotrim .. Hunting

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!
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  #2  
Old 12-19-2018, 08:20 AM
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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.
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  #3  
Old 12-19-2018, 08:20 AM
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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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Last edited by Brantel : 12-19-2018 at 08:27 AM.
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  #4  
Old 12-19-2018, 08:26 AM
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bkervaski bkervaski is offline
 
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Quote:
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.

Quote:
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
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  #5  
Old 12-19-2018, 08:26 AM
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Steve Melton Steve Melton is offline
 
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sometimes simple airplanes are not improved by a complex trim system. keep an eye on the trim to avoid a runaway condition.
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  #6  
Old 12-19-2018, 08:28 AM
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Brantel Brantel is offline
 
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Quote:
Originally Posted by bkervaski View Post
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
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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---------------------------------------------------------------------
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  #7  
Old 12-19-2018, 08:41 AM
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BCP Boys BCP Boys is offline
 
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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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Last edited by BCP Boys : 12-19-2018 at 08:43 AM.
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  #8  
Old 12-19-2018, 09:30 AM
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g3xpert g3xpert is offline
 
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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
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  #9  
Old 12-19-2018, 09:49 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Walt View Post
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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Last edited by rvbuilder2002 : 12-19-2018 at 09:51 AM.
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  #10  
Old 12-19-2018, 09:54 AM
Bavafa Bavafa is offline
 
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Location: Sacramento, CA
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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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Last edited by Bavafa : 12-19-2018 at 09:59 AM.
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