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  #1  
Old 02-19-2021, 09:43 AM
JackinMichigan's Avatar
JackinMichigan JackinMichigan is offline
 
Join Date: Nov 2012
Location: Canton, MI
Posts: 237
Default Autopilot elevator control 'porpoising' quite badly.

I have an entirely Garmin control panel in my RV-10 and one of the features is an autopilot (roll and elevator, no rudder). The roll control works great, but when I engage altitude hold the plane will begin 'porpoising', and it will get exponentially worse till, after three up-down cycles, I have to disengage the autopilot. I've reduced the sensitivity, reaction speeds and reaction forces of the elevator servo but nothing seems to help.

One thing I've noticed is when I'm adjusting elevator trim, the trim indicator lags way behind the actual trim motion. Not sure why that is, but the entire electric trim assembly is buried deep in the tail with no good way to get to it. I suspect this has a lot to do with it.
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  #2  
Old 02-19-2021, 09:54 AM
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larrynew larrynew is offline
 
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Default

I used the settings from this post. Now works great.

https://vansairforce.net/community/s...72&postcount=7
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  #3  
Old 02-19-2021, 10:03 AM
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DanBaier DanBaier is offline
 
Join Date: Mar 2009
Location: Rochester NY
Posts: 703
Default

If the tweaks don't work, you might have a problem with the static source - there can be a reservoir effect with too many gadgets hooked up and then the altitude hold does a lot of hunting and can't keep up. If that turns out to be the case, you'll want to consider a dedicated static line for the autopilot.
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  #4  
Old 02-19-2021, 10:35 AM
woxofswa woxofswa is offline
 
Join Date: Sep 2007
Location: Mesa Arizona
Posts: 662
Default

I have Dynon, but have experienced similar symptoms. Sounds to me like your servo shear screw has broken. There can still be some friction on the plate, especially if the screw broke but left a little chunk of itself behind that is scraping on the plate. It works a little bit but loses grip and keeps trying to correct itself.
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  #5  
Old 02-19-2021, 10:35 AM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
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Quote:
Originally Posted by JackinMichigan View Post
I have an entirely Garmin control panel in my RV-10 and one of the features is an autopilot (roll and elevator, no rudder). The roll control works great, but when I engage altitude hold the plane will begin 'porpoising', and it will get exponentially worse till, after three up-down cycles, I have to disengage the autopilot. I've reduced the sensitivity, reaction speeds and reaction forces of the elevator servo but nothing seems to help.

One thing I've noticed is when I'm adjusting elevator trim, the trim indicator lags way behind the actual trim motion. Not sure why that is, but the entire electric trim assembly is buried deep in the tail with no good way to get to it. I suspect this has a lot to do with it.
Disengage auto trim and try it, sounds like maybe the auto trim is running backwards.
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EXP Aircraft Services LLC
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  #6  
Old 02-19-2021, 11:11 AM
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JTJones JTJones is online now
 
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Default What Walt said,

or it could also be the pitch servo direction needs to be reversed. I had that problem when testing my autopilot. I had the auto trim off as recommended. Reversed the pitch servo direction and was good to go.


Edit: After thinking about it, it was after activating the auto trim that I had the issue. Reversing the auto trim direction corrected the problem.
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Last edited by JTJones : 02-19-2021 at 04:50 PM. Reason: After thinking about it I need to revise my post.
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  #7  
Old 02-19-2021, 05:41 PM
gfb gfb is offline
 
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Location: Madison, WI
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It’s not the shear screw, Garmin servos use a clutch.

Go through the calibration routine described in the manual.
Check that all the surfaces are deflecting in the correct directions.
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  #8  
Old 02-20-2021, 10:26 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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I agree with trying it without the auto trim to eliminate any issues on that system. On my 10 with G3X, I set up the AP with the recommended settings and found that I needed a good amount of change to the gain on the ptich axis to get it to be stable. Can't remember if it was porpoising or just twitchy. Either way, I ended up with a pitch gain that was quite a ways off from the recommended setting. Mine is rock steady.

Garmin has a very involved regiment for dialing in the AP in an appendix of their manual. Suggest reviewing that.

Larry
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  #9  
Old 03-15-2021, 11:08 AM
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Dugaru Dugaru is offline
 
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Location: Richmond VA, USA
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Default Adjusting

Do you have a link? I've got a slight porpoising problem (but only at high altitudes, 10K or so) that I need to troubleshoot. I also overshoot turns a bit.

Quote:
Originally Posted by lr172 View Post
Garmin has a very involved regiment for dialing in the AP in an appendix of their manual. Suggest reviewing that.

Larry
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  #10  
Old 03-15-2021, 11:21 AM
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Walt Walt is offline
 
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Location: Dallas/Ft Worth, TX
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Quote:
Originally Posted by Dugaru View Post
Do you have a link? I've got a slight porpoising problem (but only at high altitudes, 10K or so) that I need to troubleshoot. I also overshoot turns a bit.
Can be found in the G3X install manual.
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Walt Aronow, DFW, TX (52F)

EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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