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Old Mar 23, 2014, 10:24 PM
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EZ Money's Avatar
Nampa Idaho
Joined Jun 2006
397 Posts
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Elevator trim changes every flight

I have a PA Katana that flies fine except for the fact that I have to either add or take away elevator trim every time I fly it. Sometimes in the middle of a flight.

I thought it was excessive play in the elevator linkage but I changed all the linkage and added ball links.

I checked the CG between flights to make sure that the battery is in the same place. The elevator servo seems to center, at least on the bench.

Tried everything. Any suggestions?

Thanks in advance.

Ron
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Old Mar 23, 2014, 11:05 PM
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Doc Austin's Avatar
USA, FL, Largo
Joined Dec 2005
17,287 Posts
I just had a servo that was doing that and after I checked everything with no results, I finally changed the servo and that fixed it.
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Old Mar 24, 2014, 01:18 AM
I can't fly
camoy's Avatar
United States, CO, Parker
Joined Dec 2006
2,035 Posts
I agree it sounds like a servo centering issue when it is under load. Is it a digital or analog servo?
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Old Mar 24, 2014, 01:37 AM
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Depronicus's Avatar
Southern California
Joined May 2009
1,617 Posts
I dunno, but it seems like this has to be the arm slipping on the servo output shaft.
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Old Mar 24, 2014, 02:12 AM
GPT
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Sydney,Australia
Joined Dec 2004
1,601 Posts
Other than a dud servo, check that the servo isn't moving in the servo cutout in the fuse, and that there is no cracking in the wood in the servo hole area.
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Old Mar 24, 2014, 07:54 AM
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KE Spin's Avatar
Ohio
Joined Aug 2010
2,706 Posts
I had a similar problem and thought it was the servo but found that he control horn had actually come loose and was moving in the slot.
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Old Mar 24, 2014, 08:00 AM
This is NOT a TOY?
C_Watkins's Avatar
United States, GA, Atlanta
Joined Jul 2011
2,586 Posts
Just to make sure:
Go to full deflection, then 'ease' the stick back toward center, and see if it stays deflected some in that direction.
(Don't let the stick bang back to center. Ease it there.)
Then full deflection the other way, and ease back to center again, and see if it stays deflected some in that direction as well.
If it's really coming all the way back to center after each of those, then that's an odd one.
Possibly a dirty/bad/loose pot, either in the servo, or in your gimbal. Or some floating bit of debris, perhaps.
It's also 'possible' though unlikely, that there's some sort of thermal drift happening.
Honestly, I'd just fit a different servo first, and go from there.

Afterthoughts:
Have you given the control horn in the elevator a good wiggle, to make sure it's still secure?
Broken hinge(s)?
Loose/cracked wood in the elevator, or the joiner between the halves?
Solar flares, sunspots, zombie invasion? I dunno. Just brainstorming

Edited to add:
Quote:
Originally Posted by KE Spin View Post
I had a similar problem and thought it was the servo but found that he control horn had actually come loose and was moving in the slot.
I see someone else had the same thought while I was posting. Definitely seen the 'loose horn' thing before
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Old Mar 24, 2014, 10:34 AM
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EZ Money's Avatar
Nampa Idaho
Joined Jun 2006
397 Posts
Great responses. I will replace the servo and upgrade it to an 85MG.

Thx

Ron
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Old Mar 24, 2014, 01:03 PM
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GravyRobber's Avatar
United States, TN, Memphis
Joined Oct 2012
99 Posts
I'll second the suggestion the check the control horn. I had the same issue as you, tried everything, and eventually traced it back to a loose control horn. It's worth checking.
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Old Mar 25, 2014, 06:59 PM
KJ4WFG
modbuilder's Avatar
United States, SC, York
Joined Dec 2007
581 Posts
Am assuming that the trim changes are needed at the same throttle setting one time to the next ?
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