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Maximum play on elevator trim tabs?

rotoiti

Well Known Member
What should be ideally a maximum slack on the elevator trim tabs? When I am moving mine during a preflight, I can probably move them about 1/4 to 1/2 inch up and down (right one moves more than the left one).

I am trying to debug a problem where the autopilot would not hold altitude nor VSI of 0 but would repeatedly hunt up and down, with up to 500 fpm rates at some oscillations. Pushing the elevator slightly seems to make the oscillations less pronounced. There is no play on the elevator itself (from the stick to the surface), the play is present only on the trim tabs.

Could this play be the cause of the erratic autopilot behavior?
 
Are you saying that the trim tab will move up and down 1/4"-1/2" with everything connected? If so, I believe that is WAY too much. Mine only move a slight amount, just a wiggle.

Can you tell where the movement is coming from? Are the control cables actually moving?

Shannon
 
My free play on the trim tab is probably less than 1/16". If you are getting that much there is something wrong...
 
I suggest you check the nuts on the cable housing for tightness in their mounts.

The cable housing should be tied down securely at both ends.

If the plane was built with the old style welded nut/plate aft housing mount, they were know to have the weld fail. The machined units in this post are really nice........

http://www.vansairforce.com/community/showpost.php?p=1097352&postcount=76

Good luck, please chase this one down quickly--------a trip tab flopping around in flight is kinda scary to ponder:eek:

Personally, I would ground the plane until fixed.
 
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I'm not building an RV-10, but as a retired aerospace engineer, I've got to chime in here.

Play in a trim tab is one of the classic causes of flutter. There should be none.

Unless the plans say otherwise.

Dave
 
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Maybe check the WD-415 shown in the image above.
As others have said, I believe this to be a safety of flight issue. You have a control surface that is behaving in a way that is not intended to, nor do you know why it is doing this.
 
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Yes, there is a lot of play on the trim tabs when everything is connected. The airplane currently has 290 hours on it. It is hard to say where the slack is coming from. I have not yet removed the housing, the exposed rod is rigid.

I have not built this airplane myself so I don't have a good feel for how much these tabs should move but my impression (seems to be correct) is that they move too much.

The airplane is generally flying just fine except for this autopilot issue. On my flight back from OSH I connected the dots and figured the autopilot issue may be coming from the slack on the elevator trim tabs.

I most likely have the older style plate as the kit was purchased around 2007 AFAIK.

Thanks for all the replies. I will focus my attention on fixing the slack.
 
There is some slop in the actuating cable Vans supplies. If you have some time in ship the horns on the tabs will also wear. See the above for the cable.

I bushed the trim tab horns when they were built. As I recall, automotive brake lines are 3/16 I.D. and thin walls. Just cut a sliver about 3/16 long and glue it in with some JB Weld. Then file the tube down so the clevis fits closely. Will last the life of the airplane.
 
Manual trim or electric trim? Inherently the manual cable has some slop. Not a half inch, perhaps 1/8 -1/4 at trailing edge of trim tab.

What autopilot? Perhaps there are some gain settings to adjust.

There have been inspections that have failed due to too much slop in the trim tab.

Assuming the 10 is like the others, there should be an access panel under the elevator and you should be able to diagnose it prett quickly from there.
 
Flutter

Flutter is almost always fatal. A few examples: Cessna Conquest, Turbo Commander, Bonanza, T18,
Stephens Acro derived aerobatic monoplanes.
Get some qualified help and FIX IT. DO NOT fly it again until it is fixed.
 
Did some investigation today. Left trim tab (the one that is moving less) moves about 1/16 of an inch regardless of trim position.

The right trim tab moves about 1/16" when at full nose up position. At full nose down position it moves 12/32" and at mid setting it moves around 9/32".

I pulled the WD-415 covers, no slack there. I pulled the fairing and all movement seems to come from some lateral movement of F-1095B, which feels wobbly (side-to-side). Moving the right trim tab by hand causes the wobbling of F-1095B.

I am looking at figure 2 on page 11-9 and I am wondering if AN960-416L washers are properly installed.

Needs more work, but not today.

Bob, I will email you tomorrow, I am hesitant to fly the airplane, even to LVK at this moment.
 
On the subject of the autopilot the behavior sounds like the pitot and static input may be reversed. You can engage the autopilot on the ground in altitude hold and very gently blow in the pitot tube. If the elevator moves you have them wrong.
G
 
I would pull the whole trim system and make sure everything on the servo, arm, F1095B, etc are secure and safetied. If you have a missing cotter pin and the nut has started backing off the F1095B, that could cause what you are seeing. Also make sure the nuts holding the cables in the F1095G's and tight and have lock washers installed. That much play, as ours have said, seems like WAY too much.
 
Fixed!

On Jesse's advice, I pulled the whole trim mechanism.

There were two things generally wrong with it. First, the castle nut wasn't tightened at all, it was completely loose but secured with a cotter pin. Second, there was more space between the F-1095C angles than two AN960-416L could fill. Replaced one of the "L" (thin) versions with AN960-416 and slack was removed.

I tightened the castle nut on reassembly, made sure the servo still can run and there was no more lateral movement of F-1095B.

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Reassembled, lubricated, and there is no more play on both tabs than 1/16". Yay! Flight test some time next week.

BTW, pushing the trim cables through the horizontal stabilizer and through the elevator hole is hard. Get some help when doing this!
 
What should be ideally a maximum slack on the elevator trim tabs? When I am moving mine during a preflight, I can probably move them about 1/4 to 1/2 inch up and down (right one moves more than the left one).

I am trying to debug a problem where the autopilot would not hold altitude nor VSI of 0 but would repeatedly hunt up and down, with up to 500 fpm rates at some oscillations. Pushing the elevator slightly seems to make the oscillations less pronounced. There is no play on the elevator itself (from the stick to the surface), the play is present only on the trim tabs.

Could this play be the cause of the erratic autopilot behavior?

Great find on the trim tab! Maybe that will cure your auto pilot problem. Check that the pitot and static are not reversed at the auto pilot, if it has separate inputs. We will not discuss how I know to check this.
 
I probably should have mentioned this, but when you pull the cables out, push some thin wall pitot/static line onto the threads on each side and pull that through with them. Then leave them routed while you work on the trim, then push them back on the guide the trim cables back through. This will save a lot of hassle getting the trim back on. This can even be done for the first assembly, as it is easier to guide the pitot/static line through the necessary holes in the first place.
 
....First, the castle nut wasn't tightened at all, it was completely loose but secured with a cotter pin.....

Hmmm, might be a good idea to check all the other flight control connections on the airplane, too, just for peace of mind.

Dave
 
I tried blowing test today, gently (and less gently too), as suggested by sailvi767, no elevator movement.

Going to fly the airplane either Friday or Saturday, will report back.
 
I probably should have mentioned this, but when you pull the cables out, push some thin wall pitot/static line onto the threads on each side and pull that through with them. Then leave them routed while you work on the trim, then push them back on the guide the trim cables back through. This will save a lot of hassle getting the trim back on. This can even be done for the first assembly, as it is easier to guide the pitot/static line through the necessary holes in the first place.

So true - as I verified last night on my initial install. Now off to drill trim tab horns and rig things up so there is no discordant movement of the two trim tabs (one up/one down at same time). Thanks to Jesse and Tim O. for the respective hints/heads-up's.
 
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