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Anti-servo tab free play

How much slop?

  • 1/16"

    Votes: 20 60.6%
  • 1/8"

    Votes: 6 18.2%
  • 3/16"

    Votes: 7 21.2%
  • 1/4"

    Votes: 0 0.0%
  • 5/16"

    Votes: 0 0.0%
  • 3/8" or more

    Votes: 0 0.0%

  • Total voters
    33
  • Poll closed .

rgmwa

Well Known Member
The anti-servo tab on my RV-12 has about 3/16" of free play measured at the trailing edge. In other words I can lift the aft edge of the tab up 3/16? of an inch relative to the aft edge of the stabilator. The `slop? just appears to be the result of the small tolerances in the various hinges of the AST arm. The Maintenance Manual says there should be "very little movement" here, but what does that mean? I'm interested to know how much movement others have found.
 
Well, 15 out of nearly 500 RV-12's flying is only a small sample, but it's a useful start. Thanks to all those who have responded so far. No doubt Vans would be interested in this information too, so hopefully others will chime in over time. I don't know how much free play is acceptable before we need to be concerned about flutter or other unpleasant consequences, but there is clearly some variation between aircraft. I assume that wear will likely result in increased movement over time, so it would be nice if Vans could provide some more specific guidance in the form of an SB or Notification so we know when we need to do something about it.
 
This makes me wonder if the airplane would stay trimmed up longer if the slop in the system were to be removed?
 
I had the OHIO UNIV aviation dept chairman fly with me a couple years ago and he commented how well the controls felt We both determined that the stabilator had a slight, and I mean very slight "oscillation" on the pitch axis. Not a bad thing as he compared it to the same feeling on a Piper Warrior (and others) stabilator. Cruise flight hand flying.

I would guess that cable tension, slop in the hinges, trim setting may all come into play to attest to the movements.
 
Kyle,

I've never felt an oscillation when hand flying my 12, or, for that matter my Cherokee 180 which also has a stabilator. Did he say whether he thought it was normal?

Rich
 
He said it felt just like the warriors ... then asked if the -12 had a stabilator. Ha. I didn't even notice until he brought it up. Hands off the stick there was no visual movement but perhaps something he noticed. He has ten thousand some odd hours mostly in king airs and Blackhawk helos... military guy.

Also ... did check the play on the AST just today at it's at the 1/16 measurement.
 
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Likewise here...once trimmed, mine flys straight and true darn near hands off (in smooth air). Only had to add Van's rudder trim tab to center ball in cruise flight.
 
I assume that wear will likely result in increased movement over time, so it would be nice if Vans could provide some more specific guidance in the form of an SB or Notification so we know when we need to do something about it.

There are hundreds of different certificated airplane models that don't have a specific specification published for the amount of acceptable play on each portion of their control systems.
It is not reasonable to expect different for any of the RV's.

The burden is on the inspector to have some level of training and experience to make a determination of what is right and what is not.

For E-AB, the assumtion is that the builder knows what everything was like when it was new, so the FAA considers them eligible for a repairmans certificate (I agree with this position)

For E-LSA, an owner could have potentially purchased an airplane that was 95% completed (possible with the E-LSA build requirement rules) so they may not be as familiar with all the details of the airplane. This is part of the reason that for E-LSA you have to take a class to train you on inspecting an airplane.

If you have the training (either being an A&P or Light Sport Repairman Inspector) you are expected to have the enough knowledge to evaluate a mechanical assembly like the RV-12 AST / pitch trim system and find any portions of the system that have more play in them(ball rod end, etc.) than they probably should have.

Based on my experience with the RV-12, I would say that 1/16" play is normal.
1/8" play is an indication of some wear and should probably be looked at during the next inspection.
3/16" is quite excessive and I probably would not fly that airplane until I corrected it.
(it is very possible that the votes for 3/16" were just a guess and it is not nearly as bad as that. The only way to know for sure is to hold the stab. against the nose down stop and then measure the free play with a ruler).
 
Based on my experience with the RV-12, I would say that 1/16" play is normal.
1/8" play is an indication of some wear and should probably be looked at during the next inspection.
3/16" is quite excessive and I probably would not fly that airplane until I corrected it.
(it is very possible that the votes for 3/16" were just a guess and it is not nearly as bad as that. The only way to know for sure is to hold the stab. against the nose down stop and then measure the free play with a ruler).

Thanks Scott. I appreciate your input. I don't expect Vans to do more than other aircraft manufacturers do, and I understand that as builders we're supposed to know what we're doing and back our own judgement. That's what led me to question what I was seeing on my aircraft. The reason I started this poll was to try to find out what is normal for an RV-12, and hopefully it's provided us all with some useful information.
 
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I don't think it is a question so much of what is normal for an RV-12. There are a bunch of components in the system, rod ends etc. Those all have certain accepted tolerances. It is assumed that when new, there won't be a lot of slop in the total end to end system. This would apply to any of the control surfaces or engine controls.

In some cases it could be geometry where at large deflections and angles a very small movement of a pushrod moving a horn generates large deflections. IN a case like this a bit of slop could result in a lot of moment. But it sounds like you are getting this near neutral. So some clevis pin has more than the usual amount of clearance, or a rod end is loose, or something. That is not an issue for the mfg (usually), that is an issue for the builder or maintainer.

By questioning the 3/16" of movement you are showing that you already have a feel for what is reasonable and what is not - 3/16" seems too high to me too. So use the same mind set and go through the entire circuit end to end and identify the slop in each component or perhaps there is a bit of structure that is flexing which causes this movement. You might find that there is no individual culprit, or you might find that there is a faulty component that needs to be replaced.

I don't think anybody can tell you that some specific bad thing will happen with 3/16" of slop vs 10% less, unless there have been previous events caused by excessive slop. It comes down to judgment and trial and error. If something doesn't look right you investigate. Vans expects you to use normal practices for rigging and maintenance.

If there turns out to be a known problem with a type, like the fiasco with the Zenith airplanes that were suspected of flutter due to aileron cable tension, THEN the designer would issue specific guidance on rigging or modify the design (which they did, but that is very long story). But until a tab causes a problem you do what everyone else does and I think you are right to question that amount of movement.
 
Thanks Scott. I've checked the AST linkages before and couldn't pin it down to any specific component, but it's clearly time for another look. To respond to an earlier post, I've never noticed any problem adjusting or holding trim, or any other adverse effects.
 
I was one of the reporting 3/16, but when I did a check today the way Scott said to do it, 1/16 is more like reality.

3/16" is quite excessive and I probably would not fly that airplane until I corrected it.
(it is very possible that the votes for 3/16" were just a guess and it is not nearly as bad as that. The only way to know for sure is to hold the stab. against the nose down stop and then measure the free play with a ruler).
 
Well, there are four separate hinge points in the AST system, so to have zero net slop in the entire assembly would be unusual. Anyway, I checked mine again today. My aircraft only has 100 hours on it, so wear and tear should not be a big issue. I checked all the arm dimensions as per the plans and they were correct, and I couldn't find any obvious signs of wear and tear. However after checking everything, I tightened the lower AST arm pivot bolt another notch and the same with the servo arm pivot bolt. They weren't loose before but are now a bit tighter, and that seems to have roughly halved the tab free movement. I'm comfortable with that, so thanks to all who responded, particularly Scott McD and Scott B.
 
Learnt a lot

Thanks for initiating the survey, Rob. I was one of the ones who reported 3/16" - could have even been a whisker more - and it seemed to be getting worse. I was prompted to figure out where the problem was and do something about it. You are right in that there are four hinge points and if there is even a minute amount of play in all four it amplifiies movement in the tab. But in my case most of the play was in the clevis that threads on to the servo with one hole worn more than the other. I ordered a new clevis, and bolt just in case, but found there was zero wear in the bolt. Problem solved with movement reduced to 1/8" or slightly better. The worst hole in the clevis was elongated vertically.

During reassembly I found something interesting. While rocking the push rod assembly from side to side to find the centrepoint before tightening the jam not on the new servo clevis I found there was more movement one way than the other. This may explain why one hole was worn more than the other. My first thought was that I should order a new pushrod and the clevis plates that rivet to it, but on reflection I may only need to loosen the jam nut and realign the bearing that connects the tab. Will try that first. New clevis plates would probably be helpful, however, as there is some wear in the bolt holes - probably enough to get me back to 1/16".

I can't detect any difference in the controls but very reassuring to have it tightened up. It just didn't feel right although I'm sure the assembly was still perfectly safe.

Jack
 
Regularly checking a couple RV-12s for this, as well as a Cherokee I own, I can tell you that hinges that start to show signs of wear do so in the manner described in the OP. I recently replaced the anti-servo tab hinges on my Cherokee's stabilization for that reason. My A&P told me basically, if it moves other than the rotational axis it's a problem to keep an eye on and/or repair depending on extent. Hinges are "wear" components. Since the hinge is typically steel-and-aluminum is assembly, the aluminum will wear - kind of like the eyelets on the hinges used to hold cowls on the aircraft, something to keep an eye on.

My rule of thumb is, if there is enough play to make noise when I grab the tab and pull/push on it, there's a wear problem serious enough to plan/consider fixing.
 
Got it to 1/16"

I loosened both jam nuts in the assembly today and lined everything up properly. The push rod had been canted a little to one side and the bearing on the stab connection was also a little off. With everything dead straight (and a new clevis on the servo as per previous post) I now have no more than 1/16" movement. It's finally where I felt it should be all along.

Jack
 
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