What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

?Trim eye broken off by pushing Trim tab on stabilator

danielabernath

Well Known Member
ordered a new trim servo as I am told that the trim (F-1287A part number) has separated from
IMG_3878.JPG
the trim tab. Someone pushed on the trim tab on the stabolator. It is hanging down low and does not go up or down.
Am I on the right track?

IMG_3881.JPG


IMG_3880.JPG


IMG_3884.JPG


RV 12 Assembly Manual Section 11 http://www.vansaircraft.com/pdf/revisions/RV-12/Section_11.pdf

(the saga continues)
 
Last edited:
Looking at your pictures, it appears that your trim tab may have been incorrectly rigged. Note the indentation and wear marks on the actuator rod and mount. Something may have been incorrectly rigged and this may have contributed to the broken link. Alternatively, the actuator and mount may have been damaged when it was pushed?
 
Last edited:
Sorry to see that but thanks for sharing. I will be going back and double checking mine. When I do my preflight walk around I have a habit of taking all the flight controls through their full range of motion. In the future I will make sure I am moving the elevator and not the tab by hand.
 
It looks like the threaded stud was pulled out of the servo. That likely took quite a bit of force.
It would be a good idea to inspect the entire anti-servo tab system very carefully.....
If a lot of force gets applied to the anti-servo tab(s), this is not necessarily the first place I would expect damage to occur.
 
This brings into question another point of interest. Any thoughts about the controllability of an RV-12 in flight with the servo rod ripped out of the trim motor?? Maybe that rod to servo connection is the weakest link in that mechanism Anyway, that would be an interesting scenario to explore!!

Tom
 
A few months back we had an event with a Rans S-19 here that ended up on it's back while landing. It appears that the rod to the anti-servo tab came loose during a slightly firm landing/bounce. The aircraft porpoised 3 or 4 times, hit nose first collapsing the nose gear and flipping over. This is a similar setup to the RV-12. I think the guy was lucky it happened at touchdown.
 
This brings into question another point of interest. Any thoughts about the controllability of an RV-12 in flight with the servo rod ripped out of the trim motor?? Maybe that rod to servo connection is the weakest link in that mechanism Anyway, that would be an interesting scenario to explore!!

Tom

Edit: After inspecting a disassembled servo and getting re-familiar with its interior mechanics I realized it was not likely possible for the threaded shaft to pull out of the servo, so I went back and looked at the posted photos again. They are pretty dark, but this time I was able to see that the shaft is actually broken off where it exits the servo arm. My assessment is still the same... that some pretty excessive force got applied to the system which caused a failure.

There is always a weakest point in any system. The key is that the weakest point not be too weak.....

All the components of the anti-servo tab / pitch trim system underwent load tests for certification.
For this threaded stud to pull out, the loads had to have been pretty high.
I am aware of damage that has occurred to the anti servo tabs on an RV-12 because of someone "helping" push the airplane by pushing on them (yes, even with "no push" placards installed) In this instance, no damage was done to any of the other system components.

As for control-ability with it disconnected?
Saying it would be difficult to fly might be an understatement.
 
Last edited:
Scott,

I'm trying to imagine what the stick forces would be without the anti servo tab connected. Wouldn't the stab position be stable but with no feedback force trying to streamline it or would it being trying to go full deflection one way or the other?

Rich
 
I imagine that if the link pulled out of the trim motor, the servo tab would trail, unless the link caught on something. You might experience some tab flutter if it wasn't held in place, and had some wind load on it. It would seem that the airplane would still have pitch controllability via the stick. This is purely a WAG.

Tom
 
Sorry for the thread drift...

Lets define a few terms first....

Trim tab - an adjustable tab (in flight or manually on the ground) tab used to move a control surface in a direction we desire (to induce a trimming force).

Servo tab - a controllable tab used to move a control surface. I a sense a trim tab is also a servo tab, but servo tabs (by name anyway) are typically considered more of an active control function. Some large aircraft built prior to hydraulically boosted controls had servo tabs connected to the control system. The control system moved the servo tab which in turn drove the main control surface to the desired deflection (I think the B-29 was one airplane controlled this way).

Anti-servo tab - A tab whose movement is used to counter the movement of a control surface. This is typically done to artificially induce control force / feed back.

The RV-12 has an Anti-servo tab. It also functions as a pitch trim tab by having an adjustable neutral point, but that is a secondary purpose.

With a horizontal stabilator with the hinge point somewhere close to the center of pressure (as we have on the RV-12), there would be nearly zero control force / feed-back without the anti-servo tab. The anti-servo tab adds artificial feed back force by deflecting in a direction that tries to move the main surface back to neutral when it is moved by the pitch control system. That is why the tab constantly moves whenever the stabilator is moved.

In a nut shell.... if the anti-servo tab was non functional you would have little to no control force in pitch, which would probably make the airplane challenging to control.

I think a future release of the POH is going to cover recommendations for dealing with this. I think the primary one is to induce a different trimming force (deflecting the flapperons) that would then allow you to work the stick against the control force induced by the trimming input. It would be very similar to flying an airplane that is out of trim in pitch.... to change pitch you increase or decrease the pull force on the stick but it never gets to neutral/zero.
 
Scott, if the tab linkage failed in flight, would the tab be apt to flutter?

Not necessarily the high freq flutter we associate with structural failure, maybe just a low freq motion that would drive the stab neutral point around, and shake the stick.
 
Last edited:
It is possible, but has not been tested so can't say for sure.
There is a lot of variability in how the failure could occur (whether the mass of the controlling system was still connected to the anti-servo tabs or not), so it might be fine in one case but not in another.

There has been no indication that there is any "weakness" in any portion of the anti-servo / trim tab system.
Obey the "NO PUSH" placards and do a thorough pre-flight, and have fun.
 
Scott,

Good explanation. I would think slowing down would help since that would give some pitch down tendency like deploying flaperons. Is the center of lift for the stab right on the hinge line? If it is slightly ahead I would expect a tendency to pitch down as the stab leading edge would want to move up. Similarly if the center of lift was behind the hinge I would expect a tendency to pitch up. Does that make sense?

Rich
 
Disconnected anti-servo tab - playing with fire

The tail feathers on Comanche aircraft are near identical to the 12 - stabilator with anti-servo tab. Checking the integrity of the anti-servo tab actuating rod connections is now a vital part of Comanche pre-flights due to an occurrence in the UK many years ago. The bolt securing the tab to the rod worked loose during flight (the nut had been left off during maintenance). Fortunately, there were two pilots on board. It required the strength of both on the controls to get the aircraft safely onto the ground at a nearby field. The vibration had been such that both had blodshot eyes for several days afterwards.

The 12 may perform differently in similar circumstances, but I don't want to be the one to find out. Scott, I'd recommend against testing for this scenario. Based on the Comanche experience it seems too risky.

The Comanche maintenance manual calls for no more than 0.125" free play at the rear edge of the tab. My 12 has more than that, and I noticed at OSH last year that this is common to the type (with one or two exceptions). On mine the slop is in the bolt connecting the rod to the actuator. Seems like it needs a larger diameter bolt but not sure if the next size up would fit and drilling out the actuator clevis is forbidden by the manufacturer.

Clearly, there have been no problems with the setup on the 12 but I would be more comfortable with a tighter fit and less play, given the Comanche experience.

Jack Moore
 
The Comanche maintenance manual calls for no more than 0.125" free play at the rear edge of the tab. My 12 has more than that, and I noticed at OSH last year that this is common to the type (with one or two exceptions).

My anti-servo tab has about 3/16" free play at the trailing edge (after 85 hours). From the above it sounds as though this might not be unusual, but it would be good to know what the acceptable limit is so that it could be monitored during annuals. I don't recall this being mentioned anywhere in the documentation. The Maintenance Manual just says there should be "very little movement", but that could mean different things to different people.
 
Do you have 3/16" free play at the trailing edge when the stabilator is nearly level and the back edge of stabilator is even with the anti-servo tab?
 
Yes. When the trailing edges are in line with the stabilator about level, I can lift the edge of the tab up about 3/16" from the in-line position.
 
My anti-servo tab has about 3/16" free play at the trailing edge (after 85 hours). From the above it sounds as though this might not be unusual, but it would be good to know what the acceptable limit is so that it could be monitored during annuals. I don't recall this being mentioned anywhere in the documentation. The Maintenance Manual just says there should be "very little movement", but that could mean different things to different people.

I checked my 12 yesterday and have same condition you describe with about 3/16" play at trailing edge. I see lost motion in the horn / rod end on the anti-servo tab. I will tighten this bolt and I think problem will be resolved.
 
Add washers to the bolt

Jim -

Tightening the bolt where the F-1287E push-rod attaches to the horn may not be the fix for you .... because the nut is likely bottoming out on the shank of bolt. There was a thread about this here on VAF quite a while back.

The AN3-10A bolt used at that location is a tad too long. I ended up adding an AN960-10 washer under the head of the bolt and in addition to the AN960-10 washer that goes under the AN-365-1032 lock nut, I added an AN960-10L washer. In my case, doing the above allowed the shank of the bolt to pass through the horn and the additional washers prevented the nut from bottoming out on the AN3-10A bolt's threads and leaves about three threads showing beyond the lock nut. Also verify you have AN960-10 washers on both sides of the MM3 bearing ... (between the horn and the bearing).

Enjoy your 12
 
I checked mine the other day and found essentially zero play at 240 hours or so. The only play I can get between the anti-servo tab and stabilator seems to be from the hinge, not the trim servo linkage. Just enough to feel, not even enough to measure.
 
Jim -

Tightening the bolt where the F-1287E push-rod attaches to the horn may not be the fix for you .... because the nut is likely bottoming out on the shank of bolt. There was a thread about this here on VAF quite a while back.

The AN3-10A bolt used at that location is a tad too long. I ended up adding an AN960-10 washer under the head of the bolt and in addition to the AN960-10 washer that goes under the AN-365-1032 lock nut, I added an AN960-10L washer. In my case, doing the above allowed the shank of the bolt to pass through the horn and the additional washers prevented the nut from bottoming out on the AN3-10A bolt's threads and leaves about three threads showing beyond the lock nut. Also verify you have AN960-10 washers on both sides of the MM3 bearing ... (between the horn and the bearing).

Enjoy your 12

Went out to the plane today to resolve this issue and pretty much did as you say. A washer here - a washer there. Now virtually slop-free.

I bought my 12 from the original builder with 50TT. I knew of several discrepancies before I took delivery and this was factored into the price. I’m happy to say that today’s work, which culminated at the tail cone, completes a thorough examination of the airframe and supporting paperwork. I currently have no outstanding squawks. Remaining work are three service bulletins for prop hub, landing gear, and fuel tank. This work will be completed this summer.
 
AST followup question.

Scott, where the potential recommendations you spoke of made?

And, in plan English, what does the pilot experience when the AST becomes ineffective? Thank you! Doug in IL

Lets define a few terms first....

Trim tab - an adjustable tab (in flight or manually on the ground) tab used to move a control surface in a direction we desire (to induce a trimming force).

Servo tab - a controllable tab used to move a control surface. I a sense a trim tab is also a servo tab, but servo tabs (by name anyway) are typically considered more of an active control function. Some large aircraft built prior to hydraulically boosted controls had servo tabs connected to the control system. The control system moved the servo tab which in turn drove the main control surface to the desired deflection (I think the B-29 was one airplane controlled this way).

Anti-servo tab - A tab whose movement is used to counter the movement of a control surface. This is typically done to artificially induce control force / feed back.

The RV-12 has an Anti-servo tab. It also functions as a pitch trim tab by having an adjustable neutral point, but that is a secondary purpose.

With a horizontal stabilator with the hinge point somewhere close to the center of pressure (as we have on the RV-12), there would be nearly zero control force / feed-back without the anti-servo tab. The anti-servo tab adds artificial feed back force by deflecting in a direction that tries to move the main surface back to neutral when it is moved by the pitch control system. That is why the tab constantly moves whenever the stabilator is moved.

In a nut shell.... if the anti-servo tab was non functional you would have little to no control force in pitch, which would probably make the airplane challenging to control.

I think a future release of the POH is going to cover recommendations for dealing with this. I think the primary one is to induce a different trimming force (deflecting the flapperons) that would then allow you to work the stick against the control force induced by the trimming input. It would be very similar to flying an airplane that is out of trim in pitch.... to change pitch you increase or decrease the pull force on the stick but it never gets to neutral/zero.
 
Back
Top