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Old 05-31-2017, 03:54 PM
jliltd jliltd is offline
Join Date: Sep 2013
Location: Rancho San Lorenzo
Posts: 341
Default RV8 Elevator Trim Servo Question (curious)

We have an RV-8A here that we did the elevator spar service bulletin on to reinforce around one of the hinges where a crack developed.

Unfortunately we cannot figure how the servo is attached when re-assembling the elevator. The guy that removed the elevator is not here. So on with the questions:

The servo appears to be a Ray Allen 8A servo (gold colored part marking label is covered). This servo has a threaded rod sticking out of the thick end where a Heim joint attached through the spar itself. This is also the opposite end of the servo from where it actuates the trim tab (on right side of first photo below). Here are some photos:

The curious thing is that the Heim ball end doesn't appear to be connected to anything. So we are not sure there might be something missing here. It's as if the Heim joint was either abandoned at an earlier time or is missing a link.

Here is a shot of the cove where the trim servo attaches:

There is some protective rubber material around the base of the Heim where it passes through the spar:

Here is a photo of the Heim poking out of the front spar of the elevator:

Any insight into this configuration along with the theory behind having the threaded rod protruding from the servo would be very helpful.



Last edited by jliltd : 05-31-2017 at 04:53 PM. Reason: Caption Clarification
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Old 05-31-2017, 04:35 PM
Pat Hatch's Avatar
Pat Hatch Pat Hatch is offline
Join Date: Jun 2006
Location: Vero Beach, FL
Posts: 803

Jim, I've never seen anything like this before. Have no idea why the rod end is there. The trim servo is a jack screw type motor so, of course the jack screw moves in and out to actuate the trim. The hole in the spar with the snap bushing is there to allow the jack screw to extend through the spar. I have no idea why anyone would attach a rod end to it! I say remove it and press on!
Pat Hatch
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Old 05-31-2017, 04:44 PM
jliltd jliltd is offline
Join Date: Sep 2013
Location: Rancho San Lorenzo
Posts: 341


I am with you. I think this aircraft may have been converted from manual trim to electric trim and some Gomer re-used the original manual trim Heim to plug the hole. I have seen weirder things but this is right up there in the top 10.

Note that the actuation of this trim servo is via the plastic clevis rod link to the right in the first photo above. I don't know if maybe the threaded rod with check nut on the opposite side (subject item) is used to adjust the servo or perhaps allow another servo to move the trim servo back and forth in a more complicated arrangement.


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Old 05-31-2017, 05:58 PM
Robert Anglin Robert Anglin is offline
Join Date: Mar 2008
Location: houston, texas
Posts: 786
Default Your on the right track.

This servo does not attach to anything on the front side. You can hock up the wires to a 9 vote battery and run it back and front. I think you may be on to something with the thought that it was once a cable set up and is now electrical. The wiring diagram I think can be found on Ray Allen's Web site.
The drag link that goes out to the trim tab is all that hocks to anything on this set-up. I would check the unit with a battery before you put it back in place.
good luck, Yours, R.E.A. III #80888
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Old 06-01-2017, 06:20 AM
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Jaypratt Jaypratt is offline
Join Date: Jan 2005
Location: Hicks Airfield, Fort Worth,Texas
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Default Trim

Remove the nut and bearing. They shouldn't be there
Jay Pratt VAF #2
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Old 06-01-2017, 06:49 AM
jliltd jliltd is offline
Join Date: Sep 2013
Location: Rancho San Lorenzo
Posts: 341

We got it all straightened out. Apparently the person who installed the electric trim made the ponderous decision to use the rod end as an end cap plug for a piece of rubber boot protecting the jackscrew threads. So the rod end wasn't screwed onto anything. It was just banging around on the end of the boot along the hinge line in the open area between the stabilizer and the elevator. It was as sensible as using an anvil as a canopy cover. That's beyond bizarre and is very misleading and confusing for a new owner or even seasoned A&P who thinks they've seen most anything.

One face of the Heim was worn down from chafing as it rode along inside the D section of the elevator flopping around on the end of the rubber boot. This ignorant act is a safety issue and makes one ponder about what other parts of the airframe were touched by the same person.

I eliminated the rod end, rubber boot and check nut and installed a vinyl protective strain relief boot supplied by Deutsche for its DTM series connectors. The new boot is sealed and pokes through the elevator spar and encapsulates the jackscrew throughout it's range of motion without hinderance.

Thanks again for your comments.


Last edited by jliltd : 06-01-2017 at 09:31 AM.
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