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Elevator and Aileron Electric Trim

This article talks about installing the electric trim in a Vans RV-8.  Anyone installing a similar system in any airplane, especially another RV, will find this useful.

Electrical Installation

Zip tied trim controllersI am using a coolie hat switch on the stickgrip to activate the trim, so I had to buy two ES MK-11, combination speed controller and relay decks.  The kit comes with two rocker switches that need to be panel mounted.  If you are going to use a four-way switch instead, you will need two relays as a minimum (one for each channel).  I had heard from others that using just the relay made the trim too sensitive, and one small bump of the switch changed the trim too much.  So I opted for the combination relay/controller right from the beginning. I put these under the F-881PP-L Mid-cabin cover.  Initially I have them just zip tied out of the way.  After test flying, and getting them set correctly, I will mount them permanently there.  I haven’t decided exactly how to permanently mount them yet, but they will eventually go below the left mid cabin cover.

I used interlocking connectors from Radio Shack to connect the wires to the servos themselves.   I think this is a little cleaner than having six splices right next to each other.  Make sure you run the wires through the small hole in the EET-802 elevator trim bracket before you install the connector.  Take my word for it, the hole is too small for the connector to fit through.

I used the manual trim cable hole in the horizontal stab to run the wires for the electric trim.  It would be a lot easier to drill the holes and install the bushings before the stab is riveted together, but if, like me, you do it later, you can still install a bushing from the trailing edge of the main spar.

Mechanical Installation

Roll trim servoThe mechanical installation of the bracket was easy.  I did all the drilling and riveting before I riveted the floor ribs together. This let me use real rivets instead of blind rivets.  You could still buck the rivets if you are installing it after the fuselage is put together, but you might have problems with the four rivets across the bottom of the bracket.

I used a Dremel tool with a small grinding bit to enlarge the inside of the SB437-4 snap bushing enough to allow the F-8109 slide tube to move freely.

Roll trimInitially I riveted just the bracket and tube guide to the floor ribs.  I saved the rest of the parts until I was finally installing the control pushrods for the final time.  This was after all of the electrical system was complete, the wings were hung and the controls rigged. 

The hardest part was the final installation of the second spring for the roll trim.  I tried hooking the first end on the WD-807 and working the second onto the large cotter pin, but it ended up working easier the other way.  I hooked the cotter pin onto the spring, inserted it in the hole, then worked the second end onto the WD-807.  The first side of the spring hooked easy enough, but getting the second side hooked was tough.  I ended up using two needle-nose pliers to hold the spring as I flexed it into position. I just kept working it until it finally went.

Elevator trim pushrodInstalling the elevator trim was pretty uneventful.  It took a little filing to get the hole the right size to fit the bracket and electric trim motor, but nothing special.  I determined the length of the threaded rod by running the trim motor until it was in the neutral position.  Then I clamped the trim tab to neutral and measured the distance between the holes in the motor and the E-818EEP-1 bracket.

After everything was installed, I ran the trim full deflection one way and moved the controls full deflection in both directions.  Then I ran the trim full deflection in the other direction and again moved the controls full deflection in both directions.  That assured me that everything worked, and there was no binding anywhere.

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