Wiring Elevator Trim

Ok so my build project hasn't been the Fastest RV-7 Build in the world, but today I decided to turn over a new leaf and get cracking on what is left to do.

I also decided the reason for my stalled project was the lack of details on the avionics stage of the build, so I have resolved to document best from here on I can so others don't have to reinvent what I have done if they want to follow my example.


Wiring Elevator Trim 24 May 2016

Today I did some final wiring for the Elevator Trim.

The design was prompted by helping with an RV8 tail repair when the tailplane needed to be removed. Because of the wiring it was necessary that the the trim servo hatch was opened and disconnected then the elevators were split then the left elevator removed. Then the stripped down connector could be threaded through the holes before the remaining section of the Tailplane could be removed.

I decided adding an extra connector would be worth it even with the downside of an extra potential failure point. This would enable the entire tailplane to be removed in one piece.

Here is the extra connection fully installed:

Elevator Trim Connection

I made a little bracket to guide the cable up through the Aft Deck. This was from a strip of aluminium formed around a drill bit fitted with a nutplate. I used a few inches nylon airline to guide the cable and prevent chaffing.

I made another bracket with two plate nuts attached to carry the panel mount Plug. After spending an hour or two making it I found the hole for the plug was a little too big and the plug rattled about a bit so I backed up the push fit ratchet with epoxy glue to stop the rattling.

Others have used soldered or crimped 9 way D-types here. I chose Molex Mini-Fit-Jr. for the connector you are looking at the back of what they call the free hanging receptacle. part nr 39-01-2065 and this mates with the Panel Mount Plug PN 39-01-2066.

You can buy a kit of these connectors from RS/Allied here is mine with some extra 22-26 AWG Male/Female Crimp Terminals that you need for these smaller gauge wires:

Molex Mini-Fit Jr. Connector Kit P/N 76650-3011 with extra 22-26 AWG Male/Female Crimp Terminals

You will need also the correct crimping tool tool see in top right of next shot it is called an "Engineer PA-21 Universal Crimping Pliers" don't use any old crimp tool for this you will get unreliable crimps. It is a none ratchet type and sells for £20-£40 on amazon and Ebay.

PA21 Crimp Tool

Next I set about making off the connector and the wiring section.
There are moving parts all around here so the less unrestrained cable the better. fitting the trim servo assembly through the hatch is a challenge as there are mm of clearance if done in the correct sequance - have fun!

I decided on another bracket to restrain the connectors here. The only place to put it is on the back of the trim servo. There is extra heat shrink for the section that passes between the elevator and the tailplane.

Trim tab Servo Assembly

Trim tab Servo Connector Bracket

Spot the deliberate mistake - the tie wrap should pass under the latch clip which then stops the clip springing open even when pressed. Here it is over it which will release the latch.

Don't do this!

here is the servo fitted into the elevator

Threading Elevator trim wires

I fitted a bushing into the hole in the trailing edge of the tailplane and had to put two flats on it as it is a very tight fit - At least I know that will not work free and jam the elevator. Hole is too small for threading the connectors so had too make up pins and thread this way. you need tree hands for this offering up the elevator into the bearing joints whilst poking the wires  in through the small hole one at a time. Don't forget to have the elevator hinge pins to hand after you have done it too.
Note to self check why rear rivet hole is left open.


Here is a pic of my bracket with all pins pushed in. There are two white wires on this cable which I have designated servo up down. This is where I will come to swap them if they are wrong way round.




Now I need to produce an up to date drawing of this wiring...


Updated the Wiring Schematics 25 May 2016
So here is a draft of the wiring page for the elevator trim. This is a draft and I need to update the Roll trim details.

I have been using ExpressSch which is part of ExpressPCB this is a free design package that I have used elsewhere on this project to design circuit boards but more of that later.