Ch. 7 - Fuselage exterior - Part 5

In-foam wiring conduits (12.5 hrs)  

I have been trying to find a way to run a few wires from the instrument panel to the outer edge of the strakes, using the shortest amount of wire. I will probably be installing the magnetometers for a future ADAHRS there (Air Data/Attitude/Heading Reference System), far away from interferences, and I didn’t want the wires to run alongside other equipment wiring, thus potentially compromising the signals.

My friend Mike had an interesting idea when he built the leading edge of the strakes, burying a PVC conduit within it, giving him the ability to run cables from the cockpit to the outer strakes.

Mike glassed a conduit in the leading edge of the strake

I liked his idea a lot, but I wanted to be able to bring the wiring all the way to their final destination, the front of the instrument panel, without them ever going through the cockpit.

I chose to embed a conduit into the sidewalls, and this had to happen before glassing of the outer fuselage.

Finding a suitable conduit turned out to be a bit challenging. I had quite a few requirements in mind.

1. It needed to be flexible enough to get around the side-stick depression in the right sidewall, so the rigid PVC was out.
2. It needed to be light, so that it wouldn’t add extra weight. So copper was not an option.
3. I wanted it to be cheap.
4. It needed to be small enough to remain buried under the existing foam, but still be able to carry many wires.
5. The material needed to have low internal friction, to improve the ability to string cables over a long distance.
6. and it needed to be stronger that the foam it displaced.
7. Did I mention cheap?

Strolling trough the “Aviation aisle” of my local Lowes, I found a rubber tube that looked promising. It was all of the above, and did not collapse when bent, so I brought home a 4 foot piece for testing.

1/2" rigid rubber tube

Not wanting to test on my fuselage, I chose a piece I previously had to cut off from it.

Piece of fuselage removed in Part 3

The tube proved to be slightly too large, and I ended up having to sand it down a bit for the purposes of this experiment. After routing, microing, glassing, and drilling, I arrived at a realistic representation of what the fuselage would eventually look like.

Foam removed and tube sanded flat on opposite sides

The "big idea" part 1

Tube microed, and 2 layers of BID glassed over the foam

Cross section view

I rolled up 16 rather thick wires into a bundle, and easily pushed them into the conduit.

Hole drilled, testing conduit capacity

The "big idea" part 2

The "big idea" part 3

This experiment was very successful, but I didn’t like the idea of having to sand the conduit down to make it fit below the foam, thus reducing its strength. I decided that a smaller size conduit would be better suited for my needs, and went back to the store. 

Unfortunately, the only size they carried was the 1/2” I had already deemed unsuitable. Looking for a substitute, I found a 1/2” riser for a sprinkler system with all the properties I articulated, for just $0.99. 

Suitable replacement

Strong, flexible (without kinks), low friction, light, and less than a dollar!

I know... I had just scrapped an 1/2” tube, why would I consider another 1/2” tube?

Well, it turns out not all 1/2” tubes are the same, since the new one was smaller, and sanding was no longer necessary.

All 1/2" tubes are NOT created equal

I didn’t repeat the experiment, and went straight to cutting the outer fuselage sidewall. 

Aiming to miss the circular control stick depression

Free-handing the cut!

No damage to the inner structure! Cha-ching!

I used a heat gun to permanently bend the tube, and reduce its spring back force toward the foam

Synoptic view

After scuffing the outer surface of the tube, I microed it in place.

Scuffing the tube

Laying semi-dry micro into the hole

Tube wetted with epoxy, and pushed into the micro.

More micro added over the tube

Sidewall completed, after initial sanding

A look from the inside of the cockpit

As seen from in front of the instrument panel

The final step was drilling a hole at an angle into the rubber conduit.

Hole drilled into the tube at 45˚

Testing my theory

I did not like having a hole in my sidewall weakening the structure, so I ended up reinforcing it around the hole with 2 plies of UNI forming 2 rhomboids with their main axis oriented 30˚ fore and aft, just as the original UNI I had laid on the bare foam. 

UNI buildup around the hole cut into the sidewall

Peel-ply over UNI

Final structure

Since there’s no side-stick depression in the left sidewall, I was able to run the conduit straight to the front of the instrument panel.

No bends this time

Routing the left sidewall

A close up of the precise depth of cut achieved with the router

This time I decided to plug both ends of the tube, in an effort to keep the micro from  entering it.

1/2" aluminum rod used as a plug

Tube capped

Ready for burial

It worked well, but I had to weigh the tube down because it tended to float up to the top of the micro.

Tube trying to float on top of the micro

Screws pushing the tube down during the cure