August 19, 2008

Wing Plumbing, Pitot Tube - 4.5 hrs.

Over the last few weeks, I've been trying to reach some final decisions in my mind about just how I was going to do the internal plumbing for my wings. I want to get all this work done now before riveting the bottom skins on while I still have complete access to the inside of the wing. Three things are on my mind: the pitot tube plumbing, the autopilot brackets and/or servo installation, and the AOA sensors for the Angle of Attack instrument.

Well, there's a 4th one, but I've already figured that one out. That has to do with how to route the wiring through the wing for wingtip strobes and marker lights, my landing lights, and any antennas or whatever that might go in the wingtip. A lot of guys install conduit in the wing to run wires through. Vans even sells some flexible conduit for this purpose. Several months ago I bought some and I had planned on installing it at this stage. But I had a discussion with Randy Lervold, my technical counselor, and I was convinced to do without it. There are numerous good reasons for not putting conduit in your wings. For one, even though we can use it if we want to, PVC and polyethylene tubing aren't allowed in certificated aircraft. There's a good reason. It burns. I can do without a fire hazard in my wings, near the fuel tanks. Plus, if all the wires are inside conduit, you don't have access to inspect them or do maintenance if something goes wrong. If you want to add something to your plane later on, you would have to pull wires through somehow. And if the exit point is midway through the wing, you have to drill or cut holes in it. What if the conduit isn't big enough and you run out of room? Randy has built two RV's and he wired both of his planes using snap bushings in the wing ribs. The wire bundle is wrapped in segments of spiral wrap to keep it nice and tidy, while allowing wires to exit the bundle anywhere along the way. With the access cover plates located on the wings the way they are, you can reach inside along the way for inspections or maintenance. Finally, you don't add any unnecessary weight. This all made good sense to me. So I took the conduit back to Vans and got a credit for it on other things I needed. I installed snap bushings in all the wing ribs.

Today I finally got started on the pitot tube plumbing, after figuring out a very simple solution for the whole thing. The main thing I was trying to figure out was how to connect the pitot tube to the tubing in such a way as to allow for removal, inspection, and maintenance. Others have had a connection of some kind inside the wing near the pitot tube. I found a way to avoid this, and hence, avoid one more failure point or chance for leaks, etc.

It all starts at the wing root. Here I installed a standard connector on the inboard wing rib and ran the tubing from here through the ribs.

Here's where it passes underneath the aileron bellcrank. After taking this picture, I worked it even closer down next to the spar. There's plenty of clearance under the bellcrank and it's very solidly in place.

In the next bay, where the pitot tube is, I put in a big sweeping loop. Fortunately, the pitot tube I bought has no tubing of its own attached to it. So I don't have to make a connection inside the wing and try to figure out how to secure it from flopping around. There's no direct access to this bay and that would be difficult once the bottom skins are riveted on. With this one you just run the tubing right out through the wing into the pitot tube, flare the end, and attach it to the pitot tube. Simple, trouble-free, no connections or chances for leaks anywhere inside the wing. It's all one piece.

This tubing is just flexible enough that having a loop like this will allow me to push or pull it small amounts to attach it, unattach it, inspect it, etc. I was very careful to make sure it doesn't touch or rub anywhere.

Here's the completed installation. Nice! I'm very happy with this setup. If you're interested, this pitot tube came from Evans Aviation Products. Their main business is building fuel tanks for RV's, but he is also expanding his line of products into other areas. I heard about this from my friend and technical counselor, Randy Lervold. His comments on this product are found here. I can't believe the quality, for the price I paid. Other brands cost hundreds of dollars more. It's powder-coated, and fits together beautifully. The kit comes with everything you need, right down to the rivets. It's not a heated tube, but one is in development now, according to my conversations with them. I can always retrofit a heated tube later on, if I wish. To see more details about my initial installation of the kit when I built this wing, see my log entry for December 29, 2007.

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