Matt's RV-7 Project

Today I drilled the holes in the wing ribs through which the electrical wiring conduit will pass (Van's has a writeup on the subject here). Corrugated plastic tubing will be run through these 3/4" holes, bringing the wires for the strobe lights, nav lights, landing lights, and VOR and marker beacon antennas out to the wingtips. I'll probably also run AOA tubing through this conduit as well. Drilling the holes in the outboard ribs, where the bottom skin isn't on yet, was a piece of cake. Drilling the inboard holes was less easy – I had to use bad words and an angle drill.

The holes at the root end, where there is a root rib plus three wing walk ribs all bunched up together, were particularly challenging. I used the unibit with a series of drill extensions I found at Aircraft Depot to poke the holes, and I sort of stuffed a scotchbrite hand pad back in there and rubbed the sharp edges until they were gone.

Instead of measuring each and every hole, I made this little template to help me get the holes all drilled in a straight line. The two holes in the angle slip over the rivets in the rib flange, or are clecoed into the correct open rivet holes, and the third hole is where the pilot hole in the rib goes.

The effort of creating a drill template paid off – in the photo below, Mary is holding a shop light up to the hole in the root end, and the camera is looking down the wing through the hole in the tip rib. You can see that the line of holes is laser straight. Excellent.

I would have gone on and run the conduit material through the holes, but Van's left it out of my order. Time to call and harangue them tomorrow.

I'd also like to point out that this whole process was unnecessarily difficult. It took me hours to drill a couple dozen stupid holes. They send you these partially completed wings – which you pay extra for, I might add – and then say "oh, have fun drilling these holes in all the ribs, especially the ones you can barely reach!"… I can't understand why they wouldn't just press a couple of keys on the CNC machine and cause all the ribs to be churned out of the factory with the proper 3/4" hole already located in the correct spot. I mean, it's not like having the holes there would cause anybody any problems – they obviously have a negligible effect on the strength of the wing, since the factory has put out the above-referenced official document suggesting that builders put their holes there, and I don't know of any RVs that have fallen out of the sky as a result. Furthermore, if some builder ended up not running anything through the holes, they'd just be like extra lightening holes. (I have a hard time imagining that too many people are leaving out wingtip lighting, though) Oh well, it's just another one of those inexplicable Van's things, I guess.

Continuing work on the pitot system, I got the pitot line run inside the wing today. While some folks use a short length of aluminum tubing and then transition to plastic line, I chose to use aluminum throughout since I'm comfortable working with it and I know it won't ever go brittle and fall apart on me (a problem I had with the pitot/static lines in my last RV!)

The pitot tube itself has a short and twisty length of tubing attached to it, although not so twisty that it can't be inserted through the pitot mast from the bottom up. This will allow the short aluminum tube to be disconnected just above the pitot mast, and the pitot tube can then be removed for service. The pitot mast doesn't have to be removed in order to remove the pitot tube.

Inside the wing, the short length of tubing connects to a 45° bulkhead fitting:

Then another length of tubing runs all the way to the wing root, first bending down and behind the aileron bellcrank and then passing through a snap bushing in the next rib. An Adel clamp on one of the bellcrank bolts (one size longer to compensate for the extra stuff it's holding) anchors the tubing firmly in place. Note that the short length of tubing between the bulkhead fitting and the pitot tube can be reached easily through the access panel that will eventually cover all this.

At the root end, I put a 90° bulkhead fitting in the rib that's just outboard of the root rib:

My thinking here was that there probably wouldn't be enough space between the root rib and the fuselage to get the fuselage side of the pitot line connected straight to the root rib – not to mention the placement of these snap bushings causes the pitot line to dead-end right into the center section bulkhead – so I'll bring in flexible tubing from under the pilot's seat, through the second lightening hole aft of the spar, and onto this fitting.

And yes, it was a pain to get all that tubing in there, especially the process of flaring the inboard end.

P.S. The duct tape is to protect the primer on the aileron pushrod from getting scratched; it's not structural.

I fabricated a spacer out of 0.020" Alclad that will go between the pitot tube backing plate and the skin. Measure, mark, drill pilot holes, rough cut, Dremel, file, deburr. Here's a time-lapsed series of shots of its creation:

As for match-drilling the backing plate to the bottom skin, the Gretz plans have you clecoing the skin to the wing and back-drilling through the plate into the skin. That would work okay if I could remove the top skin to get in there, but on the quickbuild wings the top skins are of course already permanently attached. I thought about trying to get an angle drill back in there, but that seemed like a recipe for frustration and/or disaster. Instead, I came up with this approach:

I match-drilled the pattern of the spar rivet holes in the backing plate onto a piece of bar stock I found in my scrap box, to make a stand-in for the actual spar flange. Then, I clecoed the backing plate, spacer, pretend-spar, and skin to the workbench through the spar rivet holes. After that, it was a simple matter to match drill the skin to the backing plate, right into the table. Having a sacrifical layer of MDF on top of the workbench is incredibly handy.

Here's a test fit with the skin, spacer, backing plate, and mounting angle all clecoed together. Alignment is good and the skin lays down just right with the spacer in place.

On to the pitot mast cutout in the bottom skin. No pressure, it's just a $64 replacement skin and a similar cost in shipping and handling if you screw it up. First comes starter holes with the unibit:

Then a rough pass with a carbide cutting bit in the Dremel:

Lots and lots of filing later, it fits pretty well:

The gap between the mast and the skin isn't exactly symmetrical – it's about 1/16" on one side and between 1/32" and 1/16" on the other:

I could file the sides to match, but what I'll probably do instead is just blend the mast into the wing skin with a small fillet of Proseal during final assembly. I can't think of a situation outside of major wing damage where you'd need to remove the pitot mast from the wing – the pitot tube itself, sure, but the tube can be removed from the mast just by taking out four little screws and unhooking the air line and heater wires. Good enough. It might even help to keep water from getting up into the wing that way (think about it, ice + heated pitot). Who knows?

Anyway, the pitot mounting stuff is basically done for now. Now if I could just beat the head cold I brought back from last week's ski trip.