The tiedown brackets themselves are supposed to be tapped to allow the tiedown rings to be screwed in, but I don't own a tap and have little desire to buy one for this small job. Instead, I ordered some tiedown blocks that are pre-tapped from Cleaveland. They're literally the exact same piece as what's provided by Van's, just with the holes tapped so I don't have to do it myself. Worth the extra cost in my opinion.
To fit the tiedown blocks to the spar I needed to fabricate some spacers to support the blocks. These are all cut from a single aluminum bar.
Once cut to size, the pieces can have lightening holes drilled into them. I don't know how much weight this really saves, but I didn't see any reason to not do so.
I drilled a pilot hole in the center of each block, then used my unibit to slowly remove most of the material. The hole is meant to be a full inch across according to the plans, so my 3/4" bit wasn't going to be enough.
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| The bit I have now doesn't quite.... cut it. Get it? :D |
In the meantime I taped the spacers in place to keep them secure during drilling, and I clamped on the tiedown blocks so I could match drill everything to the spar. On the forums there are a few reports of issues where the tiedown blocks weren't close enough to the skin the allow the tiedown rings to fully screw into place without having some sort of spacer installed between the blocks and the inside of the skin. To avoid this, I took a flat piece of scrap aluminum and used it to place the tiedown block flush against where the bottom wing skin will sit on the spar.
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| Spacers taped in place. The blue aluminum piece is where the bottom wing skin will be. |
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| Tiedown block clamped and starting to drill. I'll definitely need to round the corners where the block will contact the bottom skin to avoid any bumps or cracks. |
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| ...and fully drilled. Nice! |
With the holes all match drilled to the spar, I just needed to match drill for nutplates that will attach the spacers to the tiedown blocks and will also attach the entire tiedown assemblies to the spars. Clamping the nutplates to the blocks and match drilling them was easy enough.
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| Screwing a bolt into the nutplate made it easy to maneuver it into place while keeping it centered. |
The spacers were then clamped to the tiedown blocks and nutplate holes match-drilled into the spacers. With all of that drilling out of the way, a quick trip to Harbor Freight yielded a nice 1 3/8" unibit, which did a wonderful job of finishing the holes in the spacers to final size.
There will be flush rivets attaching the spacers to the blocks, so I machine countersunk the spacers for the rivets. A quick deburring later and the parts are ready for priming.
Time to start work on the rear wing spars too. The inboard attach points are some thick doubler plates (2 per wing) that for some reason need to be trimmed for the RV-7. My only guess is that the 7's wings are attached slightly lower and closer to the fuselage than the RV-8's wings, so the ends have to be shaped to fit.
Taking extra care to get the marks as close to the plans as possible, I sharpied the cut lines onto the doubler plates and used the bandsaw to make the initial cuts.
From there I took all four pieces to the scotchbrite wheel to deburr and break the edges. Soon I'll be rounding all the corners and shaping the pieces to exactly match one another once they're installed on the spars.
