Final main spar countersinking, priming, and nutplates

My new countersink bits arrived so I could continue working on the main spars. The new bits obviously made for faster, cleaner cuts than the old bits. Once the cutting was done I spot primed everything and allowed it to cure for a couple hours.

In the meantime I was able to countersink and spot prime the holes for the nutplates on the forward side of the main spars.

After the primer cured, squeezing the flush rivets was a five-minute job.

With the primer for the main nutplates cured I installed the nutplates and then countersunk and primed the final holes for the screws.

FINALLY done. Glad that's over.

As much as I hate to admit it, I'm actually not finished with all of the countersinking on these things. I'll still have to countersink ALL of the #40 holes to allow the dimpled wing skins to sit flush with he spar. It'll probably be a while until I need to do that though, so I'll revel in this victory for the time being.

With that the main spars are (almost) officially done, though I have a LOT of work to do before I get to the final assembly. The next big tasks are finishing the rear spars preparing the ribs, so I have plenty to look forward to in the coming months.

Fabricating tiedowns, starting work on the rear wing spars

The last job to get done on the main spars before moving on to the rest of the wings is fabricating the wing tiedown attach points since I'd prefer to keep my airplane from blowing away in any kind of strong winds.

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.

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.

Spacers taped in place. The blue aluminum piece is where the bottom wing skin will be.

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.

...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.

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.

Countersinking, priming, and installing nutplates in the main spars

Oh boy oh boy, it's time to start the wings proper! The main spars already have a lot of work done on them from the factory. The doublers have already been attached to the main spar channel using the larger 3/16" rivets that would be difficult for homebuilders to set without a heavy duty squeezer or rivet gun, and the whole thing has been gold anodized for corrosion protection. That said, the spars still have a decent amount of prep work before they're ready to have more parts attached to them.

The fuel tanks will be attached to the wings with screws rather than rivets, so nutplates need to be added to the spars to provide attachment points. Adding these nutplates is a process that really embodies the sheer tedium involved in building an airplane. For the total of 120 fuel tank nutplates on the spars, one has to go down the line of 90 holes on each side of each spar (three holes per nutplate) six times performing a different operation each time.

The process is as follows:

1) Ream all the #40 holes that will attach the nutplates to the spar since the countersink bit won't fit unless these holes are opened to final size. I followed the example set by other builders and put tape over all the gaps on the top side of the spars to keep metal shavings from getting caught between the pieces and causing problems.

2) Countersink the #40 holes to allow the AN426 rivets to sit flush. This was easily accomplished with the Sioux air drill on loan from my good buddy Chip at the airport. The holes didn't need to be too deep since I just need the rivets to sit flush with the spar (there won't be any skins sitting in these holes), but I still needed to make sure to allow some room for a thin layer of primer in each hole.

3) Mix up some primer and spot prime the countersunk holes with a Q-tip to protect against corrosion since the anodization has been removed. I had a tiny syringe left over from my cat's flea and tick treatment that worked perfectly for measuring out small amounts of base and curing solution.

Initially I only primed the holes for two nutplates to test whether I had set the countersink bit deep enough for the rivets to still sit flush. They ended up being good enough for these two nutplates, but I set the cage to cut couple clicks deeper just to be safe.

4) Once the primer is cured (I only waited an hour or two - it was a really thin layer), rivet the nutplates to the spar. The six inboard nutplates are angled to accommodate the additional wing walk ribs. The aftmost rivets on these are easy to squeeze, but the forwardmost rivets are too close to the spar for the squeezer and have to be bucked instead. The rest of the nutplates are parallel with the spar and can be set easily with the squeezer.

5) Countersink the screw holes with a #30 countersink bit. This is done after installing the nutplates to allow the bit to center in the nutplate. These holes will need to be deep enough for the tank skins to sit flush with the spar, so I made a little gauge to check the depth. A lot more material is removed while countersinking these, so these took a lot longer than the last ones.

6) Spot prime the freshly-countersunk screw holes and allow them to cure. I also spot primed some small scratches I had made in the spar while countersinking everything else.

Once the primer is cured, repeat for each side of each spar. For each side, the whole process took roughly 4 to 5 hours.

In addition to the fuel tank attach nutplates are nutplates that will secure inspection panels on the bottoms of the wings. These are slightly smaller K1000-06 nutplates (#6 screws) instead of K1100-08 nutplates (#8 screws), but installation is pretty much identical.

Over the course of a few days I managed to get all of the fuel tank and inspection panel nutplates installed on both spars, and all of the fuel tank attach holes for the right spar are countersunk and primed. I began noticing it was taking an awfully long time to countersink the holes for the left spar, meaning the countersink bits are probably pretty dull by now. I went ahead and ordered new #30 and #40 bits and I'll continue work on the spars once I get them.

Dassa lotta nutplates.