Wing Top Skins Riveting

Last weekend I riveted the upper skins on the left wing. As I did when clecoing the skins in place, I started riveting onto the strongest parts first (4mm rivets into the main spar cap), then ribs, then stringers.

When pulling these blind rivets, it leaves an open hole that should be filled in. I borrowed an idea from Craig Maiman, who in turned got his idea from Kit Planes magazine, to use a light-weight spackle to fill in the rivets. I bought some from a local Sherwin Williams store, and filled a syringe with the material and then injected it into the rivet cavities. It comes out in a creamy/pasty consistency, and then hardens relatively quickly (within an hour) so that it can be sanded smooth.

Short GIF video of filling the cavity – almost like dental work!

I wanted to add the step skins next, but I found an issue with fit of the skin. Specifically, the holes in the top step skin (WG-SKN-003-L-C-) did not line up with the bottom skin & ribs. My assumption was that the skin was bent incorrectly, which was confirmed by TAF technical support. This skin is pretty thick material, so there’s no way I can fix this myself – and besides, aluminum doesn’t like to be re-bent. I thought about drilling new holes, but because the mis-alignment is about 2.5mm, and the holes are 3.2mm in diameter, I would wind up creating a slot instead, and the rivet possibly could pop out over time. Instead, TAF will be sending me a replacement skin.

The bend of the upper step skin tucks between the lower skin and the ribs (which is what this lower skin is attached to with clecos in this pic). But because the holes in the top skin are misaligned in the red circled area, I can’t get any clecos in there.
The red lines help illustrate the misalignment here between the upper and lower step skins (the lower step skin is the correct one)
Pretty clear to see the misalignment in this pic. The red circle is where the rivet hole is on the step skin rib.

Since I had this issue with the left step skin, I decided to check the right side skin, but that one looks to be OK.

While I wait for a new step skin, I’m going to build the ailerons & flaps, and then get started on building the right wing.

Prepping More Wing Ribs

I spent about 4 hours over the last 2 days preparing a bunch more ribs for the wings. You can see page 9 of the manual in this picture, which is where all these ribs (and more) are required. I still need to prime the edges. We’ve had an unusual string of warm humid weather with afternoon t-storms for the last week, which is not the best for painting.

With so many ribs to clean, I decided to switch to a part bath for cleaning parts, rather than using spray bottles, which has been time-consuming and kinda messy with bigger parts. These tubs are 36″ long, which will fit most of the parts I’ve had to clean so far. Simple Green Extreme (diluted 1:13 with water) on the right, plain water for rinse on the left.

I started setting up for this massive wing assembly! Rear spar is attached to the jigs; I’ll attach the main spar once I finish prepping the ribs.

Trimming Rivets

Working on the elevator trim tab now, and the instructions call for removing 4mm from the rivet body. Of course, the engineer in me insisted on a precise way to do this with a 3D-printed tool to hold a bunch of the rivets and then run them under my 3-axis mill :p

Video to follow soon

Process Makes Perfect

After about 2 days of drying, my freshly-primed parts for the horizontal stabilizer started showing spider lines in the paint, then flaking off and looking like this horror:

It took me a while to figure out what went wrong, but it looks like I failed to properly clean the parts before priming with SEM Self Etching Primer. I had been using the Simple Green Extreme precision parts cleaner to clean and de-grease, then drying the parts with a clean towel. Here’s where I went wrong – after this step I went right to priming. The important step I missed was to rinse the part off with water, to remove the cleaner from the surface of the part. Going through a Google search, I also learned that a lot of people do a final cleaning with acetone or isopropyl alcohol right before priming.

Check out my Process page for details on my revised priming process – which I can confirm now works successfully.

Mercifully, I’ve been able to fairy easily remove all the primer from the affected parts using the Simple Green cleaner and a Scotch Brite pad – still a lot of work though. I got through about half of the HS parts tonight, will have to finish tomorrow.

After following the 9 steps above, I re-primed one of the parts (a plate) and the finish came out much better. On the left is a part that I stripped back down to the bare metal, and then cleaned with the new process. On the right is a part that was reworked and was sprayed with 3 light coatings of primer. Tomorrow I’ll do a tape pull test to make sure the primer adhered properly.

I work in engineering so I know how important it is to have your process right, especially when it comes to applying coatings to metal. Lesson learned – do a test of your process on a scrap part first to avoid having to rework a bunch of parts like I did!

Primer test

Before I decided to buy a kit plane, I had no idea that there were so many choices for priming metal parts (or even to prime at all!). But given the New England weather, I decided I’m going to prime mating surfaces. I’m going with SEM Self Etching Primer for a few reasons:

  • It comes recommended from the Van’s community thread (another one)
  • It comes in a rattle can, eliminating need to buy paint spraying equipment
  • I can get it locally at O’Reilly Auto Parts, as well as on Amazon and other online sites
  • Price is moderate – about $21 a can, and one estimate I read is that I’ll need about a dozen cans in total

I made a little test coupon, first cleaning the aluminum with Simple Green Extreme and then 3 light coats of the primer; the result was pretty good. I’ll try some scratch tests after it dries for a few days. I like the green color – it makes it very easy to see what’s been primed.

To me it also feels like a real, big-boy airplane touch – I’ve spent a lot of time at the Beech factory in Wichita, where they used zinc chromate / zinc phosphate as a primer, giving the new planes on the assembly line their characteristic green color.

I also did some more wood working over the weekend: I had enough materials left over to make another work bench similar to the EAA standard work tables I made last week. This one is smaller, but I figure it will make a nice side bench for my drill press.