Month: December 2019

After a quick work trip over the last two days, I was able to complete a couple of steps today. I riveted the left top skin to the rear flange, and finished the trim tab. The trim tab is actually the first piece that I’ve built that is completely finished. All the other parts have Fiberglass fairings to shape before they are ready to fly.

The spar flange rivets were done in two steps. I squeezed the rivets holding the skin and hinge to the spar, then bucked the remaining rivets. I was happy with the results, although I need to touch up some sprint on the rear spar due to bucking bar scratches. I spent some time setting up and clamping the pieces, including shimming the ribs to avoid anything bending.

Then I went back to the trim tab. I started by squeezing the trailing edge rivets slightly, and checking for twist. Then I double-flush riveted the rivets, jumping around to avoid building in a twist. I was happy with the result, but learned a lesson to apply tape to the edge of the back riveting plate. I ended up scuffing the skin slightly, which is a bit annoying. The edges turned out near perfect, straight as an arrow. The foam ribs could have been sanded down slightly, as there is a subtle bulge in the skin in their locations.

With that done, I pop riveted the close out tabs and then stashed this piece in the cupboard.

Tonight I riveted ribs to the rear spars, along with shear clips and a gusset.

Given my experience riveting thin aluminum, and it’s tendency to bend easily, I used some tape to hold down the edges of the ribs to keep them in position. I also shimmed up the spars to get the spar web square to the bench top. Then I used some more shims to support the squeezer so I could keep everything very steady. The results were great, and all but one rivet set perfectly. On one rivet squeeze I slipped and somehow the rib pulled away from the spar slightly, and the rivet head backed off from the spar web. The result was a gap between the manufactured rivet head and the spar web, and another gap between the spar and rib. I carefully drilled out the rivet head, popped out the rivet shank, and replaced it with a new one.

The end result was perfect, and I went to bed happy.

Tonight I riveted on the elevator horns.

This was an easy task, as access was good, and the powder coated steel horns are very rigid. The only issue I had was a minor hole alignment challenge on some of the holes on the right elevator. I ended up reaming a couple of holes where the rivets were slightly too tight. Not enough to be worried about.

I used a squeezer on all rivets, and took time to carefully line up each squeeze so that the squeezer was exactly square, and fully supported. I used a combination of blocks and terry towels to aid in the alignment. I was really happy with how it turned out.

Last night I riveted the trim tab spar to the bottom skin and riveted the horn in place, and tonight I glued the foam ribs in place and riveted the top skin onto the spar.

I back riveted the spar to the skin then carefully riveted the trim tab horn in place. Unlike my first attempt I was able to complete this step without buckling the skin. It was a bit tense, and a big relief to get it done without incident. The thin skin is not very forgiving, and demands that the squeezer is exactly square to avoid cleaving the rivet over like a bent nail.

Tonight I had my first experience with tank sealant, as it’s used to glue down three foam ribs inside the trim tab. I preped all the surfaces with rubbing alcohol, then applied double sided tape to the trailing edge wedge. With one side’s backing peeled off, I located it on the bottom skin with the help of a couple of clecos. Then I mixed up the sealant, and practiced on some scrap foam and sheet aluminum. I found the sealant to be very easy to use, much like a silicone sealant, but with a far worse smell. I applied a very thin coat to the bottom of the ribs and then carefully placed then on the skin. Then applied a coat on the top, and ensured the ribs were straight.

I carefully placed the top skin on, and clecoed along the spar, then along the trailing edge and sides. Next step was riveting the top skin to the front spar, with the hinge attached. This turned out to be an easy job with the squeezer, although I almost ran out of 3-4 rivets. Finally, I weighed down the side to help the glue bond, and clecoed on some straight edges to keep the trailing edge straight.

Next step is to let this trim tab sandwich sit for a few days and then rivet the trailing edge and close out tabs.

Last night I riveted the counterweight skins onto each of the elevators. Tonight I primed the inside of the elevator horns, and started work on the replacement trim tab parts.

Each of the elevators have counterweight skins which wrap around the flanges of the tip ribs, and provide strength to support the elevator counterweights. Each skin had 50 rivets to set, for a total of 100. After verifying there was no twist in the ribs or skins, it was an easy task to squeeze the rivets, alternating from top to bottom to try to prevent twists for forming. To check for twist, I set the elevator on it’s end and ensured the sides of the skin were all in contact with the bench.

Here’s a picture of the completed step, with the two elevators.

Tonight I used some rattle-can self-etching primer to prime the insides of the elevator horns. I sprayed the primer into a plastic mixing cup, then poured it into the tube openings, while rolling the horns around to cover all internal surfaces.

Then I trimmed the new elevator bottom skin, deburred it, and dimpled the close-out tabs. I trimmed the new trim tab horns, then deburred and dimpled them. Finally, I deburred the new trim tab spar.

My goal is to finish the trim tab this weekend, we’ll see how it goes.

Tonight I riveted the tip ribs, root ribs, and spar doublers to the right main spar.

This was a repeat of the left elevator, and went slightly faster since I had done it before. Nice to finally have some parts assembled on these elevators, given how much time had already gone into this step.

Tonight I riveted the inboard rib, tip ribs, and spar doublers to the left elevator main spar.

I started with the tip ribs, which require 4 universal head 4-4 rivets. I was able to squeeze 2 of them, but the two running parallel to the ribs needed to be bucked with a double offset rivet set. Then I squished the doubler rivets, and finally the inboard rib.

On Friday night I drilled out all the trim tab rivets and inspected the parts to determine what had caused the skin buckle. It was good practice removing rivets, and It was mostly straight forward. The thin skin is so easy to bend, you need to be super careful not to damage it when popping off the heads of rivets. In this case the skin was already damaged so I just moved quickly. I decided to order a new spar too, since drilling out all of those rivets will inevitably enlarge all the holes, even if just slightly, which makes for a weaker part.

I confirmed the problem, and found that the horn was not completely square. You can see it in this picture where the silver clecos are making a v shape, rather than being parallel.

I bent the pieces to 90 degrees, and now they look much better.

On Saturday I prepared the elevator skins by measuring out the location of the foam ribs, marking them on the skins and read spars, then masking the area. A quick scuff of the areas where glue will be applied, then cleaning with acetone, and finally peeling off the masking tape. The spars were already primed, so I taped them, wiped off the primer with acetone, then scuffed and cleaned each area too.

Tonight I started assembling the elevators by riveting a gusset to a rib, then riveting the tip ribs together on each end. The only challenge was getting at the gusset rivets, where a rib flange was blocking on one side. I ended up bucking them with a double offset rivet set, which was fine, except when the gun jumped off the rivet for a moment and left a smiley face in the rivet. Looks ugly, but structurally ok.

I used a clamp and a board to keep pressure on the tip ribs while I riveted them. This was to help ensure the part came out straight.

My goal tonight was to rivet the trim tab spar onto the bottom trim tab skin, so I can work on gluing the ribs in place over the weekend. On the inboard end of the trim tab, the trim tab horn is riveted onto the outside of the skin, and in two places it shares rivets with the spar. I started by back riveting the spar, and everything was looking good. I left out the two rivets that are shared with the horn, and then came back to them at the end (masked over area in the picture below).

Because the horn is on the outside of the skin and it protrudes out, I decided to squeeze these rivets. The horn is in two parts, so I riveted on the first one, then started on the second. After setting the second of the three rivets, I gave it a quick inspection and realized that the first two rivets had twisted and on one, a small crack had opened up on the side of one of the dimples (bottom left rivet below).

Unfortunately I’ll need to order a new skin, and will need to remove the 30-odd rivets I set tonight.

I’m not yet 100% sure why this happened, but my guess is that for some reason the horn was not completely flush with the skin. Then as the rivets set, they pulled the skin and horn together, causing the skin to bend up on one side. The horn couldn’t bend with the skin because it was sitting flush against the other half of the horn that was already riveted. As I remove all these rivets, I should be able to confirm the issue.

When I do this again, I’ll try to back rivet these on the corner of my back rivet plate, which is likely to produce a better result.