Aft Fuse

Today I officially finished section 10, the aft fuselage. With the empennage attachment finished, I completed the final step in section 10, riveting the forward bulkhead to the top skin rib.

I positioned the aft fuse on my work benches, and rolled it onto it’s side. By lying down inside the fuse, I was able to rivet 2/3 of the rivets using my new “main squeeze” hand rivet squeezer. The last rivet needed to be bucked as it was too tight against the skin, and the rib. Using a double off-set in my rivet gun I bucked it. Not my best rivet, but good enough.

Then I completed the aft fuse by screwing on the inspection port covers on the aft end. After I located the correct set of 16 screws, I had a frustrating time with a couple of the nutplates, where the screw wouldn’t sit squarely in the hole and wouldn’t engage with the nutplate. Withou the covers in place I was able to get the screws in, and that seemed to fix the issue. Covers are now installed.

Tonight I drilled the last two holes with my new drill bits. The difference was amazing, the new drill bit cut straight through the steel leaving a nice clean and round hole.

With the holes drilled I removed the VS, and deburred all the surfaces.

Tonight I drilled most of the bolt holes that attach the VS to the aft-most fuselage bulkhead. I used my drill block, which worked well, but was initially too long to fit within the confined space on the aft bulkhead. A quick trim of material on the block and it worked fine. For the 1/4 inch holes at the bottom, I stepped up drill sizes, starting with #30, then #12, then finally 1/4 inch, all done with a drill block to keep things aligned. This worked well and I’m happy with the results.

I did run into a problem with the 1/4 inch holes, which I attributed to a blunt drill bit. The 3 holes at the bottom of the VS spar are drilled to 1/4 inch. They go through two steel plates, with aluminum sandwiched in the middle. The drill bit struggled to make progress on the steel, even when running at the appropriately lower RPM. I got 1 of 3 holes drilled, and half of another one before stopping. It’s frustrating to not be able to finish this step tonight, but worth waiting for a new bit so I get the best possible result. I contemplated sharpening the drill bit myself, but didn’t feel like experimenting when these are such critical holes. So I ordered some new bits from Aircraft spruce, and will wait for them to arrive before continuing this step.

I have plenty more work to do, so I’m not worried about the delay on this particular step.

Several work sessions; July 30, August 2, 3 and 4, total of 4 hours

July 30: Test fitting the top skin showed a couple of areas where I needed to break the edges a little more. A quick bend with the hand seamer, and then a re-check showed that everything fit nicely.

August 2: I started by riveting at the forward end and worked backwards, per the instructions. It was easy going bucking these rivets until I was unable to reach comfortably. Then I removed all clecos, and taped up some rivets in preparation for back-riveting.

August 3: By carefully rolling the fuse onto the top skin, and climbing onto the bench, I was able to back-rivet up to the first bulk-head. I ran into an issue on the bulkhead, where the rivets didn’t sit down into the holes in a couple of places. The problem was caused by my positioning of the fuse flat on the back riveting plate, instead of at a slight angle. I moved on, riveting the remaining rivets up to the 2nd bulkhead. This was a lot of fiddling around; starting with the fuse on it’s side, I’d insert around half a dozen rivets on each side of the skin. Then I’d position my back riveting plate, then roll the fuse onto it’s top, and secure with some tie down straps rigged up to the ceiling. By tightening and listening the straps I was able to position the side at just the right angle. For the rivets. A quick check to make sure everything looked right, then up onto the bench, crawl into the fuse, set the rivets, then roll to the other side, check positioning, climb back in and rivet the other side. Then release the straps and roll the fuse onto it’s side to check my work and insert the next set of rivets.

But the end result was worthwhile; nice tight skin alignment, and some good looking rivets. I replaced the bulkhead rivets and wrapped up for the night.

August 4: I finished the top skin by bucking the remaining 30 rivets on the aft end of the skin. I contemplated back riveting these, but decided the risk of crawling that far back into the fuse, wasn’t worthwhile. Too much chance of bending something, or losing my balance and crashing everything onto the floor. By reaching in from the access point on the aft bulkhead I was able to easily buck these rivets, and despite not being able to see the rivets, they came out great. I’m really enjoying my new tungsten bucking bar. It’s about 2 inches square, fits nicely in the hand and seems to do a nice job. Using a couple of pieces of foam on the edges makes it very easy to ensure it’s level.

Overall I’m happy with the result, and pleased to be finished with the aft fuse construction. I still need to drill some holes for the VS, but I’m otherwise complete.

This weekend I finished riveting the top right side skin. This seemed to take a long time, mostly because I ended up removing and redoing several rivets. The angle and awkwardness of access made for a hard time setting these rivets. All of them had a tendency to bend over, making for a bad rivet head. After much experimenting, and a tip from another builder, I found that pre-squeezing the rivets just a small amount made a big difference. The rivets tend to bend within the first 2-3 hits from the rivet gun. Keeping it straight for those first few hits all but ensured a good looking shop head. By pre-squeezing the rivets with the pneumatic rivet squeezer, I was able to keep the rivet straight for that initial compression. This really only worked in some of the rivet holes where I was replacing a rivet and the hole was slightly enlarged (but not enough to up-size the rivet). It also took a lot of practice to get a properly squeezed rivet; straight and not too fat to fit in the hole.

On the last rivet, my bucking bar slipped off the rivet head, and I managed to make a small mess of the bulkhead flange. Luckily it’s just a small dent and not a crack. Annoying way to finish the otherwise good session.

Last week my drill block parts arrived, so today I made the block. I cut about 3 inches off the foot long section of aluminum bar. Then I spent time squaring up my drill press, which meant fiddling with the table to get it sitting perfectly square. It had been about 2 degrees off level, it turned out. With the table square, I drilled two holes and inserted two of the sleeves that correspond to the holes a I need in the aft bulkhead. The holes came out pretty square, this should be great for helping keep the holes straight.

Finished product. On the other side is a channel for redirecting shavings from the drill.

Tonight I back riveted the right top side skin, just the bottom row. Access was harder than with the other skin, but I had success climbing onto my work bench and crawling inside to rivet. This was a “first”; I climbed inside my plane for the first time! My back rivet plate is about the same thickness as the rubber floor mats from Harbor Freight, so I was able to fully support the skin while crawling inside.

The finished result was great; nice tight skin, no oil-canning and good shop heads all the way down the line.

I clecoed on the left top side skin 2 days ago, and last night I back-riveted the bottom row of rivets. Tonight I set the remaining rivets on the bulkheads. There were a few rivets that clenched over and will need replacing, it’s tricky to get the bucking bar at the right angle because of the curved skin.

Tonight I planned to back rivet the rib, doubler, and j-channel to the top skin. Unfortunately I ran into a couple of problems, and was only able to backrivet the j-channel. The rib has a flange on the bottom edge which makes back riveting this impossible without an offset backrivet set. I don’t ah e one, and don’t feel like buying one for this job. I decided to backrivet the doubler plate rivets that were not passing through the rib, but i noticed in several cases there were some cracks forming around the edges of the dimples. I ended up drilling it out and ordering a new doubler plate. On closer inspection, it looks like I didn’t debur these holes enough. It’s also true that some of the cracks were just the primer clacking, as I had dimples this part after priming it. I set it all aside for now and moved on.

In the mail today was my longer flush rivet set. I made quick work of setting the last few rivets on the aft deck.

The next step after finishing the aft deck is to final drill several holes between the vertical stabilizer and the rear lost bulkhead. These are drilled out to #12 and 1/4 inch, and must be drilled perpendicular to the vertical stab. These are bolt holes, and the heads and nuts need to be square to develop the right clamping force. After some fiddling around, I was able to mount the VS. it was fun to see two parts coming together like this. I did have to drill out one hole on the aft fuse to allow the #30 drill bit to hold the parts in position. Flipping through the builders manual, I couldn’t find a place where this hole is final sized. I drilled it out and then everything fit just fine. I decided not to drill these final bolt holes until I can create a drill block that will help me keep the angles correct. I went ahead and ordered a bar of aluminum, and some brass sleeves.

With my benches on wheels, I was able to move them apart and install some lengths of 2×4. These can support the DRDT-2 between the two benches, creating a more convenient area to dimple larger skins. My Dad first suggested this earlier in the year, and I’ve since seen several build logs where this has been done.

With the dimpled in place between the benches, I went to work dimpling parts. Starting with the top skin and components that mount to if (j-channel, doubler and a rivet), then moving onto the top side skins. It took some moving of benches to get at all the holes, but the second one went faster than the first.

My helper! She loves to come and see what I’m doing and help any way she can.

With the skins all dimpled, I clecoed the left top side skin into position for riveting. I plan to back-rivet as much as I can, and will do some research first to see what others have done before I begin.