Aft Fuse

Today I completed a couple of steps on the aft fuse by attaching the rudder stops and about 30 nut plates.

The rudder stops turned out to be easier than I thought. I was able to start by squeezing the aft two rivets easily, and then bucking the forward two without much trouble. I did the right aft side, then the left aft side, then right forward side, then left. Both came out fine.

Then I dimpled a set of nutplates and started riveting them on. I dimpled 4 nut plates that I shouldn’t have, as I found an error in the drawings. The diagram called out for 9 dimpled nutplates and 5 un-dimpled on each side. But it should have been 7 dimpled and 7 in-dimpled, as there are 7 that sit flush with the longerons. I have a ton of these things, so it didn’t matter at this stage.

I started with all the un-dimpled nutplates, which was quick and easy. Unfortunately I had to drill out a rivet when I inserted it without checking that it was passing through the nutplate. It wasn’t, so I drilled and redid that one.

When I moved onto the dimpled nutplates, I realized I hadn’t held the nutplates steady enough when dimpling. As a result, many of them were slightly bow shaped. This was problematic, as they wouldn’t sit flush against the holes, and had a tendency to break when the second rivet was set. It took me a few nutplates to realize what was going on, and then I drilled out several, dimpled new nutplates (this time holding them level when dimpling), and then re-riveted them on.

The end result was ok. Some of my rivets didn’t set as well as I wanted, but these are not structural, and plenty strong enough to support the screws they will be holding. my main concern was making sure they were straight, aligned with the hole, and not likely to snap off.

This is a compilation of several sessions over the last week. I worked each of the last 5 evenings back riveting the rows of rivets connecting the side and bottom skins, and the bottom most rivets between the side skins and forward two bulkheads.

I decided to back-rivet in part because it gives a better finish, but also because these rivets are impossible to do solo any other way. In theory, back-riveting is easy; lay the part flat on a slab of steel with a rivet inserted in the hole, then put the rivet gun on the shop head and set it. These rivets were tricky because of the curvature of the skin. By using cushions and pillows, and small lengths of timber, I was able to roll the fuselage on it’s side to get the correct angle.

I started with the forward right side and worked aft. I was fairly successful just guessing the angle by closely inspecting where the rivet head appeared to be contacting the plate. Once I had it set up, I could quickly set around 10 rivets before needing to move the plate and re-calibrate.

Everything went well, until I got to the aft most section. I was most concerned about the aft-most rivet, which passes through three parts; the side skin, the aft bottom skin, and a bulkhead flange. By putting a little weight on the fuse, and keeping the angle shallow, I ended up with a great result. But as I worked forward, I didn’t roll the fuse enough and ended up setting 8 rivets with too shallow of an angle. Part of the rivet head on these rivets protruded from the skin, and needed to be replaced.

I spent an evening removing these 8 rivets, most came out easy but two of them gave me trouble.

I didn’t enlarge any holes, so I redid these eight the next night with the same size 3-3.5 rivets. This time, I went slowly and used my iPhone’s digital level to record the angles. 10 degrees of roll for the bottom/side skins worked great, and about 26 degrees for the next holes up on the bulkheads seemed about right.

Eventually the right hand side came out looking great. Then it was on to the left hand side.

After some experimenting, I found that the angle at which the gun contacts the rivet head is critical for getting a good looking head. Any slight angle, and the heads don’t set straight. A had a few rivets that weren’t quite level on the shop head, but I left them in and moved on. For the most part though, the heads looked great.

I had no trouble with the left hand side, I just used the digital level to ensure I was getting as close to 10 degrees as possible, and 26 on the couple of bulkhead rivets. I worked from the aft end forward, and was done in about 45 minutes. Most of the time was spent adjusting the angle of the fuselage, and checking that the hull was flat against the back riveting plate.

Yesterday and today I riveted the bottom j-channel to the side skin. I also riveted the side skins to the bulkheads where the skin curves inwards toward the bottom skin. Everything went well, although I did remove a rivet when it cleaved over.

Since I’m bucking these rivets, and doing it by myself, I can’t actually see the shop head of the rivet. So I came up with a way to make sure the bucking bar is at the correct angle, since the j-channel is attached to the side skin at a point where it’s already starting to curve inwards to join the bottom skin. By putting some clecos in “backwards” and tying a string between them, I had an angle gauge for the bucking bar. Most of the rivets came out nicely.

Next, I’m going to attempt to back-rivet the remaining rivets on the bottom, starting with the last rivets on the curved surface of the side skins. I couldn’t quite reach these with the gun and bucking bar, so I’m hoping to back-rivet them, as other builders have done.

For the last four evenings I’ve chipped away at setting the rivets on the aft fuse side skins. With only an hour or so each time, and some tricky rivets to access, progress has been slow. On the plus side, my technique seems to be improving. I did have to drill out a couple of rivets, but mostly it went well.

On Sunday evening I modified a long bucking bar so that it would fit around the tailwheel mount, and the bolts that secure it to the bulkhead. I cut this with a hacksaw and then polished on the grinding wheel, then wrapped it in electrical tape. It made bucking the rivets on the bottom aft side of the F-01411 bulkhead possible. In order to set the rivets, I turned the fuse on it’s side and carefully placed the bucking bar. I only had one rivet that I decided to replace, and that turned out to be a major pain. Unfortunately in the process of removing the rivet, I bent the flange up and away from the skin, just enough to be a problem. Without any way to hold the flange down, or bend it back, I ended up putting a BS319 pop rivet in the hole, and moved on. By cutting a piece of scrap wood into a series of weird shapes, I was able to hold the flange down and set the rivet. I need to check with Vans to see if a Cherrymax is required in that hole, in which case I will replace it.

Tonight I used my new torque wrench to set the two bolts holding the tailwheel mount to the forward bulkhead. Van’s suggest orienting these bolts with the nuts forward, and I can see why. Trying to get a torque wrench into the aft side of the bulkhead would be unworkable due to the tight access. I torqued these to 99 inch pounds, per the guidance in Van’s instructions; 85 inch pounds for AN4 bolts with self locking nuts. Plus another 14 to account for the friction of the self-locking nuts on the bolts. The wrench was easy to use and it worked out great.

I started riveting the side skins, below the longerons. I started with the bulkheads, and worked down the sides. I got through the upper j-channels, meaning about half way through this step. Hoping to finish riveting (at least as much as I can do myself) this week, so I can move on to the aft deck.

It was good to have a helper today! She did a good job putting rivets in holes, getting ready to rivet. I find I can move much faster if I pre-populate the rivet holes with a rivet and a little bit of rivet tape to hold them in place.

I like to use a piece of tape on the head of the swivel head. This helps it grip, minimizing the chance of a slip.

Happy with how these rivets worked out. I can’t directly observe the shop head of the j-channel rivets while riveting. But it’s good to have the solid edge of the j-channel to brace against.

Tonight I started riveting the side skins onto the aft fuse. I started with the longerons, where I could use my squeezer to set all the rivets. To assist with alignment, I packed up the front edge of the yoke, one for the bottom row of rivets, and a larger pack for the top row. This worked out nicely and everything set well. I did both sides, except for a few rivets down aft where I plan to leave things open until I have the aft bottom skin riveted on.

Today I fiddled around with the aft skin, and finally managed to get everything to fit just right. The challenge was deciding how many spacers I needed to fit between the tail wheel mount bracket and the bulkhead to which it is bolted. Vans supply 3 of these shims made from two thicknesses of aluminum. I started with all 3, and ended up needing just the two thinner shims to get a perfect fit. I used fresh bolts, washers and nuts, as I’ve attached and reattached these parts several times now. My torque wrench isn’t quite up to the task, as it maxes out at 80 inch pounds, whereas I need 85. Additionally, being a beam-style, there’s an access issue getting to the nuts. I’ve ordered a new tool, should be here on Friday.

I riveted the aft fuselage to the aft bottom skin, and then to the tail wheel mount. 10 rivets in total, all squeezed with a variety of yokes.

Green touch up primer on the mount, and splotches of white on the bulkhead makes everything look messy. The iterative process of shaping these parts to fit well scratched off primer and powder coating, so a touch-up was needed.