Upper cowl: first trim

Once again, this is a multi-day report; I actually started on this stuff yesterday afternoon. I’d actually put the upper cowl on and off a couple times over the preceding week, as I was thinking through my next move for the fitting process. I also swung by Home Depot this past Thursday to pick up a 2’x2’ piece of 1/4” plywood, to make a spacer between the cowl and prop spinner. That work got done yesterday – first, making a hole in the middle of the sheet to match the prop hub, then splitting the sheet into two pieces. The idea was that I could rest it on top of the hub for fitting.

Next up, it was time to start messing with the cowl itself. Before doing anything, I needed to get the fuselage into a level attitude, which meant finding the right combination of boxes and scrap lumber to raise the tail to the proper height. The good news here is that it’s a lot easier to lift the tail with the engine hanging out front – that’s really good since I’ve now put the tail on and off the boxes several times.

Anyway, with the fuselage leveled both front-back and side-side, I got my laser level set up to give me a horizontal reference line. This was mainly so I could similarly level the cowl side-side while doing the other positioning. Then I got to mess around with my adjustable supports up front to get the nose of the cowl where I wanted it relative to the spinner, as well as maintaining the side-side level I wanted. Then I checked, double-checked, triple-checked, and so on, until I was really satisfied that I had the location I wanted. Once that was done, I used the tape line I laid out on the fuselage, measuring 2” forward, and laid out the cut line for the back end of the cowl:

Next, I broke out the masking tape and laid a stripe along the marks to create a continuous reference line for the cut:

Then it was time to take a deep breath and make that first cut. I wanted to creep up on the final size, so I worked to make the cut about 1/8” or so shy of the actual cut line. Getting the final fit was always going to require a lot of iterative sanding, so no real reason to try and get super close on the first pass:

Now, the real fun part here is that before I could get to the iterative sanding, I first needed to install the hinge halves that will support the back of the upper cowl. Prior to trimming, it was resting on the fuselage, but now it would need to butt against the fuselage, and thus rest on the hinges. So starting this morning, I cut all the hinge pieces I’ll need around the cowl, then set about fitting the upper hinges to the firewall. This is kind of fun since the hinge needs to be bent to roughly the same profile as the fuselage before fitting, and then there’s just a lot of drilling in assembly. Then I cut the pins for these pieces, and pinned the other hinge halves in place.

Also, I forgot to take any pictures of this, I guess because I was so excited to lay the cowl in place and see how it looked.

Anyway, this got me into the fun iterative part. All told, this cowl probably went on and off the airplane fifteen or twenty times today. Lay it in place, evaluate the fit, figure out where the interference was, take it off, sand that area, lay it in place again…and so on. I was pleased to see that I started with only about a 3/16” gap from cowl to spinner, meaning that I had plenty of leeway to fine-tune the fit while getting the gap right. Eventually I worked up to fitting the plywood spacer between cowl and spinner while doing the fit-ups, and eventually I had a fit that I (mostly) liked at the firewall:

Up front, I’ve got a nice even gap between the cowl and spinner:

I think I’m still going to tweak that fit at the back a bit more tomorrow; there are a couple spots I’m still not completely happy with. Once that’s done, the next step will be to lay out the rivet holes on the cowl, then use those to drill through the cowl into the hinge and cleco everything together. That, in turn, will make the upper cowl stable enough that I can mark the horizontal trim lines along the bottom, and make those cuts. At least those won’t require a bunch of fitting, since they just need to be straight – the lower cowl will eventually get trimmed to match those cut lines.

The one thing that’s bothering me is that the hinge halves for the upper cowl have some slop in the fit. The plans specify undersize hinge pins for those, which are necessary in order to fit the curve of the upper fuselage. But that also means that the hinges don’t fit tight…and that means that it’s going to be hard to get the gap I want between the upper cowl and the forward upper skin. I don’t want a contact fit here; there will eventually be paint in that gap, and if it’s too tight the paint will get chipped removing and reinstalling the cowl.

My best guess for handling this is to allow the lower cowl – which will attach with full-size hinge pins – to do the primary locating work. It’ll be fixed in place, and the upper cowl will be fixed to it via the horizontal hinges that will join the halves. Maybe the best approach is to get the upper cowl fitting tightly to the fuse, drill the hinges such that the cowl can be pulled forward to establish the right gap, and then get everything fixed in place when I add the lower cowl.

We’ll see how this all works out over the next week or so, I suppose.

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