More upper cowl trimming, starting on lower half

OK, so…first order of business today was revisiting the aft flange of the upper cowl. As mentioned yesterday, while I did a lot of fine-tuning of the fit to the fuselage, I still wasn’t completely happy about it. So I spent even more time carefully sanding the flange to further tweak the fit. The good news was that things were close enough that I no longer needed to remove the cowl to sand it. With the spacer between the cowl and the prop spinner, the back of the cowl was pressed snug against the fuselage. That meant that I could wiggle the joint around to figure out where there was contact, then lift that area enough to sand it a bit with some 80 grit glued to a popsicle stick. By doing this, well, a bunch of times, I eventually got things looking really tight:

Since I was finally happy with the fit, it was time to move on to drilling the upper cowl to the hinge halves that retain it. To do this, the holes get laid out and initially drilled in the cowl, then later drilled in assembly with the hinges. As suggested in the construction manual, I laid out the rivets about 1” apart, after marking the ends of the hinge pieces on the cowl. A little work with the rivet fan and I had my holes taken care of:

I also took this opportunity to rough-cut the opening for the oil door; this opening also serves to allow access to the upper cowl hinge pines, so I needed it open for the upcoming work. I didn’t open this up all the way to the plans dimensions; I’m probably going to do something custom with the hinge and latch for this, so I’m leaving extra material for potentially working that out later.

Then the cowl went back onto the fuselage, and I very carefully got it positioned, then started drilling. My approach to managing the gap between the cowl and fuselage is to use that spinner spacer to hold the claw snug against the fuse, then I just manually held the hinge half in the rearmost position while drilling. The intent here was to use the bit of slop in the hinges to allow for moving the cowl forward for final fitting later on. About the only issue here was that while I could easily manipulate the hinge on the right side of the plane – thanks to that oil door opening – the left side was way more difficult. The end result is that I don’t have quite as much room to move the cowl forward on the left side, but I’ll still be able to do even more sanding on that later on to finalize the gap.

Anyway, at the end of all that I had the cowl fully checked to the hinge half: 

And with the cowl off, we can see those newly fitted hinge halves:

There was still more work to be done on these hinges, though; the cowl isn’t as thick as the material stickup in the fuselage, so there’s some shimming needed here to get a nice flush line between the cowl and the fuselage. The plans call for just doing a blanket .032” shim, but I noticed that the gap was more pronounced around the edges of the cowl, and less so in the middle. So I did some trial-and-error with some aluminum scraps; this resulted in using those .032” shims on the outside edges, but then closer to the middle .025” shims made for a better fit.

Once all that was laid up, I got to do the work of cutting the shims, drilling them in assembly with the hinge halves, and finally fitting everything back to the cowl. Once that was done, the cowl went back on – again – to verify that things lined up as expected. The fit still isn’t 100% perfect, but if I want to fine-tune this down the road, I should be able to do it just by adding a bit of micro to the cowl and sanding to fit.

OK, next up – it was time for the rest of the trimming on the upper cowl. With everything else in place, I could use some layout marks I already had just behind the spinner – marking the correct cut point to get things nice and symmetrical with the lower cowl – and lay out the cut lone for the bottom edge of the cowl. My approach for this was to use a laser level to first verify that my reference marks were appropriately level, and then to carry those marks around each side of the cowl.

I started with the fuselage in the level flight attitude – this would have given me a cowl split line parallel to the longerons – but that resulted in the level line dropping off the bottom edge of the cowl at the aft end. So I had to gradually lower the tail until the cut line carried the full length of the cowl. It’ll end up being a degree or two off from the longeron line, but I kind of doubt anyone will notice…

Then I got to go and carefully make Sharpie marks along the level line, along both sides of the cowl… while also making sure not to, you know, lase myself in the eyes:

Then, as with the last trim, I used masking tape to lay out the cut lines all the way around:

Which was followed by rough-trimming, sanding the cut lines close to the masking tape lines, and finally using a long sanding board to finish the entire cut line nice and straight. This was surprisingly not really a ton of work; unlike the aft cut line, which had to be carefully fitted to another part, in this case I’m creating the layout line, so there’s not so much trial-and-error. (That’ll come later when I trim the lower cowl to fit…)

OK, enough of the upper cowl…now it’s time for the real fun, the lower cowl. The lower half mostly butts against the upper cowl, except for the spinner area. Here, the lower cowl has an inset flange that need to fit snugly into the upper cowl…but that there’s no way to test-fit this intersection as provided, because the extra material the outboard cowl edges interferes:

So the strategy here is to carefully and gradually trim the forward outer portion of the lower cowl to hash out this nesting area. The marker line in the previous picture follows the edge of the flange, so that’s hypothetically where I’ll want to trim things here. Thing is, I don’t want to trim any further back than I have to, since I can’t know how what the overlap of the two halves will look until I can fit them together…aka not until I get the front to nest. It’s a sort-of maddening chicken-egg type problem.

The approach so far has been to slowly and conservatively remove material in this area, while repeatedly trial-fitting the halves. Eventually I should be able to creep up on a decent trial fit, but I’m not there yet. I got this far before deciding it was time to call it a night:

So this is where I’ll be picking up next time. I’ll probably need to rethink my localized trimming here to creep up on the fitment…that’s something to mull over for a while. As tedious as some of this is, though, it’s still pretty neat to see the cowl profile coming together.

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