Even more cowl mating

Well, this has been a tedious bit of work so far. After previously doing a test-fit and getting some guidelines to use for working on the cowl halves off the airplane, I’ve been working on getting the forward mating fine-tuned. The first issue was finding the best way to work on the cowl; my previous approach of putting both halves on the floor, aft side down, just wasn’t working for lining them up reasonably.

I decided to try laying the lower cowl up on the work table, and sort of going from there. A jug of checks placed inside the lower cowl, plus some wadded- up old sheets beneath, manages to keep it in a reasonable working position. Then, the upper cowl can sit on top, with the aft ends held together in rough alignment by just a couple cleco clamps. The result actually works out pretty well:

That was the simple part, however. The basic task in getting the forward mating surfaces right is to carefully massage the four points of contact: the two sides of the prop spinner area, and the outboard corners. What I’m going for is having all four points of contact mating once and evenly, while also ensuring that the two air intakes are also the same size. It’s entirely possible to remove material asymmetrically and end up with a lopsided nose, which I don’t want.

The air intake aspect actually made this slightly more straightforward; when I compared the two, I found that the left intake was about 1/4” taller than the other one. Not enough to really be obvious without using a measuring tool, but that meant that I could essentially consider the right outboard corner junction to be my reference point, and do all the material removal on the other three points. This would have the effect of tending to shrink the left intake to make things more even.

The process is simple, but tedious. As long as an unwanted gap exists somewhere, go find the spot where the halves are making the tightest contact, and sand that area until the contact becomes light, or even until there’s a bit of separation for larger adjustments. Then find and sand the new tight contact point, then find the next one…over and over.

At first I was making relatively dramatic adjustments, using a sanding block with 40 grit. As things started to get closer, I switched to using bare sheets of sandpaper, either a bit of the 40-grit – which is relatively thick, cloth-backed material – or some 60 grit I had lying around. The 60 grit is nice thin paper, and works really well for truing up a gap. The approach here is to just pull a strip of sandpaper in the joint to be worked, apply a little pressure to the joint, and work the paper back and forth. An uneven gap will naturally tend to even out, since the tight contact point will put the most pressure on the paper. Once the paper moves with about the same resistance throughout the joint, it’s trued up pretty well.

There’s still a little more work to be done, but overall things are looking pretty good:

Really my only gripe at this point is that the prop spinner joints still aren’t trued up very well – they make good contact on the forward-facing portion, but the flanges have a bit of a gap that opens up towards the back. I’ve about decided that I don’t care about this on the inboard side – this won’t be visible at all when installed on the airplane – but I do want to even up the outboard portion a bit.

Also, the above photo somewhat displays another middle annoying issue I’m dealing with up here – all four points just won’t line up at the same time in terms of the outward-facing areas. If I line the outboard corners up, then the lower cowl sits a little far back on one side of the spinner, and a little bit more on the other side. Note the bit of shadow on the right side of the spinner above. If I close the gap at the spinner, the upper cowl corner overhangs by 1/8” or so, and there’s barely any material left to trim at the back of the lower cowl. On the other hand, if I line up the corner, then I have a similar big gap at the spinner.

In the photo above, I’m basically splitting the difference, which I decided I liked best after trying various approaches. I have to remember that the end goal here is to set the joint on the front of the cowl, so I can then hang them and trim the lower. Sometime down the road, I can worry about fixing the misalignment – I’ll be spending time on the entire exterior of the cowl, and adding some micro up here and reshaping everything will just be part of the fun.

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