Upper cowl rework – making some molds

So…yeah, it’s me. Last time we were here, I made an offhand comment about the cowl not lining up properly near the front corners. I think I said something about it “maybe requiring a little work” to address. Since then, I have spent a lot of time both thinking of the best way to address this, and trying to avoid the inevitable need to cut out chunks of the cowl and completely redo the layup. It also hasn’t been difficult to talk myself out of working on the airplane with weeks on end of triple-digit heat.

This post is sort of a culmination of a bunch of small assorted work sessions over the past week or two, but with some actual consistent work over the past couple days.

The basic deal here is that I need to find a way to create a mold of the upper cowl as it needs to be shaped – then I can adapt this to the existing cowl and work out scarfing in new fiberglass to fix the areas where fitment is an issue. For while I was intending to start with pink insulation foam board from the home improvement store, carefully sculpted to the right shape. The problem with that is that it would require a lot of work to get the shape right, especially since some part of it would have to match the existing cowl. I did go so far as buying the board and starting to work on it, but that just made me even more aware of how much work would be required.

I’d also considered experimenting with two-part expanding pour foam. I’ve seen folks use this for molds as well, and it has the advantage that I can mold the inside of the cowl as a starting point, then build off of that. So I bought some of the stuff, mixed up a test batch, and…didn’t like it. It wanted to collapse after curing, and didn’t get very hard. I was ready to abandon that, when I talked to someone who said that’s definitely not how it should cure. So I decided to mix up a second test batch, and when I did I saw that one of the component bottles was a lot lower than the other. This stuff is supposed to be mixed 1:1, but apparently I somehow mixed it more like 2:1. No wonder it didn’t turn out well. Test batch #2 was much better, and I finally felt I had something I could try out.

So the next step was to prep part of the upper cowl to essentially take a cast with the foam. This just involved me spending a lot of time laying down plastic packing tape inside the cowl, covering every surface where I’d be molding to keep the foam from, uh, becoming one with the cowl. Then I carefully cut cardboard pieces to create a dam around the area I wanted to mold. Nothing here is straight, so there was a lot of fiddly work to get to this point:

Eventually there was nothing left to do except mix up a batch of foam, pour it in, and hope I’d sealed everything up decently. Then I mixed up a second batch, because I’d vastly underestimated how much I needed to fill this space. Then I mixed up a third batch, because it still wasn’t enough. The result is kinda weird looking, but that’s OK, I expect to cut off a lot of what’s seen here as part of shaping the mold:

Then came the real fun: seeing if this mass would actually turn loose of the cowl. It definitely took some muscle, but it finally broke loose. I also had to spend a bunch of time peeling off all that packing tape, which let go of the cowl in favor of sticking to the foam. I was a little worried about the foam filling in the space around the cowl inlet, but it did pretty nicely:

I did some rough trimming and then decided to leave this piece alone for the night (this was Tuesday, I think). The “outside” of the mold is a little irregular, thanks to the packing tape wrinkling on the various compound curves, but I figure once I’m done shaping this, I’ll hit it with some microballoon slurry to fill out the voids and make a nice surface:

So that was one of the two corners I needed to do this for. Tonight, I spent another hour or so going through the same process for the other corner. So at this point, I have the starter molds for both sides of the cowl. I think the next step will be drilling the upper cowl to the hinges as far as I can, then I can start thinking about the fun part of cutting some big chunks out of that upper cowl.

My current plan  isn’t to completely redo the layup in this area – I’d like to keep those corners with the really tight compound curves, and then basically splice them back into the rest of the cowl. I’m not sure how that’ll work in practice, but I’ll give it a try. The cloth I have should be fine for handling the compound curves, but I’d still rather avoid that fun if at all possible.

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