Cowl

So the general to-do item right now is to redo the cowl hinges. But before I can figure out the length of the hinges and their layout, I need to decide where to locate the hinge pin covers. This has been a source of quite a bit of head-scratching and second-guessing over the past, well, six months. All that time when I wasn’t making blog posts, I was still occasionally taking a look at this issue and trying to figure out how to solve it.

These nice machined covers from Aerosport Products are intended to match the profile of the cowl, but that kind of depends on putting them in the right place. And while they have specific instructions for locating the covers on some RV models, the RV-8 is not among those. I’d previously tried using the RV-7 instructions as a starting point, but they didn’t really work due to apparent differences in the cowl design. So that left me with trying to reason the whole thing out.

To save some time, I’ll skip to the end here – I decided the best way to experiment with location and fit was to make a female mold of one of the mockup covers (these are used for fitment instead of the actual covers). So I covered the, um, cover with plastic wrap and then laid up a few plies of fiberglass over it. Once cured, I sanded the mold to shape, and now I had a way to truly visualize how the perimeter of the cover would rest against the cowl.

This still wasn’t the panacea I might have hoped for – while it gave me a good look at fitment, it also showed me that there was apparently no location where the cover would perfectly match the cowl. That at least allowed me to stop trying to think the problem into submission, and I just chose the best apparent location and moved on. Later on, I may add some micro around the covers to make them blend in better, but I’ll cross that bridge when I get there.

In the meantime, I moved on to laying out the hinge on the lower cowl. I figured out the overall length, laid out the end locations and the outer rivet holes, and marked my guide points for laying out all the other rivet lines with the rivet fan. I shifted all the holes a bit in order to try and put them in between the previous holes that I filled. This is probably overkill, but…well, I am still me, so here we are.

Drilling will need to wait until another night; first I have to remove the lower cowl again and modify the vertical hinges that attach it to the firewall. Those will now interfere with the new horizontal hinges, so I’ve got to solve that problem before I can get to actually drilling the horizontal hinges – but I have a good sense of how I want to handle the situation.

Just more chipping away at the cowl hinge. Last night, I worked on modifying the lower cowl-firewall hinge. In order to accommodate the new horizontal hinge location, I needed to remove one eye from the cowl half of the firewall hinges. Once those were trimmed, cleaned up, and reinstalled, I got the two lower horizontal hinge halves cut to length and those cut ends cleaned up as well. I also broke out the rivet fan and marked all of my rivet hole spots on the lower cowl.

Tonight was drilling time – just clamping each hinge half in place and doing a lot of drilling, moving clamps, adding clecoes, and so on. Once the two lower halves were drilled and flecked, I marked the upper halves and got those cut to length as well. Last order of business for the evening was to lay the upper cowl in place again and mark the area where I needed to remove some material to accommodate the hinge pins during fitment – that’ll be where I pick things up next session.

In the meantime, hey look, it’s a hinge:

Straightforward stuff again this time. Last night I opened up a small passthrough on the upper cowl for the hinge pins, then laid out the rivet locations. Tonight I made some bracing blocks from wood, to support the aft end of the upper hinge half while drilling, then got the upper cowl in place and did the actual drilling.

I’m now trying to decide whether I want to go ahead and mark/cut the openings for the hinge pin covers, or if I should go ahead and rivet the horizontal hinges in place first. I’m kind of leaning towards the latter approach – I’ll likely want to do some sanding/shaping at the horizontal cowl joint, especially towards the front where the covers will be, and I can’t really finalize all that sanding/shaping until the hinges are permanently attached. The main reason for that is that things always fit differently with actual rivets instead of clecoes, and I don’t want to do any finish-fitting (eg setting the gap between cowl halves) with temporary fasteners in place.

On the other hand, just cutting the cover openings is pretty low impact, and would make installing/removing the pins a lot easier for the time being. The point at which I’ll want something resembling final fit is when I add the backer flange for the covers…but that can wait a little while. Maybe I’ll rough-cut the covers tomorrow and then switch gears to attaching the hinge halves.

So…I was prepared this morning to go forward with my plan to rough-cut the pin covers, then pivot to working on the hinges. Instead, I took a brief detour. As I was considering the placement of the covers and marking the cut lines, I decided that it’d be a good idea to patch up the previous pin passthrough cuts I made. These cutouts are just a bit forward of where the new cover cutout would be, close enough that I figured I was likely to mess up the cover cutouts if I went back and patched the old cutouts later on.

So I pivoted to working on those. I sanded the perimeter to a decent taper, then laid up four strips of cloth across the inside of each cutout. I pressed the cloth just a bit into the cutout, mainly to ensure good contact with the taper I’d sanded. The basic idea was to bridge the gap with cloth, then come back later and add flox to fill things out (the latter bit was done this evening).

In the meantime, while the first layup was curing, I pulled the horizontal hinges and deburred all my rivet holes. I also countersunk the rivet holes in the lower cowl…upper cowl will wait til later, ie once the current layup/patch work has cured.

So tomorrow I’ll get to do more sanding (it never ends with fiberglass work), then I’ll probably take an initial look at the gap between the upper and lower cowl. While I don’t want to try to get it precise until the hinges are riveted, I’d at least like to get a rough fit done. In particular, right now there are a couple spots where the upper and lower cowl halves make contact and interfere a bit.

I think I’m also going to add some material back to the upper cowl and rework some of the gaps there. There’s one spot in particular where I ended up with a much larger gap than I really wanted.

Worked on a few different things today, all of it looking towards getting those hinges riveted in place. First item of business was laying out and rough cutting the cowl pin covers. With both cowl halves in place and pinned together, I used the cover templates to trace the outline off each side of the cowl:

Then I super rough cut the openings with a spiral bit in the Dremel – that worked mostly OK but boy does that thing like to wander around. Next I used a rotary file bit to shape the edges and get them a little closer to the cut line. The final cut will wait until after I’ve riveted the hinges so I know everything is fixed in place:

Well, mostly. After working on the layup on the upper cowl (more on that in a moment), I went ahead and sanded the cutouts in the lower cowl to snugly fit the template covers. These cheap covers are slightly oversize compared to the actual ones; a snug fit for these templates will allow for a nice paint gap around the final covers:

OK, so about that layup on the upper cowl. I decided I didn’t like how the gap ended up between the right side of the upper cowl and the firewall, so I wanted to add some material so I could rework the gap. The basic idea here was to sand a scarf joint and lay up a bunch of layers of cloth to overhang the edge. To get the contour of the cowl right, I clecoed a piece of aluminum scrap in place on the outside and covered it with packing tape to keep the fiberglass from sticking:

Then I laid up eight layers of cloth on the inside, which should give me about the same thickness as the original cowl edge. Once this cures, I’ll have a lot of sanding to do to clean up the scarf joint here, before I even get to working on the actual edge:

Finally, while working on other stuff, I had an idea for improving the cowl alignment at the firewall. The basic issue here is that the very upper edge of the lower cowl is unsupported, along with about a 6” portion of the upper cowl in the same area. So while everything else is pulled into nice alignment the upper edge of the lower cowl sits a bit proud of the fuselage skin:

The construction manual suggests possibly drilling a hole in the firewall for the hinge pin to slide into, this providing some additional support for this area. That seems like a decent idea, but I’m not a big fan of unsealed holes in my firewall, no matter how small they are. But I realized that I might be able to add some aluminum blocks inside the firewall flange, which could in turn force the pins inboard and pull the cowl edge into alignment.

Some experimentation with scrap from my panel cutout looked promising, so I cut a couple pieces and match drilled them. I also added a taper on the forward edge of the spacers, to help guide the pins into place as they’re inserted. These pieces still need some final trimming and cleanup, but the idea seems sound:

Much better alignment with the spacer in place:

I guess the next order of business will be cleaning up that flange extension – as mentioned, lots of fun sanding in the works there. Then maybe I’ll see about riveting all these assorted hinges in place…

So the last two nights’ work has been doing lots and lots of sanding on that extension piece I grafted not the cowl. The basic idea here was to sand a scarf, lay up multiple layers of cloth, and then sand the overlapping area back down flush. This was, to put it lightly, quite a bit of sanding. Especially since resin and cloth makes for some pretty hard stuff that doesn’t exactly sand easily.

It also makes it more fun that this is a contoured surface, though at least it’s not a compound curve. Still, that means my normal tools for trying to sand a straight line, like a nice sanding block, are no good here. I ended up wrapping sandpaper around a deep socket – yes, you’re reading that correctly – to make a sort of round sanding block. Then I went to town with that thing, for…a lot of time. I think it was about 45 minutes in that I really wanted to go tell Past Philip that that old gap really wasn’t that bad after all…too late now, though.

Tonight, I finally got things sanded down to a relatively flush point, so I switched to a soft foam block and 60-grit paper to finish the blend between the old and new glass. The result of all that work was this:

Next up was getting started on re-trimming that extension. I basically repeated the initial trim steps – laying the cowl in place, marking a conservative initial cut line, then gradually sanding the edge to fine-tune the fit. After a couple iterations, I was able to pin the two cowl halves together again. The gap still needs some work, but I think I’m going to go with the approach of riveting all the hinge halves in place before I remove any more material:

Oh, and I’m still not done sanding here, even ignoring the gap itself. I took a wild guess that eight layers of cloth would be about the same thickness as the rest of the cowl, but that looks to have been too many layers. The new grafted edge is a decent bit thicker than the rest of the cowl, to the extent that I can’t pin the upper cowl to the firewall right now – too much misalignment. So I’m going to have to work on thinning that extension down some before I can put this whole task to bed.

In retrospect, it probably would have been smart to do some test layups to get an idea of the finished thickness of different layer counts. But, much like the whole “that gap wasn’t so bad after all,” that ship has sailed, and there’s not much to do except…more sanding…