Cowling

August 26, 2014 - 3.5 Hours

Begin Fitting the Top Cowl

One of the things I love about summer is the nice weather. My garage is only 20' x 20' and at this stage of the game, there's not a lot of room in there. So whenever possible, I like to pull her outside to work. That's what I did today, to begin the fitting and trimming of the top half of the cowl. The idea is real simple. You just lay the cowl down on top of the engine, leave a 1/4" space in front between the front of the cowl and the spinner. Then you can mark the aft edge for trimming to fit.

I had to admit, it was pretty exciting to see the final lines of the airplane for the first time! At least across the top of the cowl. I made a bit of a mistake here. In my anxious state of mind to get started, I simply used some 1/4" sticks and clamps to hold things in place at this point. This led to some trouble, as I'll describe shortly. There's a better way to go. I cut a piece of black PVC tubing as a spacer, to put in between these spinner plates where the prop will eventually go. That way I could put the prop bolts on to hold it all together and simulate how things will actually be. Turns out, there's too much flexibility in these spinner plates, by themselves. To really stiffen things up, the spinner should be clecoed in place as well. I learned that the hard way, too. There's another good reason to have the spinner in place, and that's to set the height of the cowl, up or down in relation to the spinner. Most builders want to account for some engine sag, and they leave the aft edge of the spinner sitting up above the cowl about 1/8" or so. My engine has been mounted for quite some time, so I may not get much sag, if any. How can you really know? This is one question that is somewhat of an unknown for me, and I'm not really sure what to do. I think I'll split the difference, and maybe leave about 1/16" for sag. If it sags more than that, it will still be okay.

Before laying the top cowl down, I had previously measured carefully and drawn a line with my fine sharpie, exactly 2" back from the front of the top skin. Then you lay the cowl down, measure 2" from the line and mark the cowl for trimming. I also marked the cowl for the center, so I would make sure to get it accurately placed each time it goes on. It's hard to see below, but the cowl is lying on the front skin (all clecoes removed for now underneath it), ready to mark for trimming.

Here you can see my marked trim line, and the cut has been started at the top.

Here's my die-grinder with the cutting disk, used to make the cut. It reminds me of cutting the canopy. With a lot less stress! Cut well behind the trim line, so you can carefully sand up to the line and not go too far. So far, so good!

Here's one of the most valuable, important tools you'll need for working on the cowl. A nice, perfectly flat, sanding board. You can use a board, a 2 x 2, or in my case, a leftover piece of stiff aluminum angle. I used part of a sanding belt from a belt sander, cut to fit and glued to the aluminum with some spray-on 3M adhesive. I left it oversized until it was firmly glued in place, then trimmed it to the sides of the aluminum. You need this tool to do accurate sanding of the edges of the cowl, to avoid any highs and lows, or waves. It's essential to get a very straight, perfectly flat, good edge. I use my fine wire brush and shop vac to clean this up when it gets clogged with fiberglass, and it greatly extends the life, before pulling the sandpaper off and replacing it with new material.

After carefully sanding down to the line, I put it back on the plane. Clecos back in place now. The initial cut and sanding looks pretty good! It was on and off several times as I sanded my way carefully down to the line.

The left side of the cowl, looking pretty nice!

When I first marked this line, I continued the trim line from across the top, in a straight line down to the edge. But you can see it takes a joggle or turn here, along the side of the fuselage. You don't know precisely where the "corner" of this joggle is until you get the top edge sanded so it will lay down flush on top of the hinge. Then you can mark it and draw this second trim line. This is another reason why I marked the center on the very top of the cowl, so I get it positioned carefully in the same position every time. You also see the fine line 2" from the front, down the side of the fuselage. Same thing as across the top. Measure 2" forward from this line, mark the cowl for trimming.

That pretty much wraps up this session. I feel like a lot has been accomplished so far.

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