September 21 , 2014 - 6.5 Hours

Work Begins on Lower Cowl; Trim Top & Bottom to Fit Each Other

I'm now ready to begin work on fitting the lower cowl, as I did to the top half. But there's a serious problem here that needs to be dealt with first. In order to get the bottom and top halves of the cowling on the plane, they must first fit together at the forward end. As they come from Vans, they do not! Some serious trimming and sanding needs to be done first, before you can even consider putting them on the plane. Now when you look at this picture, it looks like they should nest together in the middle. But they don't even come close.

Here's why. The top and bottom halves overlap as seen below, and will not nest together. Both right and left sides are like this. They're both the same size and profile, and they're not meant to nest. You can see a line I marked for trimming on the lower cowl. But it's worse than just the sides running into each other. Even though the center areas look like they should nest together, and they do appear to be molded that way, they don't even come close either. Some serious sanding needs to be done before these pieces will nest together. But first, the trimming of the sides. I had already trimmed the top cowl at this point, even or straight-across with the center area.

So how do you go about trimming the sides? How much do you trim? And where, exactly? How do you know where to mark them? And how do you get the center to nest and be just right? I put a lot of time and thought into this before cutting on my cowling. I finally realized that what I want to end up with is a perfectly round center, right behind the spinner. So that's where you start. I began by cutting out a round disk of stiff paper, just the size of the cowling. Here you see it, laid on the bottom cowl:

Now if you fold this in half, you'll get a perfect half of a circle, a semi-circle. And you can use it as a template to mark the cowl. Ideally, I wanted to trim the cowl so the top and bottom pieces were the exact same size semi-circle in the center. The cleco pliers are just there as a paperweight of sorts. Now, you can mark the center of the cowl along this paper disk, but you DO NOT TRIM THIS OFF! This is the lower cowl, and this is where the top and bottom pieces nest.

I used a straight-edge clamped down on the marks, to extend these marks over to each side, so I knew where to mark the left and right sides for trimming:

And here are the trim marks. These trims must be made, before these parts can nest together in the center. You don't have to trim them all the way down the sides. Just "around the curve" on the front. The sides can overlap and be trimmed later. They're plenty flexible.

Here's the trimming I did on the lower cowl, on both sides. Again, DON'T trim the center extensions! I had previously trimmed the top cowl, straight across, using this same method. No center extensions there, so you can make the top half a perfect semi-circle. Precise, semi-circle marked in the center, extended to the sides, and the sides trimmed and sanded straight with my sanding board.

Now, once the sides are trimmed, it still won't nest together because the center areas need a lot of sanding. The top cowl is supposed to fit down over the lower cowl's center extensions. But as these come out of the mold and from Vans, the center areas need a ton of work. At least mine did. Mine were very thick and irregular. I found the best tool for this was my Dremel tool with a small sanding drum. It rips through this stuff like crazy! So be careful, but just sand and examine it closely until the parts nest together in the center.

You can sand the inside of the top cowl, seen below, and the outside of the bottom cowl extensions until they fit together nicely. I was watching so I didn't sand anything too thin and lose structural integrity.

My goal was nice smooth sanded areas, evenly thick, and a nice fit when you nest them together. I used the Dremel tool for rough hogging out, then switched to the sanding board and other sanding tools for a flat smooth finish. Through a lot of trial and error, many test fittings, and more sanding, it will finally begin to come together.

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