Letters from Flyover County

Cutting plastic

by Bob Collins

One of the things that fascinates me about building an RV airplane, is the opportunity to work with a variety of materials. By this stage of construction -- the finishing kit -- most builders (and even me if I think about it long enough) are pretty darned comfortable with sheet metal work. In fact, by now it becomes our security blanket. Nervous about working with fiberglass or plexiglass or electrical work? No problem, do some airframe work on ye olde aluminum.

But these "other" materials that I get a chance to work with also give me an opportunity to learn about their properties. In essence, they have their own personality and character, and "meeting" them is almost as much fun as strolling down a hangar row and finding a few open doors, with a pilot inside.

That's where I am with my plane project right now. "Bob, meet Mr. Plexiglass." I've been working on the canopy -- off and on -- for a couple of weeks now and have made my initial "practice" cuts and now I'm getting down to the point of making smaller cuts as I near a final shape.

Fortunately, the Earth is about to implode with heat -- at least in Minnesota -- so it's been warm enough to cut plexiglass with a minimum of risk. Oh sure, there's always the chance I'll do something stupid, but knowing a replacement costs $1,200 motivates me to employ the "measure a gazillion times, cut once" philosophy.

Last night it was 83 degrees in the garage at 9 p.m., and what with everyone still firing off July 4th fireworks, it wasn't like I was going to disturb anybody by cranking up my compressor and die grinder.

I have to say I've enjoyed working with the plexiglass so far. Last night I learned there is a huge difference when cutting plexiglass when it's 80 (the temperature when I've done most of the cutting) and when it's 83. Last night, the cutting wheel sliced through it like it was butter. Three degrees certainly made a big difference.

Sanding the edges, too, is good for giving one a feeling of accomplishment. I've treated the edges on every cut so far -- even the "practice cuts." It teaches me patience and reinforces the notion of doing a quality job, not just charging ahead. I start with an 80 grit and then a 130 grit roll on the new belt sander, then switch to a manual sanding block at 150, 180, 240 (or whatever) and 320, before finishing it off with 400 grit. I'm not sure just how smooth the edges need to be when done but they're smoother when I treat them this way than the edges were when the canopy arrived from Van's.

I had made some drawings on the fuselage to try to get a symmetrical cut, or at least to try to translate a very poor outline of the canopy on the forward fuselage in the Van's Aircraft plans.



Online sources had basically told me to cut where the horizontal part of the canopy meets the vertical part, but you know what? I'm still not sure I want to do that. I did cut, last night, the very center part right up to that point. But looking at the plans, and transferring the measurements to the forward skin, gave me a different cutting pattern from the center to the side.



Here you can see the mark on the fuselage and the curved cut I had in mind. But I didn't make it. I overshot it in some spots (I actually was staying close to the point where horizontal meets vertical as I got nearer the edge, and that conflicted with what I had envisioned based on the plans), and left material on in another.

Now look at the right side.



Again, you can see the planned line vs. what the canopy shape was telling me to do. It's not symmetrical with the other side, but now that I've made the sort-of-but-not-quite final cuts in the front, I'll measure it off to get both sides correct... or at least symmetrical. The line paralleling the forward skin is the 1.75" that the forward edge should touch. I like it in the center there and I'm not inclined to cut anything more off. The plexi -- and it's impossible to really tell -- seems like it's within a half to an inch of the rollbar and other sites have indicated that Van's says that's fine.

The question I have now is how much more to cut before making the "big" cut; slicing the canopy along the rollbar.

For example, the front sides still need to be trimmed to allow it slip between the "ears" of the skin, but should that be done now or after the "big cut." I have a flush fit with the forward skin and in the instructions, Van's says that's what I should be working toward.

But the instructions aren't clear when this should be done. In fact, they say to tape down those ears when fitting the canopy and -- a few paragraphs later -- admonishes that I should not "attempt to do any trimming to final size other than around the base of the windscreen, until after you have split the canopy at the roll bar."

Does that mean I don't chop anything more off the sides until then? Do I leave the "ear flaps" taped down? Am I just to be concerned about the front fit?

What's fascinating -- at least to me -- is by taking my time doing this (folks had said it could be done in one day.), it gives me a chance to think more about what I'm doing. Sometimes that's good, sometimes it's not. Give me more time to think, and I've got more time to get nervous about doing something wrong. The more small trims I make, the closer I am to not having any room for error. Frightening? Sure. Exhilerating? You bet!

As you can see, I still have lots of questions before the next step (feel free to answer them). But I'm running out of plastic.

I love building my airplane.

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