Canopy Notes
Now that I'm further into buiding the canopy, I thought I'd write down some of the tips I've picked up and the lessons I learned the hard way. I'll continue to update this page as I learn new things.
Figure out how many clamps you think you'll need, double it, and buy twice that many. You'll still need more later but this will get you started.
Always wear a dust mask and eye protection when cutting plexiglass. I also wear hearing protectors to keep the dust out of my ears. Wear long sleeves if you can stand the heat, or at least wear gloves to keep molten pieces of plastic off your hands while cutting.
Use a separate laundry basket for your plexiglass-dust-covered clothing so you don't get grit all over your good clothes. I had to wash my work clothes multiple times to get all the grit out of the fabric.
An inexpensive laser level like this one is handy for marking a centerline on the canopy bubble.
Always tape up the edges of the protective plastic film before making a cut. This keeps grit from getting under the plastic and scratching the plexiglass. It also keeps hot molten pieces of plexiglass from landing on the canopy and burning little pits in the virgin material. As you trim the canopy, cut the plastic film about two inches back from the new edge and reapply masking tape.
Duct tape leaves a residue on plexiglass that's obnoxious to remove, but masking tape comes off cleanly.
I was told by a composites guru that isopropol alchol is safe to use on plexiglass, but anything stronger (e.g. acetone) will melt it. I found that you can use a sharpie to mark directly on the plexiglass, then wipe it off with alchol. After I was finished with all the canopy cutting, I read somewhere that a dry erase marker works even better.
You can use a regular #40 bit to drill the first hole through plexiglass. It's only when enlarging an existing hole is a special plexiglass drill bit absolutely essential. When enlarging a hole, back it up with a piece of wood to keep the drill from breaking through and grabbing.
Under no circumstances should you try to adjust the bend of the slider frame after the canopy has been drilled. The plans seem to advise you to do this, but every source I've asked has said it's a recipe for a cracked canopy. Instead, get the canopy to roll smoothly with the glass clamped to the frame, before you drill anything. This will probably require bending a substantial preload into the frame to counteract the tendency for the glass to spread out the frame.
The plans have you trim the bottom edges of the bubble even with the bottom of the square canopy frame tubes before making the big cut to separate the bubble into separate canopy and windshield pieces. I found that this leaves you just barely enough windshield material later on. Next time I will trim off the mold flanges, make the big cut, and only then start removing material from the bottom of the canopy.
A Norton cutting disc in an air drill can be used to make fairly precise cuts if you wear a leather glove on your off hand and use it to steady the drill chuck while it's spinning. A pneumatic cut-off tool is less accurate but can be used one-handed. Both tools are useful at different times.
A small handheld belt sander does a dynamite job of smoothing out cut marks and generally cleaning up edges of cut plexiglass. Go easy to prevent heat buildup in the plexiglass (heat leads to stress leads to cracks).
The Avery edge scraper is very useful. I use it in conjunction with sandpaper to smooth and round off edges.
The method recommended in the plans for using masking tape to find the centerline of the canopy frame really works. I found it works even better if you use two layers of tape so there's more stuff to compress.
When drilling the canopy to the frame and the windshield to the rollbar, always use shims in places where the plexiglass doesn't want to lay down properly on the tube. The last thing you want to do is force the plexiglass to do something it doesn't want to do, because you're setting yourself up for a crack later. I used small nylon washers for shims.
Be very particular about how you initially position the canopy frame inside the bubble before you drill the hole for the canopy latch tube. Once you make that hole the fit of the center spine to the canopy will be locked in, so you want the fit to be as good as possible. I managed to get mine to fit quite well.
You can drill all the holes in the slider tracks and the center slider rail when you fabricate those parts, but wait to drill all the holes that attach them to the fuselage until you're finished with all canopy construction. If you attach the tracks and rail with only two screws each (one forward and one aft) you have some leeway later to tweak the final position.
After making the big cut, never set the canopy down without supporting it somehow, or you run the risk of letting it crack. I found that a couple of boards clamped to the table were a handy way to keep the canopy sides from spreading while it was sitting upright.
Pay close attention to the relative height between the aft edge of the canopy glass and the fuselage skin. Ideally you want to end up with the glass and skin at the same exact height, so the aft skirt will flow smoothly from the canopy to the fuselage. I didn't fully grasp the importance of this until it was too late, and ended up with the aft edge of my canopy being too high. I covered up the mismatch by building the aft skirt out of fiberglass, but it would have been better to get them level with each other to begin with.
Leave about an inch and a half of space between the slider frame side tubes and the bottom-most holes in the front and rear canopy bows, so that none of the pop rivets will go through the side skirts. If you don't, you may end up with edge distance problems like this.
When fabricating the parts for the side skirts, if you follow the spacing given in the plans for cutting the lightening holes in C-791, you'll end up with edge distance problems like this. Instead, mark your own lightening hole locations to avoid interference with the rivet holes in C-660 (pictures here and here). Also leave yourself some extra material at the forward end of C-791 so you can cut a relief notch to clear the nut that attaches the roller brackets to the slider frame (photo).
I didn't like the idea of attaching the canopy directly to the forward bow with pop rivets, so I made a little cover strip that goes along the forward bow and gives the plexi some room to grow and shrink without the rivets bearing on it directly.
