I decided to hold off on glassing the bulkheads until I was able to type up more detailed steps for doing it. I was finding way too many things in the plans that I might miss if I didn't have them written out step by step. In the meantime, though, I decided I could move ahead and cut out the firewall pieces.
I tried to pin the firewall templates to the plywood sheet, but the birch plywood was too dense for that, so I taped it down instead. I then tried drawing a line around the edge of the template with a pencil, but keeping the line on the very edge of the template got very tedious very quickly. I went ahead and cut the firewall out with the template still attached. I made my first cut an inch or so from the edge to see how the jigsaw I was using would handle the thin, dense plywood. When I saw how much the wood blade was tearing up the veneer, I stopped and switched to a metal blade, but it didn't seem to want to change direction too well. I switched back to the wood blade and used it to rough cut the firewall shapes, leaving an inch or so around the edges. I decided it was time to buy myself some new jigsaw blades and use those for the final cut, so I called it a night.
I came back the next evening with a new set of blades made specifically for plywood and got to work making the final cuts on the upper and lower firewall pieces. I found that my hands weren't as steady as my eyes wanted the lines to be. After awhile I moved to cutting out the shapes about 1/8" from the edge, then came back again for the final cut. The straight edges were more difficult than the curves. I clamped a straight edge along one side and tried to use it for a guide, but the jigsaw still wanted to wander off on its own. When I made cuts near an edge, the blade would start to bend away from the cut, giving me a beveled edge that I obviously didn't want. I ended up spending a lot of time using my wood rasp and sanding blocks to finish the edges and get them reasonably straight and square.
The next night I cut out the two side pieces. These pieces were smaller and all straight cuts. I scored along the cuts this time before cutting them with the jigsaw to give me straight edges to work from and to keep the wood from splitting. I used the jig saw again because I didn't want to have to haul out the table saw only to find that it was shredding the wood with its blade. The jigsaw has a laser that marks out a straight line for you to follow, but I found I couldn't see the blade clearly when I used it, it so I ended up turning it off. The cuts were a little better than before, but I still had to spend some quality time with the wood rasp and sanding blocks to get straight clean lines.
I cut out the temporary firewall a few days later. I wasn't sure how to handle the sharp turns around the sides, so I just rounded them off. I'm not sure what any of the holes are for yet, and I'm a little reluctant to start cutting them out until I'm sure. I couldn't find any fir plywood, so I used pine. I figure I'll have to add a couple of stiffeners to it as well.
I waffled over the engine mounts; several builders were uncomfortable with how close the bolts were to the edge when installing the engine mount and suggested making the mounts 2" square rather than the 1" called out in the plans. The blank that I ordered was 2" by 4", so it would have been easy enough to do, I guess, but I decided to stay with the 1".
After my troubles with trying to cut straight lines on the firewall with the jigsaw, I didn't want to repeat the experience on the engine mounts. I asked around on the canard aviation forum about cutting the aluminum, and got several ideas, but I ended up getting the urge to DO SOMETHING again, and simply had at it with the jig saw using new metal cutting blades. The blade did wander a bit, but not as bad as I had feared, and filing the edges flat didn't take nearly as long as I expected. I had marked the lines with a permanent marker, only to discover that they're not permanent at all on aluminum. Next time I'll use a nail to scribe the edge, as someone in the forum suggested. Of course, they weren't exactly 1" on each side, due to the kerf of the blade. They weren't exactly square, either, but I figured if I kept filing them down to try to fix it, they'd end up being too small to use.
I also asked around the forum on securing the screws to the firewall, since there seems to be a strong consensus that the plans method leaves a lot to be desired. I liked the idea of using pieces of welding rod that fit into a slot cut into the firewall, but I don't have any idea how to weld them to the screws, and I'm not keen on having to learn welding for such a simple task.
A week after building the engine mounts, I had the day off, so I decided to go see what more I could do on the bulkheads. I had never really studied what all the holes in the firewall were for, so I spent some quality time with the plans trying to figure it out. Hmm... seems there are FOUR engine mounts required, not just the two I has built earlier - doh!
I pulled out the jig saw again and tried to cut two more mounts out of the blank. I got about halfway through the first cut before I ran out of room on the workpiece - either the jigsaw shoe would hit one of the clamps I was using to hold the blank down with, or the piece would wander off while I was cutting. I scratched my head for awhile, then decided to try mounting the jigsaw upside down in my Workmate stand and using it like a band saw. I was VERY nervous about doing this even though I had seen it used before. The workpiece was pretty small, and I'd have to get my fingers pretty close to the blade. The first cut went okay, and the second was doing alright until I noticed the workpiece getting hot - as in too hot to hold on to! I slowly backed the workpiece out and shut down the saw, then put on a pair of auto mechanic gloves. I also decided to swap my regular glasses for a pair of safety glasses. I carefully started the cut again, but it started getting hot again, and even with the gloves I had to back the workpiece out again to let it cool. When I slid the workpiece back into place, it caught the blade wrong and jumped out of my hand. The scared me enough that I gave up the idea of using the jigsaw altogether and resigned myself to spending half an hour or so with a hacksaw to make the final cut. I clamped the workpiece into my Bench Buddy vise and had at it with the small hacksaw. I was amazed when it cut through the workpiece in less than two minutes. So much for the best laid plans - I could have done all of the pieces by hand in less than half an hour if I had tried the hacksaw in the first place! I had cut the pieces oversize so I could file them down to the right measurements, which took less than ten minutes. They weren't perfectly square, but I knew that if I kept trying to fix them, they would just keep getting smaller and smaller.
I marked the locations for the engine mounts on the firewall. I have to say that it took awhile to find a good reference to measure from. The plans spell out the B.L. and the W.L. for the mounts, but there isn't a zero level W.L. to work from. I measured the dimensions on the plans to find the W.L. for top edge of the bottom piece and the bottom edge of top piece and used those to mark off the mount locations. Since my hard points weren't exactly square, I traced the outline for each one, then used a wood chisel to make the cuts. I worked pretty well and I got a good close fit with each mount. I did get some tearout on the back side of some of the holes, but I can probably fill that with flox or micro before I lay the glass down. I also numbered each hole and each hard point to make sure I matched them up again when I was ready to lay the glass.
I screwed a 1x4 to the back of the temporary firewall to straighten and stiffen it up, then cut out the longeron holes. I'll probably have to open the holes a bit when I attach the sides to the firewall, but I cut them as close to the correct size and location that I could. for now.
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