Dave’s RV-7A Building Log

I re-checked my measurements from yesterday and drew another line 1/16″ of inch behind the one I drew yesterday. I trimmed to that line with the plexi cut-off wheel. The original line was really very close to the correct cut off. I used a belt sander and then sand paper and block to get closer to the cut off line. I put the cowl on and off several times, sanding in between. I got it to the point where it looks pretty good. I could do some more tweaking and get it exact, but I think I’m going to fit the bottom cowl before I do that.

Working with the cowl is much nicer than the canopy. The cowl is lighter, less prone to breaking, easier to cut/sand, and it’s easy to fix any mistakes. I was singing its praises to Julie. She started to worry that I’m going to build a plastic plane next. I just think it’s a nice contrast to working with metal or plexiglass.

Today’s session was a bunch of measuring, tweaking, clamping, and marking. I started off by cutting the slot in the bottom cowl for the nosewheel. It’s fairly difficult to determine the middle of the cowl when the corners of the thing are all rounded. I finally got it figured out and cut a 12-1/2″ deep slot with the canopy cut-off wheel. Initially I didn’t realize that I had a regular cut-off wheel in the grinder. That thing didn’t work at all. You definitely need to use a plexi cut-off wheel. Cutting epoxy fiberglass makes a nasty-looking cloud of fine dust. Definitely different than the canopy snow storm. I didn’t make the slot pretty yet, as I’m sure it will eventually need to be longer.

I was then able to hold the bottom cowl in place to get a picture of how the cowl halves will line up with the spinner back plate. It then dawned on me that it would be much easier to figure this out with the cowls on the ground. I fit the two halves together as best I could, and clamped them in place. I then measured where I thought the spinner back plate should end up and put some marks on the cowl.

With the top cowl back on the plane, I centered the cowl side-to-side and make a registration line on the front top skin and cowl. Using some perfect width blocks (1/4″ plate to cowl gap), I clamped the front of the cowl to the spinner back plate, lining the plate up with the marks I made on the front of the cowl and making sure the cowl was centered using the registration mark at the back. It sounds complicated, but it was actually fairly easy if not time consuming.

I let the cowl sink down about 3/32″ relative to the mark I made. This is to allow for engine sag. I’m not yet sure if that is a good amount or not. I’ll check around.

Using the lines I made on the fuselage skins previously, I then marked on the cowl where it needed to be cut off to be flush with the firewall flange. It’ll take about 10 minutes to cut it, but this is a good point to step back and make sure I have everything right. I’ll sleep on it.

I drilled the top cowl hinges and shims to the firewall. They turned out pretty nice.

I did a little more work later in the evening. I beveled the hinge eyelets so the two halves of the hinge would fit together despite being curved. I also cut the hinge pins and attached the other half of the hinges. I placed the top cowl in place and clamped it to the spinner back plate with some wood blocks. It still needs some tweaking, but it’s looking good. I’m getting close to the point of trimming the aft edge of the top cowl.

I unfluted the firewall flange without any problem. I also filed a couple of the flange tabs that stick out just a millimeter past the top forward skin. I also measured the thickness of the top skin plus firewall flange and compared that to the cowl thickness. It seems like a 0.15 shim would work best. I found some some stock that was 0.16 and made some shims. It was pretty time consuming to cut and deburr them.

I also placed the top cowl into position just to check it out. Looks cool.

I continued prepping for fitting the top cowl half. When I tried to put in all of the structure under the forward top skin, I realized I hadn’t cut a hole in the middle rib for the canopy release mechanism. It seems that many builders ditch the mechanism, but I’ll keep it if at all possible. I cut the hole and trimmed the rib as needed. With the top skin clecoed back in place, I measured and cut the hinge for the cowl/firewall joint. I realize now that I shouldn’t have fluted the flange on the firewall. I’ll have to flatten it out. Hopefully the holes still line up OK.