Dave’s RV-7A Building Log

I didn’t get much done tonight. Helping somebody with her book report is more important. I did sand the edges of the cowl a bit more and widened the slot for the nose gear leg. The slot is almost wide enough now. I’m just going to make a couple of small semicircles right around the leg to finalize the fit. The edges are getting very close now as well. It’s almost time to do the horizontal hinges. I made up a new sanding board as well, as my old one is fairly worn down. Fiberglass is easy to sand, but it wears down the grit pretty quick.

I started making a check list of items to do on the cowl. Working in little 1-2 hour sessions, it’s easy to forget things.

I finished up the shims I started making last time for the sides. The bottom hinges didn’t need any shims, but I used some 0.016 along the top and sides. With the side shims installed the cowl seems perfectly flush with the sides.

I attached the bottom cowl again, got it exactly into position, and drilled the holes along the bottom hinge. I then laid out and drilled holes along the sides. A trick I figured out for holding the hinge steady while drilling is to put some masking tape over the loops. That holds the hinge in position well enough for drilling.

With all of the hinges along the firewall drilled, the cowl is very stable. I removed the top cowl just to check out the clearances below the engine. The starter is pretty close to the cowl, but not too close. The Mattituck alternator looks close as well, but I won’t know for sure until I install the belt and position it correctly.

I did a bunch of little things fitting the cowl. I trimmed one loop off the bottom hinges at the outboard corners. They were keeping the corners from fitting well. I made the slot around the nose gear leg wider. I marked some lines on the bottom cowl using the bottom skin rivet lines. I then used those lines to straighten up the nose gear leg slot. I trimmed the horizontal edges of the bottom cowl to within 1/16″ of the desired line in stages. I laid out and drilled holes in the bottom of the bottom cowl for drilling to the hinges.

I think that was it. I try to get as much done for each iteration, but I still end up removing and re-installing the cowl a few times per hour. It’s not difficult to do–just repetitive. This is a stage where it definitely pays to be careful, though.

Another long day of fitting the cowl. I cut and countersunk hinge material for the sides of the fuselage.

I must be missing something, but it seems like Van’s didn’t supply enough of the larger hinge pin material. I have gobs of the thinner pin, but am about 4″ short of the thick stuff.

The bulk of today was many iterations of removing the cowl, tweaking the edges with the sanding board, putting the cowl back in place, and marking for further tweaking. I worked slowly and deliberately, but I think that paid off. The cowl now fits pretty well all the way around at the firewall. I still need to make a paint gap between the cowl and aluminum. I’ll do that before riveting on the hinges. I’m thinking 1/32″ will work fine.

I drilled a few keeper holes along the aft edge of the bottom cowl to stabilize it. I also sanded the horizontal edge of the top cowl until it was straight. I then marked a trim line on the bottom cowl.

It seemed like a long day, but the cowl is really shaping up nicely. This pic shows the final state for the day with just the horizional trim left to do. You can also see the various chunks of duct tape I’m using to hold the cowl where I want it.

Today was sort of a blur of taking the cowl off, trimming and/or sanding, and putting the cowl back on. The sanding I did around the spinner area helped cure the “over-bite,” but I needed to drill new keeper holes at the front. That improved things immensely. The old holes won’t be a problem because they get drilled out to a bigger size anyway. Here are side views from the left and right of the area behind the spinner. You can see that the recess is a little large than needed. I’ll have to fill that in later.

With the top cowl exactly in its final position, I drilled four holes at the back along the top to put keeper clecos in. That helped stabilize the position quite a bit. I then marked and trimmed the bottom of the cowl.

I followed the advice of another builder and made a longer sanding board, about 2 feet long. I glued sand paper to both sides of it with wood glue and weighted it down to dry. Contact cement would have worked better, but I didn’t have any handy. I sanded the aft, bottom edge of the cowl until it was fairly close to final and then marked and trimmed the sides of the bottom cowl.

I didn’t spend too much time on the project tonight. It was already a long day. I did work on the fit between the cowl halves in the spinner area. I sanded the inside of the top cowl and outside of the bottom cowl to get them to fit together a bit better. I reduced the “over-bite” quite a bit. I should be able to make some good progress this weekend on trimming and fitting.

After re-reading the instructions last night, I realized I missed a step. I removed the top cowl from the plane and fitted it together with the bottom cowl. I was mainly concerned with getting the area behind the spinner into a perfect circle. I cut a 13″ disc out of a pizza box to use as sort of a template. Once I had it set, I drilled two keeper holes. I then put the cowls back on the plane using clecos in the keeper holes to hold the bottom cowl in position. I also used some duct tape to hold up the back of the cowl.

The fit looks pretty good, although the bottom cowl seems to be sitting back just a tiny bit too much. It’s like the cowl has a little bit of an over-bite. I’m going to try to fix that before doing any trimming. I mainly just spent a bunch of time examing the cowl from all different angles, examing the fit and where trimming will be needed.

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.