Matt's RV-7 Project

Now that I have the upper cowl fitting fairly well, it's time to start fitting the lower half. First I had to grind some relief notches into the outboard corners to clear the landing gear legs. This isn't spelled out in the plans but obviously has to be done in order to get the thing onto the airplane.

Duct tape helps keep things in place while you wrestle the lower cowl around. At this stage, the rear flanges are oversized and are overlapping the fuselage by around three-quarters of an inch. Note that I have the lower cowl tucked under the upper cowl – I found that things fit better around the front using that arrangement.

I used the now-familiar offset line method to transfer a cut line to the bottom of the lower cowl. Lacking any specific guidance on the matter, I chose to leave the cooling air exit ramp at its original length and make diagonal cuts along the sides to match the shortened underside of the cowl.

Several rounds of sanding and trial-fitting later, I had the bottom cowl edge fitting the fuselage well. I had previously drilled three #40 pilot holes in each mounting strip, which I transferred to the cowl using a strong light from behind.

Once the bottom edge fits well, you can mark and trim the sides. I used a shop light inside the cowl to double check the cut lines I laid out.

The lovely Mary appeared long enough to get her picture taken. Now that her residency is complete, she's promised to spend some time helping with the airplane project now and then, so you may start seeing her turn up in the occasional photo once again.

I got the initial cuts made and sanded straight, but I ran out of time to make it really fit well. It looks like it's pretty close, though. I left myself some little notes to remind me where to start sanding next time I get a chance to put in some airplane time.

Almost four years ago, I riveted hinges to the bottom firewall flange, as shown in the plans. But ever since then, I've felt conflicted about these hinges, since I've seen multiple RV's that have broken hinge eyes in this area, and I've read accounts from many other folks that have the same problem. It seems that some combination of airflow and exhaust pulses causes the underside of the cowl to get beat up more than the rest, and the aluminum hinge is perhaps not quite strong enough to put up with that amount of abuse over the life of the airplane.

The final straw was when I ran into an acquaintance who's just recently started flying his RV-6A… when I asked him how the airplane was behaving, he said "Great, except every time I fly it I find another hinge eye broken off the bottom of the cowl!" It wasn't very long after that before I found myself drilling the rivets out of the lower firewall flange, and cutting some solid mounting strips out of 0.063 alclad to replace them.

These got match drilled and dimpled to fit the holes that previously secured the hinges, and then I gave them a slight bend to follow the upward taper of the cowl.

I'll use either nutplates or camlocs here – I haven't decided. There should be just enough room to squeeze the rivets that will attach them to the firewall. I am keeping the hinges along the vertical sides of the lower cowl, since they don't seem to suffer from the same malady as the bottom ones (and also because the engine mount would have to come off to replace them!).

Between a long vacation, a week at Oshkosh, and hundred-degree temperatures in the garage, the airplane has been neglected for a little while. However, I have managed to make some progress on the upper cowl. This involved lots of iterative steps of sanding and fitting which don't make for good photos, but I'm fairly happy with how it's turned out.

I pulled the spinner out of storage and temporarily clamped it to its backplate so I could check the cowl-to-spinner transition. And also – I'm not going to lie – to look at how cool it looks with the spinner on there.

I shortened the paper spacer slightly, lowering the height of the upper cowl behind the spinner. This gave me a nice continuous contour between the upper surface of the cowl and the spinner, and it also helped shift things around so I could fix a small fit issue at the lower outboard corners of the top cowl. The engine will naturally sag a fraction of an inch as the rubber vibration isolators wear in, so I wanted to slightly bias the cowl fit to take this into account.

No more putting it off… time to start fitting the top cowl. Much like with the canopy, the instructions are frustratingly vague (although at least with fiberglass, if you take off too much there is usually a way to put more back on). I have been reading everything I can find on the usual RV sites, soliciting advice from friends near and far, and generally just trying to get an idea of where to begin.

One trick I discovered during my research was the use of a book as a spacer to set the height of the front of the cowl. I thought that was so clever I stole it for myself. As a rule I generally hate defacing books, but in this case an old outdated copy of the FAR/AIM had to give its life for the greater good.

I duct taped half the book to the top of the engine case between the spinner and the fuel injection distributor, and added and subtracted pages until I had the thickness I needed.

Here you can see why this works… the book-chunks on top of the engine naturally assume a shape that's helpful for centering and positioning the cowl, which has a similar contour.

There are no trim lines molded into the cowl, so it's hard to tell if you have it level, or if it's even symmetrical as-delivered. I leveled the fuselage and then used my super cool new self-leveling laser to find the centerline of the cowl and check the relative lengths of the sides. It turned out that one side is about 3/8" taller than the other. I'll use this tool again when I'm finalizing the top/bottom cowl split line.

Before you fit the top cowl to the fuselage, you have to make the top and bottom cowls fit each other at the forward end. On my cowl, I had to cut about a quarter inch off the lower cowl at the outboard corners in order to get the two halves to mate around the spinner opening. This is one of those frustrating problems where you don't quite know where to start removing material, and you get paranoid about cutting off something you shouldn't. I stewed about this for quite some time, and eventually just decided to start cutting and hope for the best. It turned out fine… just go slow and it will be okay.

The fit gets progressively better as you sand off more material…

Turns out I do have a use for that wooden copy of the spinner backplate I made – measuring the circularity of the spinner opening on the cowl. I ended up making the opening about an eighth of an inch taller than it is wide… the extra space will be at the "chin" of the spinner opening. This will hopefully give the spinner some room to sag downward as the engine mounts break in, without making the spinner-to-cowl transition look ugly.

The inboard side of the left inlet turned out pretty well. A little filling and sanding later on will make this practically seamless.

The right side fits okay, but not great. I will have to do some more involved work to get this to match up, but it shouldn't be too bad.

I locked the two halves together with a single cleco on each side. Later on I'll put real screws and nutplates here, but for now it's okay to let things float a bit.

Checking for side-to-side alignment:

The plans call for an idea gap of 1/4" between the forward face of the cowl and the aft edge of the spinner backplate. I'm going to shoot for 3/16" to begin with, then sand the cowl edges until I get closer to 1/4". That should keep me from accidentally making the gap too wide. Here I've positioned the left side of the cowl so a 3/16" spacer just fits through the gap:

The right side is a different story… the gap is more like 5/16" here. I guess that's not a huge amount of variation for such a big piece of fiberglass. I will have to even this up with filler later on, but I have done that before on another airplane and it's no big deal.

I drew a reference line on the fuselage skin exactly two inches back from the forward edge:

…then taped the cowl to the top skin to keep it from walking around:

I clamped the forward face to the spinner backplate, with a 3/16" spacer on the left side and a stack of scrap pieces on the right side as required to fill the wider gap. The key thing here is to keep the cowl from shifting around, not to try to force the gap to be even.

I very carefully marked a line on the cowl, two inches forward of the reference line:

Then I got out my cutoff wheel and trimmed the cowl back to the line. Since I was doing this solo, I didn't get a picture of the process, so you'll have to be happy with this self-portrait.

It doesn't quite fit perfectly all the way around, but it's nothing that hours of tedious trimming and sanding won't fix.

I made this two-foot-long sanding board out of scrap wood I had laying around, the better to sand the long straight edges of the fiberglass cowl. It's just a dowel screwed to a very flat piece of MDF shelving material.

I filled the screw holes to keep the paper from getting punctured and torn.

I attached some 60 grit 3M paper using spray glue. Purple, Mary's favorite.