Matt's RV-7 Project

I trimmed the upper edge of the bottom cowl along the lines I prevously marked. Then I spent about two hours sanding the cut edges straight and progressively taking off more material. You can't tell how it's going to fit until you have it on the airplane, fastened and/or taped into place, so it takes many repetitions of cowl-on, cowl-off, each time removing a few thousands with the long sanding block. The other thing that makes this frustrating is that the fit will change later once you fasten the top and bottom halves together, due to the way the cowl gets pulled inward along the horizontal split line. At this point, you're just shooting for a straight edge and a consistent gap, knowing that the spacing will change and need to be fixed again later.

At some point during this process, the aluminum pins that came with the side hinge material started to get kinked, as I knew they eventually would. The plans call for you to replace them with stainless steel wires, so I got out the torch and bent some new hinge pins.

Eventually I got to the point where I couldn't improve the fit any further, so I moved on to actually joining the top and bottom cowl halves. The plans specify more hinge material here, but I had that on my previous airplane and I had no end of trouble getting the long curved pins in and out. Instead, I intend to use quarter-turn fasteners and screws/nutplates just like I did along the firewall.

Since there is not a joggled flange molded into the lower cowl, I need to make my own. I gave serious thought to the idea of laying up some fiberglass flanges, but since it was a three-day weekend and I didn't have any of the glass tape I'd have wanted to use laying around, I decided to make aluminum flanges instead. These started out as 3"x30" pieces of 0.063" alclad, drilled for a double row of rivets and shaped to the proper contour with a rubber mallet.

After getting the shape right – another whole set of iterations – I drilled enough holes to let me cleco the flange plates securely to the bottom cowl. The upper fastener holes are spaced about 3 1/8" apart, starting 3/4" from each end of the flange.

Here you can see the cross section of the flanges. Compound curves in 2024-T3 aluminum are interesting.

After quadruple-checking that everything was fastened correctly, and with duct tape in strategic places to hold the cowl in alignment at the nose, I used the shop-light trick to drill through the upper cowl into the pilot holes in the flange.

The edges of the top cowl get pulled in once they're clecoed to the bottom cowl flange, so the gap between the top and bottom shrinks after you drill the fastener holes. So, it was another hour of try-and-fit work to get to the point where I ended up… a fairly straight seam, with a pretty consistent 0.020" gap for paint.

Big milestone… the two halves of the cowl are properly fitted and attached for the first time, even if it is just with clecoes:

Somewhere during all this, I noticed that there is only a finger-width of clearance between the alternator pulley and the inside of the bottom cowl. This is pretty typical for RV's… I will keep an eye on this, and maybe think about switching to a slightly smaller belt to let the pulley move inward.

I don't intend to undertake all the glass-finishing work at this stage, but I do need to carry out a few tasks with the gooey stuff. Since I haven't used the epoxy for a few years (!!!) I mixed up a test batch just to see if it's still good.

If this hardens by tomorrow, there will be more to report soon. If not, well, it's nice outside and I have the day off from work!

Not pictured in this update: several hours of tedious sanding and fitting! It's messy and fiddly and it doesn't photograph well. Just imagine lots of messing around with sanding blocks and dust getting everywhere and you'll get the idea. Anyway, the fit at the lower cowl to firewall junction is pretty good all the way around, and the bottom edges of the top cowl are straight and parallel.

I cut two pieces of 1/8" hinge material to attach the sides of the lower cowl:

After drilling a series of #40 holes with one-inch spacing, I put both halves of the cowl on the airplane and used a shop light to illuminate the holes from behind. It's pretty easy to locate holes in fiberglass this way, as long as you can work the light back behind what you're working on.

In between each pair of rivet holes, I drilled a 3/16" hole that will later help increase adhesion when the hinge is glued to the cowl.

I clecoed the cowl halves to the fuselage, carefully taped things where I wanted them to end up, and marked a line on the bottom cowl where it needs to be trimmed to fit against the top cowl.

I ran out of time to make these next cuts, but at least I have a nice line to work with next time I get a chance to do some work:

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.