Matt's RV-7 Project

Here's what the corner cowl flanges looked like after being trimmed to shape. (They aren't really that white, it's just the flash)

The top cowl overhangs the bottom cowl by about 3/32" at the forward outboard corners, giving it a sort of "overbite" appearance. To build up the lower cowl, I first put packing tape along the mating edge of the upper cowl, then clecoed the two halves together on the workbench:

Then I mixed up a batch of epoxy and microballoon filler:

Filler is applied with a popsicle stick over a light epoxy wipe, then roughly contoured with a rubber squeegee:

After curing, most of it gets sanded away as the new shape of the cowl is formed:

Then comes another round of filling and sanding, and then another. After three iterations, it's starting to get where I want it:

You can see where I filled in some low spots on the bottom of the left air inlet. I think the vacuum bag got partially wrinkled here when the piece was being molded, but it's all smoothed out now.

The fit is pretty good now – no more overbite. I will wait to do the last bit of sanding until I have the cowl back on the airplane and locked in to its final position.

To sand the filler to shape during the above steps, I used a variety of different tools. Each one has its own strengths and weaknesses, kind of like a team of superheroes. Let's run down the dossiers:

Team Member: Two-Face

Background: Was an ordinary piece of aluminum angle until unfortunate particle accelerator accident. Then had sandpaper glued to it.
Zodiac Sign: Gemini
Strength: Squaring corners
Weaknesses: Bad for large areas, sometimes sands where it shouldn't

Team Member: The Strip

Composition: Stainless steel, adhesive sandpaper
Thickness: 0.020 inches
Strength: Good for sanding gap between cowl halves
Weaknesses: Limited applications, sharp edges
Ouch My Hand: What did I just say

Team Member: The Rod

Origin Story: Wooden rod with sandpaper taped to it. Also, bitten by radioactive spider.
Cover Identity: Nuclear-plant employee
Strengths: Inside corners, curved surfaces
Weakness: Flat areas

Team Member: The Block

Alias: The Preppin' Weapon
What Seriously: Yes, that's the actual product name
Yikes: Tell me about it
Strength: Most ergonomic sanding block ever
Weaknesses: Bad for inside corners, goofy name

Team Member: The Saw

Manufacturer: Xacto
Application: Adjusting kerf between cowl halves
There Is No Such Word: Yes there is. Kerf is a word.
Philosophy: Every problem in life can be solved by cutting
Hobbies: Cutting, slicing, windsurfing
Weakness: Not every problem in life can be solved by cutting

Team Member: The Hand

Technique: Holding a piece of sandpaper with your hand
Main Power Source: Engineer's hand (L or R)
Strengths: Contours to any surface; can also open cat food containers
Weaknesses: Stamina, precision, sports

Sanding team, assemble!

While waiting to see if my epoxy will still cure properly, I drilled a bunch of #30 holes in between the existing rivet holes in the metal side flanges. When I later bond and rivet these parts to the lower cowl, the holes will help the epoxy get a good grip.

My epoxy turned out to still be good, so it's time for what will probably turn into several weeks of glass work. The first thing I want to do is build up some flanges on the inside of the lower cowl, at the outboard corners. Eventually these will be used for a single screw and nutplate per side, similar to this example. First I applied clear packing tape to the areas where I didn't want the epoxy to stick, and clecoed the two halves of the cowl together on the bench:

I laid up five plies of cloth from my box of scraps and stuck it to the inside of the cowl, after first cleaning the area of interest and wiping it down with epoxy.

This is how it looked after it cured, immediately after removing the top cowl:

I should be able to work with this. It needs to be cleaned up and trimmed/sanded to shape, of course. Then a bit of flox on the front to ensure a good fit, a few more plies on the back if it needs stiffening, and I'll have my flanges.

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!