Matt's RV-7 Project

Had a bit of a work interruption recently due to a painful injury that knocked me off my game for a couple weeks, but I'm back on top now. So, remember those great-looking cowl intake duct closeouts I made last time? Yeah, about that… it seems that it would have been a good idea to check the fit with the flywheel installed before I got all crazy with the fiberglass. Grinding away a week of work was no fun at all.

I traced the line of the crankcase baffles onto the cowl so I'd know where to build the shape back up:

With the inboard ends of the inlet ramps cut further back, the cowl fits over the engine like it's supposed to:

Here's another problem area I've been putting off dealing with. I had to cut the lower cowl farther forward in order to allow it to clear the engine air filter when it's being installed and removed, and in doing so I lost space for the third attachment screw.

I cut the upper cowl to match and sanded the edge straight. For now I'll just live with only two screws on each side of the spinner here. If it causes problems down the road, I'll come up with a fix. In retrospect, it was a bad idea to follow the plans and drill all three holes early on… I should have drilled only a single hole at the very front, and left the others for after I had the cowl trimmed to fit the baffles.

At least the inlet ducts are fine at the outboard ends. I trimmed the forward cylinder baffles to fit the inlet ramps using the paper clip method.

I built up the new inlet duct closeout shapes I wanted using floral foam. This time made sure to leave clearance around the flywheel, and to allow a more natural transition of the crankcase baffles as they run across the inlet ducts and up to the apex of the cowl.

I laid up three plies of glass over the foam:

Then I followed up with the usual sand-and-fill process. Once again, this looks like hell, but it's fairly smooth to the touch.

Here's a view looking inside the cowl to show just how close the flywheel comes to the cowl and inlet ducts. There's a gap of about 1/2" all the way around, now that I properly shaped the fiberglass to fit.

Now I guess I'm back where I was a month ago, except this time the cowl fits properly. I guess that's progress, of a sort. Oh well – if this was easy, everybody would be doing it.

The baffles have been fitted to the cowl, but the cowl isn't finished yet. I still have to attach the intake ducts to the inside – these will help the air enter the cowling volume and slow down before flowing through the engine.

I put the cowl on the airplane, then reached in through the inlets and traced the position of the baffles onto the inside of the upper cowl:

The lines make it fairly straightforward to locate the ducts on the cowl. I also trimmed back the lips of the cowl inlets slightly in order to make them symmetrical.

I drilled and clecoed the ducts to the cowl to temporarily hold them in place:

Then I glued the ducts in place using epoxy and flox:

I used more flox and microballoons to fair the ducts into the cowl. Even though you can't see this area from outside the airplane, the oncoming air sure can, so I figured a smooth transition would help cut down on cooling drag.

This area was particularly interesting… the cowl is relieved here to clear the #1 cylinder on the bigger 200 hp engine, which prevents the intake duct from fitting properly. I used a heat gun to reshape the corner of the duct, and filler to bring it all together.

The space between the cowl surface and the intake duct forms a tunnel between the high and low pressure parts of the cowl, which is known to cause a loss of cooling air and resulting high cylinder head temperatures. Many folks end up closing off one end or the other in order to force the air to go through the cylinder fins and oil cooler, where it will do some actual work. I figured I might as well go ahead and make this modification now while it's easy.

I started by cutting some endplates out of balsa, and coated the soon to be hidden surfaces with epoxy to make them resistant to oil and water.

Then I glued them in place on the inner face of each intake duct "tunnel".

Once the wood was glued securely, I laid up some glass over the top to provide the real strength and close up the remaining gaps. Hmm, since wood is composed of fibers that are made of carbon, do you suppose I can get credit for this as a "custom carbon fiber part"?

As I went along, I wiped down the inside surface of the cowl with whatever unused epoxy I had left over in each batch. Eventually the entire surface will get an epoxy coat to seal it, so I figured why let perfectly good glue go to waste.

Finally, I squeegeed on a coat of filler to smooth the edges of the glass layup on the endplates, and sanded it all smooth. It looks like hell, but it's smooth to the touch, which is the point. This might be overkill, but when you stick your hand in and feel around for how I did the baffles (which is something all builders do) hopefully you'll say "hey, craftsmanship!" Not to mention, maybe all this effort will make the airplane 0.001 knots faster… every little bit helps.

Next: Trimming the baffles to fit the cowl, again!

On the inner faces of the air inlets, the cowl halves are joined together by three screws on either side of the spinner. Here I'm marking and drilling pilot holes using the spacing provided in the plans:

This is an area where you typically see a lot of wear on RV's that have been flying for a while – no surprise, since the screws only have a thin layer of fiberglass to bear on. I decided I wanted some reinforcement here, so I fitted a piece of 0.063" alclad to each side of the upper cowl. This will give the screws something else to wear against, and actually helps in another way by filling some gaps in the area where the top cowl overlaps the bottom cowl flange.

Then I glued each one to the inside of the cowl using an epoxy/flox mixture. After curing, it's now pretty much permanently attached.

After the doubler plates were affixed, I put the cowl onto the airplane and match-drilled the pilot holes into the lower cowl.

Oops! The dimensions in the plans will cause the aft-most screws to interfere with the engine flywheel and ring gear. No problem, I just filled up the holes with flox and redrilled new holes farther forward.

I figured it couldn't hurt to bond some more doublers inside the lower cowl too, so I did that before riveting on the nutplates.

Nutplates and doublers permanently attached to the bottom cowl flanges:

Test-fitting the screws – everything lines up, which is a relief. These screws are only barely threaded into the nutplates, due to all the stack-up of extra material. I'll probably have to use 5/8" screws instead of 1/2", but that's okay.

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!