Matt's RV-7 Project

I started fabricating and fitting the rubber strips that seal the baffles to the cowl. These allow the engine to move independently from the cowl, while preventing the incoming air from escaping past the baffles. In effect, the whole top of the engine becomes a plenum chamber with the top cowl as the lid, and the cooing air is forced down through the engine cylinders.

Before I got the tools out, I did a bunch of thread-reading on VAF, then went to the airport and looked under the cowl of as many RV's as I could find (five). It turned out that every one of airplanes I encountered had its baffle seals done in a different way, but it was interesting to see which ones fit well and which ones had leaks. In the end I came away with some good ideas that I'll try to put into practice.

In places where the baffles are curved, I used multiple overlapping strips to avoid wrinkles that could cause leaks. Each one overlaps its neighbor in such a way that the incoming airflow will tend to flatten it against the baffles and top cowl – sort of like bird feathers. The fastener holes are 3/8" from the edge of the baffles, spaced approximately every 1 1/2". Eventually the rubber seals will be riveted to the metal baffles with large-head blind rivets.

Since the seal material (rubberized fabric) comes rolled up, it has a natural curve to it. I took advantage of this by cutting parallel to the axis of curvature and orienting each piece of material so it naturally wants to bend inwards.

A leather punch is about the best way to put the fastener holes in the rubber strips. The rubber doesn't drill well with a normal twist bit, but the punch makes a clean hole without tearing.

Looking into the cowl inlet during a test-fit. I purposefully left the rubber strips overly long so I could trim them to fit later. The forward end especially will need some work, since there will also be a separate set of rubber strips attached to the lower cowl.

I managed to fit rubber seals to all the outboard and aft cylinder baffles before I got tired. The prop hub and flywheel will have to come off again in order to do the crankcase baffle seals anyway, so this is a good stopping point.

After trimming the baffles to their final shape, I finished riveting together all the parts that I had been leaving clecoed together for fitting purposes. First I made some doublers out of scrap to reinforce the outboard corner of the #3 cylinder baffle:

Here's what it looks like riveted together:

Another view. I'm not sure if reinforcement is strictly necessary here, but it was easy enough to do and adds almost no weight.

Here's the previously-seen #1 cylinder baffle, newly riveted to the starboard side crankcase baffle.

Closeup view of the joint… flush rivets towards the front where the rubber seal material will go:

And now the #2 cylinder baffle, also riveted to its corresponding crankcase baffle. I later used RTV to fill those visible gaps where the parts come together.

A view of the #2 cylinder baffle from the aft side. The angle reinforcement piece has almost no rivets through it, since it will primarily be secured by the screws that hold the air filter retaining ring.

All the baffles are now riveted together permanently, and back on the engine temporarily:

It's a bit hard to see in a photo, but there are substantial gaps between the forward baffles and the nose of the crankcase that will have to be caulked up when I put them on for good. You can also see some of the RTV previously mentioned.

The plans tell you to rivet little tabs to the forward cylinder baffles where they overlap the aft ones, to make a sort of expansion joint. I wanted these to be removable without drilling out rivets, so I sized the holes for #6 screws instead.

Screw heads are flush on the inside to accommodate the rubber seals:

I cut the tabs slightly oversize so they would seal up the tooling hole that's visible in the previous photo:

One small step at a time…

The #3 cylinder baffle, as supplied from the factory, has a big round hole cut in it. You're supposed to attach a hose flange here and connect a scat hose to pick up air for the cabin heat… problem is, that only works for a vertical induction engine, and mine has horizontal induction. So I had to fabricate a cover plate to make the hole go away.

I put a bead of RTV between the baffle and the cover plate to keep air from leaking past. I'll have to wait till I buy the heat muff to figure exactly where I'm going to relocate the heater air supply.

After a seemingly never-ending process of measuring, marking, trimming, and test-fitting, I finally have the forward baffles trimmed to fit the cowl inlet ducts.

I ended up totally re-making the center crankcase baffles from new parts. It was really tough to figure out the proper shape here, so I'll post plenty of photos.

The key thing here is to try for a nice smooth transition up the side of the inlet ducts to the inside surface of the top cowl. This is highly dependent on how you shape your inlet ducts so there is a feedback effect as you work on the shape of the various parts.

Close-up of how the parts join together just behind the spinner:

All the various baffle parts are cut about 3/8" behind the aft edge of the cowl opening:

A view from the other side:

I bent the forward edges of the crankcase baffles inward to help the rubber baffle seal material bridge the gap:

Trimming the outboard baffle on the #2 cylinder was fairly easy. I already made the corresponding modifications to the outboard #1 cylinder baffle in a previous work session.

I had to grind away more baffle material in front of the #1 cylinder to make room for the back of the flywheel:

A view with the cowl on… the gap between the crankcase baffle and inlet duct looks huge due to the camera angle. In real life it's between 3/8" and 1/2".

Same area on the left side. This photo shows where I still need to do some fiberglass work to square up the face of the cowl behind the spinner… that can wait till next summer, though.

Looking in through the spinner cutout. This is the best photo I could get of how the crankcase baffle runs up the side of the inlet duct towards the apex of the cowl. It's almost impossible to see in real life – you mostly have to figure it out by sticking your hands in through the inlets and feeling around. Naturally this makes trimming the baffles a long and tedious process.

On to the next step…