RV-7 N890GF

Now that the baffles are all trimmed, riveted and in place on the engine, I started work on the plenum. The plenum is an alternative to the baffle seals that Vans provides. The original design uses the seals on the baffles to press up against the top cowl creating a seal to prevent air from escaping outside the baffles. The plenum acts as a seal, but is fabricated from fiberglass and attaches directly to the baffles like a lid.

There are many sources for how to build a plenum online. I’m going to combine several methods I’ve seen. One of them is from Dan Horton on vans airforce and another from Pete Howell as well. Both describe how to build a model above the engine to allow for laying up of fiberglass.

I started by laying down some plastic on the engine. I got some two part expanding foam, and mixed up a small test sample before doing the big pours. Once that went well, I mixed up a batch and began pouring it on the engine.

Working on batches makes this a lot easier. The foam takes about 30s to mix, and then about 10 minutes to fully expand and harden. Here is the fully covered engine. I need to use the plastic in places to prevent the foam from spilling over.

I then used a hand saw and a rasp and sanding blocking to begin shaping the foam. The final result is a plenum shaped mold. I also began covering this with packing tape. I will continue shaping the inlets after the plenum is made.

The foam around and in the cowl inlets will be trimmed away before molding the fiberglass. The distance from the plenum/baffles to the cowl will be opened up to 3/8″ to allow for enough room to fabricate gap seals.