Plenum Tug-of-War

With the -7 build in Michigan while Dolly and I are in Florida, a project for the winter was to fabricate a fiberglass plenum lid, the oil fill door and some HDPE firewall and baffle wire pass through. I brought engine measurements (32″plus by 18″plus) and the top Vans cowl with us.

The oil door was fun, really. First try was two layers of 6 oz cloth cast onto the cowl over clear packing tape and Mother’s Brazilian canauba auto wax. Two layers was too thin both in flexure and contour with the surrounding cowl surface. The second try at seven layers was stiff, but too thick. Like ‘Goldilocks and the Three Bears’ the third try at four layers was just right but will need some anti-flex reinforcing.

I purchased yards of cloth and West Systems epoxy at the new West Marine store located about a mile from here. It seems that from now on the WM stores will only carry smaller packages of cloth, not large rolls.

The outer surface of the RV-7 cowl received from Vans had pits reflecting the shape of the hexagonal cells in the sandwiched layer. After three thin screed layers of epoxy/microlite and two layers of rolled on clear epoxy the surface is ready for spray primer. Of course each layer was preceded by block sanding, but we don’t like to think about that.

After all that I felt ready to tackle casting the plenum lid on the inside of the upper cowl. Clear packing tape and auto wax were again used for mold release. Six pieces of 6oz cloth were cut oversize (I only used five), along with a layer of 2mm Soric core material.

Blue sharpie lines on the cowl marked where the composite layers were to be placed. A single large piece of nylon sail cloth peel ply was laid over the waxed release tape and the blue lines traced. Epoxy was poured over the first layer of glass, and a bit on the second glass layer. Air was worked out with a plastic spatula. The third glass layer also required some epoxy. Then the Soric was placed and a 3″ paint roller used to force contact and squeez excess epoxy to the edges. Corners of the Soric were lifted to verify epoxy was covering the bottom of this layer. The next two layers of glass and peel ply were applied in a similar manner.

The plenum lid easily popped from the mold after twelve hours. It was left for a day before our Tug-of-War and trimming loose cloth from the edges. The finish is beautiful. Weight at this point is 34.4oz. This is undoubtedly heavier than if the assembly had been vacuum bagged and resin infused. However, we can tolerate forward weight to offset the relatively light weight lithium battery and Catto prop.

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