Building a Cowl Blister

Our F1 Rocket project is now in paint, and when I removed the cowl after a few engine runs in preparation for the paint job, I noticed a bad sign—a small burn spot on the lower cowl, clearly from the exhaust. Sure enough, the exhaust system ball joint had remnants of fiberglass on the lower edge, and there was no way to move it relative to the cowl—so the cowl needs a blister. Blisters are annoying but easy to explain at fly-ins and shows … “Yes, that is creating space for the Turbo-Encabulator that adds 20 hp when engaged … “

Building a blister isn’t all that hard once you reconcile yourself to the task. In this case, the blister was going to be centered in a “valley” of the lower cowling, so I couldn’t just mold one on the workbench on a flat surface. So I taped off the area with packing tape (to keep the fiberglass resin from sticking) and then drew a nice aerodynamic shape on the cowl. I then pulled out the modeling clay and transformed the outline into a pleasing low-drag blister. Next, I pulled out some scraps of fiberglass cloth, wetted them out, and built the blister directly on the clay and tape. A drop light underneath the cowl promoted quick curing—we are, after all, in the middle of paint, and time was of the essence.

With the layup cured, I popped it off, removed the clay and tape, and shaped the edges of the blister to a nice thin taper. I cut a hole in the cowl (from the inside) to match the blister’s inside dimension. If I had more time, I would have dropped the blister into the hole from the inside and left the flange to settle into a scarf carved in there, but I had molded it to fit the outside, and I was working quickly. I mixed up some resin and flox to glue it in place and filled the joint inside and out, once again using heat to cure it quickly. A little blending followed with resin and micro, followed (after a further cure) by sanding to shape. And just like that—we had a blister to accommodate the exhaust!

I always tell people that I have a love/hate relationship with fiberglass—I love what I can do with it, but hate the mess. This was a quick project, executed from experience without plans, and it only took about six hours from start to finish. Don’t be afraid to make things fit—and do it before your painter lays down his pretty colors!