Update #27

How to Build a Custom Cowling

This is what happens when you make your own engine mounts . . .

     In a perfect world we would have enough money to buy a 100hp Rotax engine. Bolt on, hook up, go fly. In this world we don't have the money to do that but were fortunate enough to find Mr. Mel Ellis of Hillsboro who donated an aviation-conversion 2110cc Volkswagen engine. The first problem was to fabricate an engine mount. With that accomplished now we have to create a cowling that will enclose that engine.
     Cowling design and construction is complicated, as many things about aviation are. The cowling must be strong enough to keep its shape, and yet light enough so that it will not adversely affect the center of gravity. It must allow easy access to engine components for preflight inspections, service, and maintenance, and yet must be able to withstand 140mph dive speeds.
     We decided to create our cowling out of fiberglass. Fiberglass is problematic: it can be expensive to create a single piece, it can be heavy if the fabricators are not careful, it is messy to construct, and the fumes of the process are objectionable, if not unhealthy.
     On the other hand, fiberglass construction allows for complex curves that would be possible but extremely difficult to accomplish in aluminum.
     Fiberlay, Inc of Seattle provided us with discounted and surplus materials and resins which will create our molds and cowling pieces. Their website also provides excellent tutorials for anyone wishing to learn about the process of composite (fiberglass and resin) construction.

	Our first step was to wrap the engine, mount, and accessories with plastic kitchen wrap. We then started covering the front of the aircraft with building foam. This is the stuff sold at building supply centers for filling cracks and insulating houses.
The top of the engine was easy as gravity was our friend. The bottom proved to be more difficult. We eventually came up with the idea of spreading foam on a sheet of plastic, then raising it into position and taping it to the bottom of the plane.
	The foam took 24 hours to fully harden, so this part of the construction took over a week. We had to add foam incrementally in many of the more difficult spaces.

	We used several home-made tools to shape the foam. We needed to create air intake holes for oil cooling, engine cooling, and carburetor intake air.
We were warned to cover the foam with acrylic latex house paint. The reason was that we intended to put a skim coat of auto body putty on the outside and the story was that the putty would dissolve the foam and leave a huge, useless, sticky mess covering the engine. We wanted to avoid this. As we added putty, though, it didn't seem to affect the foam we had used in places without paint. No harm done, and better safe than sorry.

	Once we had the basic form of the cowling, we covered it with Bondo, a common auto body putty. We had to add enough to create and hold the shape, but not too much. This stuff costs $20 a gallon and a shape of this size uses about 6 gallons.
Once we had the basic shape, sanding began. We began sanding with 80 grit, then moved to 100. After 100 we will use 150, 220, 320, 400, and 600 grit paper over the entire surface.
	A lot of ingenuity is required to get the shape just right. It needs to be smooth, functional, and if possible, attractive.
Usually one slathers Bondo on a surface with a putty knife. Small surfaces use 1" knives, broad areas use 4" knives. Concave areas, though are problematic. Here we filled a construction-paper-and-tape piping bag with putty and piped a bead around the surface.
	Here we're starting to get close to the shape we want. But this is only the first step in making a cowling. What we're making is a 'plug'. It has the shape we want but is far too heavy and weak to consider using in the air. Our next step is to build a mold around the plug. Once we have the mold, we will use it to make a piece that will have the shape we need, but will be both light and strong. We will document those steps in the next two updates.
"Wax on, wax off," is a magical phrase. Students know just what you mean and they know what to do. They sand. As we moved through the finer and finer grits, the boys got excited. Each grit went quicker than the one before and they could feel the improvement. Once the plug was sanded to 600 it felt like glass. Next comes wax, PVA, and mold fabrication.