I needed to make 3 Cowlings to fit on the same airplane. One (1) for Electric, one (1) for 2 stroke engines, and one (1) for a Four stroke cycle engine. I had read an article several years ago and so, I thought I would try their method.

I carved the Cowl Plug from hard Balsa. After checking the size and making certain that it looks about right, I mounted it on a holding stick and Base. Pic # 1

You will need LARGE Balloons that are at least 4 times the cross section of your cowl plug for the inside, and LARGER Balloons for the outside.

Small cowls are easier to make than the larger cowls. This process allows you to make several cowls of the same size from one plug.

I then inflated a large balloon (~ 18" Dia) and forced it over the plug while slowly letting the air out of the balloon. Pic # 2 & 3 Making certain that the Plug is inside the balloon and the balloon is overlapped all around the base. Pic # 4

Pull on the stem and cut it off, at the same time pulling it down and tucking it under the base of the plug. (Firewall end) Pic # 5 (It's easier to do than to write it all out) see Pic # 6

If you have any Mold Release agent, spray it all over the balloon and let it dry. Pic # 7

Cover the balloon with strips of Fiberglass dipped in Resin or epoxy. Be sure to get two (2) layers on all surfaces, even three (3) layers if using light weight 2-3 oz glass cloth. See pic # 8 & 9.

Now we blow up another larger balloon, maybe 20"-24" dia and repeat the process as in Pics # 2-3 & 4.

The plug with the resin and fiberglass is now all in-cased within the larger balloon. Pic # 10 & 11.
The balloon will slowly apply pressure on the glass cloth and resin forcing the resin into all nooks and craneys and providing a fairly smooth surface on the inside and outside of the cowl.

Allow 24 hours for the resin or slow cure epoxy to set and harden before attempting to peel the outside balloon off of the form.

Very little sanding is necessary after it is removed from the plug. Of course you will still need to cut off all the excess glass shards that extend past the firewall surface. See Pic # 12

This process is a little messy with the glass and resin or slow cure epoxy, but it does work. If there were any air bubbles in the resin, there will be voids in the surface of the cowl and need some filler to patch it over before the final sanding and touch up.

Merry Christmas Everyone,

Ken AMA 1528