Fabricating a Light-Weight Cowling
2005, Steve Mahoney

The carburetor in my new XP-360 is located slightly farther forward than was on my old narrow deck Lycoming 0-360  so my carb heat box that I had built no longer clears the cowling

New XP-360I built from a kit mounted to the airframe

 My solution is to buy the carb heat box kit that Van’s sells. This requires moving the engine air inlet to the front of the cowl with a ram air scoop. I always liked this design as it is simple, and do a much better job of pressure recovery and should provide a slightly higher manifold pressure than the old system did where I took the intake air off the left cowling inlet. (as captain Kirk on stark trek would say more power Scottie).

With all these changes needed to the cowling I decided scarf them on to my existing cowling then pull a new mold.  This way I could keep my fiberglass one homogenous piece and keep the extra weight out.

My first Job was to bond the new Van’s air scoop to front of the cowl. My dad had always claimed that the air inlets on my old cowl where way over sized, so I took this opportunity to close them off a bit. They are ½ the area of the old ones. This should reduce my cooling drag some.

New Vans aircraft scoop bonded to the old cowl ….Air inlets reduced to ½ the area.

I added  2” of extension to the cowling so I had more material to work with.

I hired a guy with a Chopper gun to spray on a layer of gel coat and then lay down a thick layer of chopped matt.

Lower cowling mold

Top Cowling Mold.

Waxing the mold with paste wax….. getting it ready to make the first part

I made these test coupons up to test the different types of lay-ups

I have been doing quite a lot of thinking as to what material I will mold the new cowling from. I really wanted to keep the weight down to less than 15lbs total  so I have been doing quite a bit of work looking and testing different fiber glass lay ups. This is what I decided to use.


For the resin  Gougeon brothers (same manufactures as west systems)  pro set 125 resin with 229 hardener. I purchased the resin from Fisheries Supply in Seattle. 

 This is the best epoxy I have found and has superior properties even with out being baked out in an auto clave. I later found out that this is what Rutan used on Spaceship one….. If it’s good enough for Burt (the epoxy guru of plastic airplanes) …. Then it’s good enough for me.

What I like about this resin is that it has an incredibly low viscosity (thin like water) so it is very easy to wet out the glass or carbon fiber cloth material without dry spots. This thin resin makes it easier to control the amount of resin used so you can achieve very good Glass to resin ratios without vacuumed bagging.  It also has a very long cure time, so the pot life is more than 2 hours. This provides enough time to really squeegee the excess resin out of the fabric provide a very light lay-up.  If you’re careful about how much resin you use, than it’s not that expensive to use these hi-tech materials. I only used a little over a gallon to do both the top and bottom cowl.

For the Lay-up schedule I decided I would only use 3 layers of material. The outside layer would be bi directional glass cloth (bid) . This is a very flexible cloth and will form around corners well.   One layer of 5.7 ounce carbon 2x2 twill weave cloth for the inside. This is the most amazing material I have found …it is very light weight and incredibly stiff when cured in a resin matrix. If I had it to do over I would use 2 layers of this and skip the glass. It’s not cheap though I paid about $25 /yard for a 50” wide roll. It takes about 4 yards to do one layer of the BD cowl.

The fabric lay-up schedule - bid glass – core mat – 5.8 ounce carbon fiber

For the center layer I am using core mat (bulk tek mat) this is the stuff with the holes in it. This is interesting stuff…it looks like felt… but is much lighter…

It’s a low-density, no woven continuous-strand laminate bulk containing 45% by volume of micro-balloons. Excellent wet-out and a laminated density of 40 to 50 lb per cubic foot—about half of fiberglass laminated mat. Because it’s made from micro balloons it’s mostly air and fairly light while adding .1” of thickness this will do a lot for stiffness. You can buy it from Tap plastics for $9/ yard.

I will not be using any foam at all in this lay-up as the core mat servers its function and is much easier to wrap around curves in the mold. This should make a fairly light cowling.

Cured Cross section showing the sandwich lay up   only 3 layers….. glass cloth – core mat - graphite/carbon.

Installing core mat   each piece is cut to fit in the mold like a mosaic.

Core mat gaps are filled with Micro balloons …now ready for Carbon fiber layer.

Hey…. this thing is light!

The finished top cowling after removed from the mold and trimming weighed in at 5lbs

It turned out that this was way stiffer that my old cowl which was made from S glass – Kevlar – S glass, and lighter too!

Adding a flange and nut plates to the cowl