The cowling was another 6+ month project. I started on the rough fit before I moved the plane to the hangar once I had the Rotax prop hub extension from Sonex. I had an issue with the cowling being too far aft of the hub face even before trimming material.
After discussing with Sonex, the trim on the cowling was too short (I also have a vertical spit cowling that had enough length but doesn’t work with the Cooper cooling kit). I pointed out that when you add the dimensions of the engine mount, bushings, length of engine, and prop hub length that it would put the prop hub face 0.6" ahead of when an Aerovee flange would be and wasn’t told that those dimensions were incorrect. The solutions were to wait for mold modifications, graft additional fiberglass on the aft edge, or use as is with a shorter prop hub spacer. Since I was good but maybe a little tight on exhaust clearance I switched to a 30mm prop hub spacer figuring I could always add a blister.
The Cooper cooling kit had 4 fiberglass pieces to add to the cowling, a scoop for the coolant radiator, two exhaust drafters, and a duct that holds the oil cooler. For the radiator scoop I made the hole a little different than the instructions. I mounted the cowl and traced the radiator with blue Sharpie, then removed it. I then offset those marks 0.75" and cut the hole so if I was a little off it would be OK. After that I mounted it again, I used a square on the radiator to make the hole larger making the width correct first. Then working the fwd/aft edges to pivot it in place and aligned with the radiator. When in place I used some thin angle to temporarily hold it.
For the exhaust I made my first hole by measuring and making them larger so the exhaust could stick though when mounted. I then clecoed the exhaust drafters in place, traced the final hole, and cut. I found like for the canopy, the half moon cutter on the multitool gave nice controlled cuts. Mounting the oil cooler duct is in a post above and with the modified position I extended it a little to the front of the cowling. For the cylinder head intakes, I used the time dimensions from the Sone Jabiru installation manual.
After everything was in position I removed the gelcoat and roughed up the mating surfaces using my die grinder with a 3" sanding disc and my multitool with a detail sanding head. For cutting fiberglass cloth into strips, I found that a paper cutter worked great to cut without distorting them.
From there it was a lot of sanding between applying fiberglass, filler and glaze. A large and small detail sander for my multitool worked for most of the sanding. For some of the really tight places I used a Dremel with a small drum and a 1" disc for the die grinder that I found at Harbor Freight. Sanding was a bit tedious but having power tools fit everywhere made it not so bad. I used about 1.5 qt of resin but could have came out with less if I wasn’t learning.
Some of my sanding tools
Between shaping and just needing the cowling in place for fit up, the cowling has a lot of miles in my car and has been installed and removed at least 20 times. Using my lift table worked good to maneuver it into place. So far there are no signs of any contact and the engine doesn’t move a ton besides startup and shut down. When I did my weight and balance, the shorter prop hub spacer put the tip of the prop spinner 1/2" forward from the drawing dimensions which makes sense with the 3.5" thick propeller.
I enjoyed the fiberglass work more that I thought I would for how tedious and messy it can be. It’s a neat material that I’ve never worked with before and being able to fix mistakes makes it a little more relaxing cutting it and drilling holes. Even though the cowling took a long time, it wasn’t inhibiting getting to the end of my project and was something to work on at home. Also, the scoop looks quite cool in my eyes as someone who always wanted a car with a hood scoop.
