Jabiru 3300 Cowling Modification

By design, all Sonex-approved powerplants place the prop flange in the same location. This allows for Sonex to use their latest design "universal cowling". In my case, the prop flange was nearly perfectly placed, except that the engine was perhaps 1/4 - 1/2 inch too far forward. This caused the oil sump on the front of the Jabiru engine to bump into the lower front portion of the cowling. Apparently, the Jabiru 2200 engines also have this problem, and Sonex sells a pre-molded fiberglass "blister" that can be added to the front of the cowling to create the needed extra room.

NOTE:  photos link to full size image

The first step in converting the cowl was to remove the offending section of fiberglass that interfered with the oil sump.
The opening has to be big enough to clear the sump, otherwise the cowl will not fit properly into place.
The blister comes slightly oversized. Trim the edges so that it mates up properly, especially around the prop hub opening. I've also split the blister into left and right halves, and fit a section of piano hinge in order to join that section of cowl together.
I suppose you could wait to split the blister until after it has been glassed into place, but doing it first allows the seam to be perfect before the fiberglass work in started.
With the blister trimmed to fit, split down the middle, and joined with a hinge, it was ready to match up to the rest of the cowling. This part is simple. Hold the blister in place and trace the perimeter with a sharpie. Once the outline has been transferred, the opening in the cowl sections can be adjusted to provide a smooth transition from old cowl to new blister.
After the blister has been fiberglassed into place, the outside seam gets a layer of dry micro (micro balloons and epoxy mixed to the consistency of cake frosting) to help smooth the transition.
Working with micro gives you a lot of flexibility in making smooth, pleasing contours. With a little work and sanding, the transition will be excellent.
The inside edge of the transition is reinforced with one or two layers of fiberglass. I used strips of medium-weight cloth, cut into 2" wide tapes. The tapes bridge over the transition and add much-needed strength and stability to the joint. This will prevent the joint from flexing and cracking around the micro on the outside.
The universal cowling requires new exhaust tunnels to be cut into the bottom. The laser-cut baffle kit provided by Sonex includes the needed parts for the right and left tunnels, and they go in without much problem.
The AeroVee exhaust exits were at the aft edge of the cowling, and needed to be closed up. To accomplish this, I added several layers of fiberglass to close up the gap. As you can see, this had to be done before the exhaust tunnels could be completed.
At this point, the oil cooler inlet can be located and cut, and any other remaining sanding can be finished. After cutting the oil cooler inlet too large on my AeroVee cowling, I was very cautious on this one. I intentionally cut it undersized, and then opened it up a little at a time during flight testing. As it turned out, the dimensions indicated in the Jabiru Installation Manual provided by SOnex were perfect.
The side profile shows jut how much room is created with the addition of the blister. All in all, it really doesn't look out of place, either. If you are having second thoughts due to the change in appearances, don't worry about it. It'll turn out fine.
As before, I used the Contego Fire Barrier Paint on the inside of the cowl. One or two coats are sufficient to add some fire protection, without adding significant weight.
At this point, modifications to the cowling are complete.


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Updated: 27 May 09