New Plan for the Aft Cowl

Last week Thursday, before our trip to Kanab, I got the layup on the lower aft cowl done. What an exercise in frustration. I have done quite a few layups in my building history, but this one made me feel like it was the first time I had ever seen epoxy or cloth.

The plan was to break the lower cowl into four sections, each with their own plies of glass. There would be left and right above the wing & left and right below the wing. Each of these sections would have some overlap. The above the wing pieces are easy. No so much with below the wing. Long story short, this may have worked had I had 6 pairs of hands, but not by myself.

Trying to get pre-wetout fiberglass plies on plastic to stick to the bottom surface of the cowl plug, all the wile working against gravity is a loosing battle with only 2 hands. I was using vacuum bagging film as my plastic backer for wetting out the glass on the table, which made things worse too. It is really nice smooth plastic, but it is heavier than you would think. So the additional weight of this plastic made each ply heavy enough it was impossible to get it to stick to the cowl plug. Oh yea, it is also 100 plus degrees out and the epoxy is kicking off quick enough that each piece was sticky to the point of difficult to manipulate before I could get a ply in place.

I tried everything I could think of to get it to work, and eventually came to the conclusion that I had to not only cut the plies into smaller pieces, but also get rid of the plastic to get it to stick in place. This made it really difficult to get the plies on in any kind of organized manner or with any concern for ply orientation, fiber straightness, etc. It was certainly NOT an aerospace grade layup. Luckily, this only needs to be a tool.

I eventually succumbed to the fact that the layup wasn’t going the way I wanted and it was best to piecemeal it. So I took about 1 foot square pieces of cloth, stuck them in place with overlap, and stippled on epoxy. It was messy, sticky, time consuming, and aggravating. It was not pretty, but it got the job done. And a fair bit of epoxy all over me.

By the time that layup was done, I was glad to be heading to Kanab for the weekend. I needed the time away from glass work, some R and R, and some cool flying. We know how that turned out. Instead I got more glass work, some R and R (just not the R and R I wanted), and a little bit of the cool flying.

Since getting home from Kanab, it has been more of the same process you saw for the top cowl on the rest of the aft cowl surface. Contour sand the fiberglass. Add filler. Sand. Repeat. Once it is to the stage where you think you have the shape, start spraying with primer. Sand. Find flaws, pinholes, etc. Fill Sand repeat. And then get Joe-ed.

While I was sanding on the lower cowl, I was thinking about how I was going to make a mold with a wing root into it etc. Any way I sliced the problem, I figured it was going to be a complex mold with multiple pieces. You certainly can do multi-piece molds, but it adds all kinds of challenges for vacuum bagging etc. So I took a step back and asked, why am I parting the cowl where I am? The simple answer is, thats where the original cowl parting line was. I was just doing the same thing Brian did.

Turns out that there would be significant advantages to moving the cowl seam to the plane of the top surface of the wing. One, there is a significant improvement in serviceability of the engine just with the top cowl off. You can get at just about anything you need. Two, it makes the molds easier. Top cowl can be one mold, so can bottom cowl. Three, it moves the cowl seam to a less obtrusive location for improved aesthetics. The downside is I have to make a new mold for the top cowl. But If the lower cowl was going to be 3 pieces, I am actually making 1 less mold than with the original plan.

So this week I glued top TCSN2 (tool cowl serial number 2) to the bottom TCSN1 and jigged it back on the airplane. I have the full shape now in primer, and for the first time looking very representative of the final parts. I am liking it more and more. A little more polishing and pin hole filling and we will be ready for tooling flanges and making molds.

To me, this cowl project has taken a long time. I am ready to move on to other parts of the project. I am ready to be able to claim victory over a part. In actuality, at just a bit over 5 weeks, its moved at a pretty good clip. I just can’t wait to have the satisfaction and MVP (major visual progress) of having actual finished cowl parts. That will be a lot of fun to see.

5 thoughts on “New Plan for the Aft Cowl

  1. I’ve made several cowls and know the frustration of fiberglass cloth pull off The bottom mold and resin running down your arm I admire your persistence! Bruce

    Sent from my iPad


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