The last post we started a right side lower aft engine cowl, say that 5 times fast, and got it to a point where the foam shape mostly replicated the left side. Since then, I spent a bit more time adding, sanding, and shaping foam using my handy templates derived of 3D scanning to improve the symmetry. When I got to the point where I was happy with the shape, and couldn’t tell if I was improving symmetry or making things worse, I moved on to the next step, drywall mud.
Same story here, slather on the drywall mud and sand to shape. Foam sands to a pretty coarse contour, the drywall mud has more finesse. The temperatures here in Arizona have give us a little respite, which has been wonderful. It does, however, make the drying time for the drywall mud longer. My impatience had me getting the heat gun out to accelerate the drying. It worked fine, but you need to be careful that you add heat slowly. If you dry the exterior and the underlying mud is still wet, it will crack easily and still won’t sand.
As is to be expected, especially if you have every done any drywall work in your home, it is again many rounds of mud, sand, mud sand, mud sand. If you find a place that is really low in contour, you can always go back to foam. I had the opposite problem in a few areas, I needed to sand through the mud to foam and go deeper into it. This can be a risk because you may not have much foam in that area to work with. Luckily, I had barely enough and while that area has become flimsy, it is the shape I need.
Again, with the mud, you use the templates to get close. From there, I tend to stand back and look at the shape from all different angles trying to compare sides or areas that are a known shape. You need to get high, low, look from the front, the back, sides, underneath to compare all the different areas.
One of the things that I continue to find interesting is that when you get the foam to the place you are ready to move to the next finer medium (drywall mud), you would swear that the shape is pretty darn good. Then you put the drywall mud on, and even before the whole thing is covered, oy vey, awful flaws in the shape. So you work the mud until you get it to the point where it looks as good as you can get it and move on. This time when you wax it for a layup, as soon as you put the wax on you start seeing things you didn’t before, and round and round we go.
Today I got to the point where I couldn’t get any/much better with further rounds of drywall mud. It was the point of diminishing returns. So I decided to move on to the next process. That means a fiberglass layup to make the plug which I’ll refer to as Lower Aft Tooling Cowl Serial Number 1(TCSN1). We do love our acronyms in aviation.
In preparation I did 5 coats of wax on the drywall mud surface, then 3 coats of PVA. Time ran out to get a layup done tonight yet. Kevin and I were working hard on getting all the materials together, cloth cut, plastic cut, etc. But by the time we were ready to start mixing epoxy I decided it was too late. There is nothing worse than trying to do a complex layup, against gravity no less, as it is getting late, you are getting tired, and it is way past dinner time. Maybe I am getting soft in my old age. So I will do the layup first thing in the morning. It is starting to get to the fun stage for this cowl. I’m excited to see how it is going to turn out.