Pour Foam/Sand, Repeat, Drywall Mud/Sand, Repeat

It seems that I haven’t been able to get much time in the workshop lately, and when I do that I don’t get much done. This cowl fabrication has been consuming my brain power, and I have not been able to give much thought to working on other parts of the airplane.

In the last post I was still working with the foam core to get the basic shape that I wanted for the cowl. Once I was pretty much “there” with the foam shape, I started working with drywall mud, as suggested by James. Drywall mud is cheap, sands easily, and washes off with water. The only down side has been that it takes 24 hours to cure. I have helped this cure along with lamps, heat tents, and fans, but you cant rush it too much or it cracks.

Again many iterations were done with the drywall mud to get a decent shaped, smooth surface. I have lost count how many coats I have done, but I have used up three, one gallon buckets of drywall mud. Much of which was sanded off and swept up into the trash.

I started working the right side first, and got that to a shape I was happy with. I then made cardboard templates every six inches along the cowl. These templates were used to match the left side as closely as possible (the engine is not symmetrical) to the right side. At worst, the left side is approximately 1/8″ different at one station, most places it is less than half that.

Once I got to this point, I was starting to loose my patience with the minor, if at all measurable, improvements I was getting from each subsequent layer of drywall mud. I was attempting to change the shape ever so slightly that I ditched the templates, and have been finessing small areas by feel. In actuality, I consider the shaping to be done, and am mostly refining the surface so that it is ready for laying up the actual part.

There are only two big things left to do before making the top cowl. One is to add a depression for the leading edge joggle. The joggle will create the clearance for the fuselage side attachment flange for the leading edge of the top cowl. The second thing is to move the jig popsicle sticks to the bottom side of the cowl and clean up the horizontal pieces that mate to the wings. Once those two tasks are done, it is time to prepare the cowl form with mold release, cut carbon and peel ply, and layup a cowl. I can’t wait.

Some Photos of the cowl form.