Bagging a stab

This site was last updated on February 15, 2008.

Howard thinks my carefully sanded balsa stabs are too wimpy, so we're experimenting with covering them in fiberglass, using tricks suggested by Bruce Kimball. Here's part of the process. Later we'll get some more documentation.

We start with a piece of very light fiberglass, cut a little oversize. It's supposed to wrap around the leading edge. In the picture above, it's laying on a piece of nylon bagging material while I wet it out with epoxy (System Three, Phase 2).

I tried a couple of techniques to spread the epoxy. We need to be careful not to squeegee too much out; need to leave enough to get a nice finish.

We decided to add a couple of strips of carbon tow on each side. When we were done, they made noticable lumps in the fiberglass. Next time I will take more care to make them as flat as possible.

After putting the stab in place on one half, we fold the other half of the material across the stab and smooth it in place (a messy job).

The picture above hints at the kernel of our approach. We'll have to add more pictures later.

On the left, you can see the stab, wrapped in a couple of pieces of heavy mylar (to help achieve a smooth finish). They've been waxed, for a clean release. Each piece of mylar was trimmed to extend from the leading edge back past the trailing edge. We place the mylars so that the leading edge of the stab protrudes about 1/8". This will cause a little ridge that we'll have to sand off later, but the mylar is too stiff to wrap smoothly around the leading edge.

On the right, you can see a small, stab-shaped, wooden frame, with two parts. We'll put the stab with mylar skins into the bag, then clamp the assembly between the two halves of the wooden frame. The frame will support the stab when the vacuum is applied, hopefully preventing warps.

Into the bag, which is sealed on each end.

Here's the bag in the frame, weighted in place.

Looks pretty nice. Kind of heavy though, at 30 grams. Unfortunately, I neglected to weigh it before bagging. I'll do better next time.

Up close, there are some flaws. In the picture above, we can see where the finberglass bridges the gap at the edge of the 1/64" plywood. The mylar isn't flexible enough to force it into that small a gap. Next time, we'll leave off the plywood and try a little rectangle of CF cloth.

For our 2nd attempt, I measured a weight gain of about 8 grams. This time, the surface had even more dry spots, perhaps implying that I was too vigourous when squeegeeing the epoxy. On the other hand, a perfectly smooth and glossy surface may imply excessive weight. I guess I need more practice.