RE: Sailplane Wing Design Round II
IMHO wrapping the spar like that would be a lot more trouble and a lot more messy than simply making up a jig and cutting out the proper vertical grain webbing.
Nodd, the width of the spruce you used should have been more than enough to hold the flight loads. So the failure mode was something else. Two possible causes that come to mind are;
[ul][*] The face joints of the webbing failed because you used too light a grade of balsa in the higher stress area near the center of the wing. For this area the webbing should have been very hard balsa or even 1/32 plywood. And to gain more joint area I'd have used the vertical webs both front and back. By putting only one piece on the back of that wide a spruce spar the failure where the upper spar wants to collapse inward to the lower spar could easily have started along the front where it was unsupported. And when it started to twist the forces could have split apart the webbing glue joint and then total collapse followed very fast.[*] The spruce used for the spar may have a high angle grain runout in that area I can't tell from the splinters in the failure area.
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Consider this. You put the carbon fiber over the area of the spars at the center joint where they had the benefit of plywood joiners as their "webbing". And, I'd have to go back and check the other thread, likely you put plywood braces both front and back?
On the wing you broke over your knee examine the webbing to see if you can get a better idea of what part failed. I'm pretty sure it was so easy to break due to the nature of the balsa webbing or the glue lines. Couple that with a rather wide C shaped spar instead of a front and back webbing box spar and I can see it breaking that easily.
Anyhow, going with the carbon top and bottom caps with full width vertical webbing a la Mark Drela's designs will certainly do the trick. Just keep in mind that you still want to bridge the center joint with long fibers in a way similar to the carbon caps you used in this first wing.
At least you don't need to do new tip panels. Even with the weaker C shaped spars and webbing the loads out on the tips are a lot lighter.