All right. I get it now. If you are correct that the stuff shrinks significantly more in the roll direction than across the roll, then I would agree that given that the ribs are curved and the leading and trailing edges are not, you would want the shrinky way to go spanwise. Otherwise, the increased shrinky between the leading edge and trailing edge would tend to pull the covering down in the area between the ribs. (Did that help anyone?)

Still. I can't help but note that I've got a beautiful wing here.

One thing that I'm convinced of is that the different colors shrink differently. That makes the science very difficult, because even if you were to build identical wings, you might not be using the same color each time.

Another thing I learned from my experiment is that the stuff shrinks surprisingly little. I would have expected 1/8" or more over 6", but in fact, I've now measured more carefully, and it's a 1/32", if that.