On Monday, July 29, 2019 at 3:27:07 PM UTC-7, Charlie M. (UH & 002 owner/pilot) wrote:
OK....stupid question....most FRP works better in tension (from what I know) than compression.
Sooooo.....why is top layer of spar thicker than bottom...

As Uncle Hank says, the upper spar cap is generally in compression, and the lower cap is in tension. The compression force in the upper spar cap is about the same as the tensile force in the lower cap. But since composites are generally weaker in compression, you might want to give the upper spar cap more cross-sectional area so that the stress (force per unit area) is lower. Or, another way of looking at it is that you are leaving some unnecessary material out of the lower spar cap since you don't need as much there to react the tensile forces.

The result, of course, is a set of wings that can support a greater load factor in normal flight than while inverted. But since most gliders are neither intended nor used for high-g inverted flight, it's a reasonable compromise, especially when you're dealing with relatively low strength-to-weight materials like wood or fiberglass.

With my gliders, I use the same amount of carbon in both the upper and lower spar caps. It makes it easier to fabricate the wing spars, and with the high-strength Graphlite material the weight penalty is negligible.

Thanks, Bob K.