bdk suggested this topic, and I also like to know more about it. Despite all the difficulties with composite, there is no contest of appearance (and aerodynamic drag) between a metal airplane and a composite airplane. So we have to deal with composite.
The textbooks on composite place a lot of rules on the "right" ply orientations and sequences. For example, you should have symmetry so the structure will not warp under tension. Another rule is that you should not have only uni-direction plies of the same orientation, otherwise the laminate has poor load bearing properties in the transverse direction.
I have seen both rules violated in spar and skin of wings of small composite airplanes. In one airplane, the skin has a glass ply, a carbon ply, a foam layer, and a glass ply (from the outer to inner skin), there is no symmetry in the laminate. The spar caps are solely made of uni-directional carbon of the same orientation, sometimes up to 20+ plies.
Are these ply sequences typical for small airplanes? I would like to know more examples of wing skin, spar cap and fuselage skin ply sequences. What are the "penalties" for breaking textbook rules?