Schneewittchen will be made of a mixture of carbon composites and wood. In more detail: laminated layers of uni-directional carbon profiles for the main and aft spars, a carbon sandwitch for the leading edge (torsion box) and wood/composite ribs for the rest (aft of main spar). Due to the mixture between wood and composites, I analyzed the desgin of the ribs in detail.
In principle, the ribs are similar to usual wooden truss ribs found in vintage gliders. This type of ribs is elaborated, however, very light and strong at the same time. The idea is to modify this traditional method and use carbon composites for the truss elements, in particular, the curved upper and lower profiles. These elements define the shape, which should be as precise as possible, also at loaded conditions.
Usually wood strips are bent into form and glued together with the other elements to form a truss. This works fine, however, the strips are stressed and the shape always differs slightly. Instead of using wooden strips, a composite sandwitch profile can be cured in a thread-milled form resulting in an almost stress free profile of perfect shape. At least in theory…
So much on the theory. Some figures on strength, stiffnes, etc. are needed before ribs can be build ribs this way. I started by making different sandwiches for the upper and lower profiles and testing their ultimate strengths. After having found the way to build the main profiles, I moved on to testing the joints. Having all needed figures, I built two rib prototypes, which are still waiting to be loaded and tested for their ultimate strength.
A picture is worth a thousand words:
Prototype of rib number 10: