BFoote

Dissimilar materials = dissimilar deflections and dissimilar load distribution. Assuming bond strength is there. When talking balsa or any other wood, the bond strength will surpass the wood in question so your dissimilar materials concerns vanish. Gets back to Deflection. Likewise your statement about a stiff spar and your so called monocoupe balsa "problem". What is a problem with your moncocue balsa wing sheet build is transferring the lift from said skin to said spar. I have seen this in that there were no ribs to transfer said load to the spar. Therefore the wing will shatter where it is attached to the spar as the numb nut brained person building said wing did it wrong. You want as stiff a spar as you can get. Stiffer the better.

Take the 787 for example where I worked on the LE/TE slats/flaps along with the systems/sensors which was being contracted out and my brother worked Main wing and Wingtip structure. 787 CF main LE/TE spar. It was deflecting so much and creating near impossible aerodynamic drag that they almost gave up on CF entirely and went with an aluminum spar structure and CF skin as said CF spars were loading the skin up way too much creating monstorous stress risers that are not present in an aluminum skin due to its allotropic properties. Then found out they couldn't go with a CF skin due to said stress risers for same weight. Instead said skin had to be heavily increased in weight to compensate. In fact the 787 CF skin is heavier than an aluminum skinned wing because of this. Where they saved so much weight is in the major structural components. Add in the problem with lightning strike and the giant amount of copper that had to be laid into the skin and the weight further increased drastically. In short, there were no weight savings in the wing due to Carbon Fiber. Its far more complex to create/work than aluminum and it was barely lighter due to the very large wing structure weight in comparison to the skin. The last 40 feet of wing would have been far lighter to build out of aluminum but it was deemed impractical to change entire plants to work both materials. IE Moog didn't want to do it in Japan as it wasn't in the contract... Its why the next 737 will have aluminum wings and a CF fuselage or aluminum/glass wings and CF fuselage.

Back to dissimilar materials issue it has everything to do with edge shape(How loads are applied) between structures and stress lines not aligning in a structure. Just another stress riser and is something we 787 engineers have had to deal with daily and how is done on all composite structures as it is Not an allotropic material. It all depends on how the orientation of the fiber and how one runs this into the next piece/joint. In short every layer of a CF build is a dissimilar material as its deflection orientation and load orientation are ALL COMPLETELY DIFFERENT! All structures really, but is far more apparent when designing/manufacturing with composites.
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