Alex, Here is an answer to your foam query offered by a friend of mine who happens to be a well respected aeronautical engineer and manufacturer.
"The answer to your question is simply based on how the foam is used and what you hope it will do for you. I am assuming that in the construction you describe you are envisioning a construction process similar to that used by the Vari-Ezs and others, where the foam acts primarily as the shape plug, which you then cover with a structural material. In that application the purpose of the foam is to provide aerodynamic shaping and to provide surface stability for the covering materials, in order to reduce the chance of buckling or localized crippling when the structure is under load. There will be some loading in shear within the structures and so any application should be analysed in order to verify that the application is within the realm of the material selection. As a general answer though, the loads tend to be very small in this case and so the bond achieved between the epoxy and the foam should be more than enough. There is however a misconception in your post - the strength of this structure is not based on the foam "absorbing" the epoxy. As a matter of fact you actually do not want this to happen as this will make the structure substantially heavier. This is why the extruded foams are preferred to the expanded ball styrofoams - they tend to be stronger with more surface stability, they shape and/or sand easier, and they do not absorb needless amounts of resin. Yes, the laminate will peel off the surface quite readily, but in service peel is not the type of loading you see. If designed correctly, there will be no exposed foam edges from which the surface covering could separate - the entire foam substructure is enclosed in the composite shell. But the bottom line in all this is the proper design of the wing. The foam is not structural beyond providing surface backing to the structural shell and a moderate amount of shear resistance. In a normal wing the shear resistance is provided by the ribs so the loads are concentrated just in the area where the rib flange contacts the skin. In a foam cored wing, the skin is in full contact with the foam interior and thus the large amount of surface adhesion results in very low shear loading. In order for the structure to behave predictably, it will be important to design the wing in such a way that the primary loads are absorbed by the skin, any intermediate shear webs, and of course the built-in spar caps, and any loads transmitted to the foam are kept to a minimum. The details of all of this will be a function of the design requirements and the detailed engineering analysis." I hope this helps you, and it should also provide some valuable info to others on the list. Cheers, Peter Bancks. stranged...@dodo.com.au
