Sure your point is valid and there is always modeling to be done. My point was more for the wrinkling issue than the membrane stresses. As a research uni project I would first build a FEA cross check with any available theory and then embark on testing program and then create an empirical theory/curves for practical design. For a one off average amateur design testing is more fun and realistic.
Testing is fun, but I have a strong preference to make one test article instead of a bunch.
Wrinkling/buckling/crippling. Hmm sounds like a trip to Bruhn and then take apart the plate theory for homogenous material, then put it back together again with composite theory to get even close. I am not doing it, but it might be useful to folks talking about doing it.
btw I have the sixth edition of Roarks and 11.4 does not map. I have a 12.3 that refers to combined membrane and bending in thin pressure vessels. Is that what you were referring?
Nope, Roark's Seventh Edition Chapter 11 is about flat plates. Chapter 13 is Shells of Revolution, and could well be useful for this same purpose. Chapter 11 is for flat plates, not the gently curved plates in one direction that are our wing skins, but they are thin (3/8" on my roughly 12" x 30" panels) and constrained on the edges, so it seems a reasonable approximation. Chapter 13 is for shell of revolution, so they are curved in one axis similar to our wing skins, but there are no appropriate edge constraints. I picked Chapter 11.
To recap there are two issues with holed core: membrane stresses due to pressure differential and wrinkling of skin due to limited foam support. The former (membrane stresses), I do not believe, is an issue. The KR series airplane flew with one sided faced foam about 1" to 2" thick. In other words there was a giant hole as wide as the distance between the spars and a few feet span wise between the ribs. As long as you leave about an inch or two between the facings and the holes and leave the spar area clear you should be fine. The latter will need testing to be sure, though frankly if you make the holes aft of the spar I think you should be fine.
I had forgotten that the KR's were not skinned on both sides of the foam. Now that I am thinking about it, I recall that the KR wings were wooden structurally - wood spars, wood ribs, then filled in with foam and finally glassed with 4 oz cloth. The originals were not even glass, but Dynel, which is acrylonitrile and vinyl chloride copolymer of modest strength and stiffness, and is a sponge to resin. Many suppliers recommend using it only with vacuum bagging to keep weight down Think of the KR's as a fabric covered wing, but with thick foam backing up the fabric. It does give us some hope that a reasonable amount of foam can be left on the ground without being glassed on the inside...
Billski