Hi Phil D,
Apologies for not responding to your earlier posts - things got busy for me and I'm afraid I put you on the long finger!
A foam core, of almost any thickness, will definitely be stiffer than my three-laminate D-tube leading edges, as thicker generally equals stiffer. I wish I had seen Rol Klinberg's videos before I started my CD build, as I think he has developed a really good system for his build. My D-tube skins are very tough, but they do tend to 'oil-can' buckle under stress whenever I fly into a big thermal, resulting in a lot of 'bonking' and 'popping' noises! It's not dangerous, but it is disconcerting for the first couple of flights each year, as I get used to it again! I also omitted to install the non-structural foam 'false' ribs between the main structural D-ribs in my leading edges (to safe a little weight), but this has lead to the oil-canning in turbulence.
If I were to build another CD I'd be tempted to at least fabricate a test section of D-tube using Rol's method to check its weight and strength before committing to a full size part.
The Windrose wings are designed to be fabricated with solid foam cores skinned with S-glass (S=Structural - E=Electrical - see Wikipedia
en.wikipedia.org/wiki/Glass_fiber), but I am considering hot-wiring out the cores of the wings so that the foam thickness is reduced to around 1/4" or so, and skinned inside and out with S-glass. The idea being to lighten the wings (especially outboard, so as to increase roll rate) and provide space inside the wings for water ballast towards the root section. I want to be able to experiment with both very light and nominal wing loadings to see if the Windrose can also be made to operate in the micro-lift region... and keep the empty weight under 120kg for regulatory purposes :-)
I've almost finished building my 3-axis 4'x4' CNC mill for cutting out parts
(link here)
and will shortly start work on a 4-axis CNC hot-wire foam cutter
(link here)
for cutting out the wing cores.