OK, let's talk about foam.
First, a bit of theory. The purpose of the layer of core material in a composite structure is to create spatial separation between the outer and inner fibrous layers, and to take up the compressive load when the finished structure is being stressed. The thicker the core layer, and the higher compressive rating (measured in psi) it has, the stronger the whole composite sandwich will be.
However, for our applications, there are limitations, and some compromises have to be struck.
First, the thickness: the thicker the core, the more it will weigh, no matter the material (foam, balsa, etc). Also, thicker material is harder to bend to small radii which are found in places like the trailing edge, some wingtips, etc. If the core can't bend, it will crack, and we generally don't want that. I empirically determined that the foam thickness of about 1/16" is about right for my choice of material and my layup schedule (i.e. the number and gage of fiberglass layers).
As it turns out, not all foams are created equal. There are different types of foams (XPS - eXtruded PolyStyrene, EPS - Expended PolyStyrene, polyurethane foams, expanded polymethacrylimide (PMI), and many others), and then there are different densities within the same type. Everyone knows the large pink foam sheets made by Corning and sold at Home Depot, but did you know that they come in different grades? For most people the R-value (thermal insulating value) of the foam is most important, but for us it is density and compressive strength. Here's a table detailing different grades of Corning Foamular pink XPS foam:
https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/EIS/10015703/10015703-FOAMULAR-SI-and-I-P-Units-for-Selected-Properties-Tech.-Bulletin.pdfI had a sheet of blue XPS foam from Lowes that I bought long time ago, and it is similar in properties to the Foamular 250, which means it is a 25 psi foam with a roughly 1.5 lb/cu.ft density. Read that again: 1.5 lb foam - can you find balsa this light? And if you did, how much would you need and how much would it cost?
When I sliced half of this 4'x8'x2" board into 3/32" slices the other day, I got over 60 wing panel -sized sheets. The entire board costs about $35 plus tax. Not bad.
Here's another interesting fact that I've learned from the Corning foam table. While the foam density (lb/cu.ft) increases by only 16% when going from Foamular 250 to Foamular 400, the compressive strength goes up by 60%! Which means it is more benefitial to use stronger foam. If you can find it, that is. Regular home improvement stores don't carry those foams, they are considered specialty material and are sold by specialty stores, and cost quite a bit more.
Don't even get me started on specialized brand name foams like Rohacell, Herex, Airex, etc. A 2'x4' sheet of 3mm (~1/8") Rohacell is listed at $99 in small quantities. Some of these are difficult to find, too, and I've never seen them in thicknesses that would work for us (1 - 1.5 mm). If you have a source, let me know.
Anyway, once you hotwire your blue or pink foam to about 3/32" thickness, it comes out with a layer of crust (melted foam on the surface) covered with long whiskers. Both the whiskers and the crust need to be sanded off.
Here Gabe is sanding away using a cheap Harbor Freight sander.
I highly recommend doing both the hotwire cutting and the sanding outside (pick a nice day), if you don't want to be evicted by your wife or develop some disease as a result of exposure to toxic fumes and foam dust.
I will continue when I have more pictures.