Foam Cutting, Sandwiches, Hinges and Horns.

I used the CNC foam cutter to carve the e297 airfoil skin molds out of 4” thick blue Styrofoam. The two female mold pieces are for the upper and lower surfaces, while the male piece can be used to mold root ribs and tip ribs. I decided not to use any intermediate ribs and instead rely on the molded shape of the glass-epoxy sandwich panel between the forward and aft spars. The sandwich core material I'm planning to use is Dow’s special Styrofoam grade called High Load 60, which I sliced into flat sheets about 0.25” thick on the CNC foam cutter. This foam is also blue in color, but very different than the run-of-the-mill Dow blue or Owens-Corning pink foam insulation grades you’ll find at home improvement stores. High Load 60 is an extruded foam grade that has its cells stretched in the thickness direction for much higher directional strength and stiffness. It’s rated at 60 psi compressive strength and about 6,000 psi compressive modulus in the all-important thickness direction, as this resists skin buckling far better than common foam grades. The density of High Load 60 is about 2.3 pounds/cu. ft., only slightly higher than the common grades offered at 1.7-2.0 pcf. You might be able to buy High Load 60 from an industrial insulation contractor, since it is most commonly used to insulate the bottoms of heavy refrigerated storage units that impose high compressive loads from their shear weight. If not, you'd have to buy a 100 sheet minimum from the distributor.. not an option for us not-so-rich folks. It's probably easier to just buy Divinycell H45 pre-cut to thickness...I have some of that, and may yet use it... that's good stuff too, almost the same stiffness, and maybe a bit tougher and easier to bond, but you can't hot-wire it!. Stiffened wing skin panels are a sandwich of two plies of 0.005” thick 2x2 twill S-glass cloth impregnated with high-toughness epoxy resin and separated by the 0.25” thick foam core. This sandwich panel wing skin construction is used throughout except around the leading edges, which are solid S-glass/epoxy laminate without any foam core. The whole sandwich panel has about half of the areal density (pounds per square inch) of 0.016” thick aluminum sheet.

I'm not that pleased with how the upper surface hinged ruddervator looks in cross-section.... rather severe bends at the 21 degree max deflection shown in the picture above... I'm afraid the flow will not stay attached, even with turbulator strips and curly Mylar hinge seals . Maybe I'll try a centerline hinge, or a flex-hinge like Zenith Aircraft uses on the ailerons of their metal CH601 Zodiac plane .. sort of a hinge and spring all in one:

http://www.zenithair.com/images/kit-data/ht-aileron.html

I also might use balancing horns on the tips.. thanks to Ted on the brand new "modiflyer" Yahoo group for inspiring me on that with a couple of excellent posts... it seems stability is improved and pilot workload reduced in rough air conditions especially... and I was thinking horns were only for reducing stick forces and flutter.. ( and for Chuck Mangioni ;)

More Y-tail design sketches & composite fabrication reports coming soon.