Okay, another way to remove "lazy" foam (assuming it exists) from a monolithic XPS core. In this case the foam is removed in chordwise chunks leaving XPS ribs in the "normal" chordwise orientation. Like a solid XPS core, the composite skin would be 100% supported by foam. (This could use pictures, and is probably harder to describe than to actually do):
1) Cut solid core to desired airfoil shape
2) Using a hotwire, split the core from LE to TE
3) Mark the future location of the spar on the inside of the split face of the top and bottom core half. Also mark the future rib locations (say, every 6 " on center)
4) Use a foam cutting hoop to remove the foam between the ribs. You'll use a guide/template to assure the hoop goes the right depth into the foam leaving an adequate foam supporting layer under the skin.
a) Foam cutting hoop: A modified soldering >gun<. Described
here by Pops and others. Make a squared-off hoop out of suitable rigid wire. One dimension of the hoop is the same as the open space between the ribs, so 5"in this case if our ribs will be 1" thick and they are 6" on center. The "depth" of the hoop is the deepest cavity you'll need to make, leaving the desired foam thickness under the foam. The foam cutting gun/hoop has a flat foot/sled so it can ride on supports while in use and have the hoop hang down.
b) Make a tapered template of particleboard/XPS etc for the foam cutting gun/hoop to ride on. It is placed on the inside face of the "split" core surface to guide the foam cutting gun/hoop along the correct line for removal of foam in each rib bay. The main purpose of the template is to guide the cutting depth of the hot loop tool. The small end of the tapered template is at the spar side (so the cutting hoop goes in deep at this thick part of the wing) and as the cuttting hoop goes toward the trailing edge the template raises it up to keep it the desired distance from the skin (1"? 3/4"?). So,the cutting hoop will exit the foam some distance from the back of the core, to allow for the desired foam thickness under the skin.
c) Cut out all the rib bays aft of the spar. Then, use another appropriate template to (with the proper taper) to remove the foam from the bays forward of the spar (if desired).
5) While the top and bottom core halves are apart, make any desired small spanwise cutouts for
wires, pushrods, cables, etc.
6) Re-glue the top and bottom core halves (all ribs and the place where the wire left the skin at the LE and TE). Assure everything is still aligned and no twisting has occurred due to internal foam stresses (consider putting everything back inside the original female top and bottom halves of the foam blank and apply weight/pressure while the glue cures).
7) Next, use a hotwire to cut out the locations for the spar caps and to split the cores top to bottom at the spar web. Build the spar webs and cap on the foam core.
It is more trouble to do things this way than to cut out spanwise channels as described in the OP.
Pros:
- Like the OP method (spanwise cavities) and a regular solid core wing, the outer laminate skin is bonded >everywhere< to XPS foam for support against buckling.
- Faster construction than cutting, aligning, and attaching "a thousand ribs" to a spar. Less adhesive weight, and all the foam junctions will have a filleted edge (from the cutting hoop) rather than a 90 degree junction with adhesive bond.
- ?? Better support against skin buckling/foam failure than the spanwise ribs-and-voids in the OP??
Con:
- Takes longer to remove the foam in this fashion than to remove spanwise chunks with a simple hotwire. Still, once the taper of the template is figured out and the foam cutting hoop is made, I'd think the cutting would go quickly.
- Unlike the method in the OP, there's no easy way to leave the to-be-removed foam in place to support the skin temporarily during vacuum bagging. This might be a fatal flaw.