Preparing the Wing Cores Now that we have a spar, we need to make the rest of the wing. It was decided that this year's model would have a foam-core wing, and so I cut a set of core using my modified FeatherCut system. If you've never cut cores before, I'd recommend submitting an order to one of the many core-cutting services that are in business both in Canada and the United States. Otherwise, crack out the templates and have-at it. This series won't attempt to explain the intricacies of foam cutting (though I'm happy to report that I got three cores with no wasteage...a first!).

Above Left: First, cut a series of endplates out of very thin plywood (1/32" or the like). Score the endplates where the spar will be inserted, and flex BUT DON'T CRACK the plates. The slots will be cut free after the plates are glued to the cores. Above Middle: Epoxy the plates to the end of the cores (use a very thin brushing of epoxy), then cut the spar slots free with a razor saw, exposing the foam underneath. You can see in this photo that the cores are covered in fuzzy residue from the hot wire cutter. This is a good time to brush the fuzz free from the core. Above Right: By exposing the foam in the spar slot, you can use a hotwire bow to cut the material away from the spar slot. This shot shows my FeatherCut bow setup to cut the spar slot. Note that it is suspended from the ceiling on bungee cords. By tilting the core slightly, the hot wire will follow the side of the now inclined slot, and the foam in the spar slot can be removed.

Above Left: Here the hotwire is slowly passing through the core on the leading edge of the slot. Cutting the leading edge first will make it easier to manipulate the core for the second cut. You can manually manipulate the foam bow to keep the wire pressed against the plywood endplate if it isn't being cooperative. Stop the power to the wire before it cuts through the bottom core shuck/bed - scoring the shuck with the wire won't affect the laminating operations later. Above Middle: Here you can see the finished core. Since this is a tip panel, note that the spar isn't full-length. The remainder of the slot will be filled with end-gain balsa. The foam from the spar slot can be discarded. Above Right: Here is the center panel. Note that the spar here will run the full length of the core, so no filler balsa is required.

Above Left: Here you can see the bolt blocks that will be used to guide the wing hold-down bolts, and prevent the foam from being crushed. They are cut oversize to spread the load on the wing skin, and are made from spruce. They are only installed in the center panel of the wing. Above Middle: The spar is glued between the two sections of the foam core, making sure to prevent the foam from bowing by using appropriate weights (I keep old gell-cell batteries on hand just for this task). The bolt-blocks are also added at this time. Above Right: Since the spar is rectangular in cross section but the spar slot is likely not, after the spar is glued into the core it is grouted flush with the surface of the wing. I use Poly Filla lightweight spackle for the task, which is strong enough to bear the compressive loads between the spar and the wing skin. Try to avoid sanding into the core when sanding the spackle. With that, the core is ready to be skinned.

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