LAST EDITED ON Oct-13-04 AT 06:45 PM (CDT) Foam core fuselages are pretty easy and forgiving.
For 40 size planes make the front end out of 6 lb. 3/4 in. stock. Extend it back to behind the spars an inch. Use 3/8x3/4x5in. maple motor mounts. Extend the balsa block back between the spars to carry the bellcrank. It is a good idea to cut some offset in the motor mounts to bring the thrustline to the center of the fuse at the spars. For 25 size motors the fuse can be thinned down to half an inch.
Reduce the thickness of the front block 1/16 in. on each side at the back to allow for the 1/16 in. fuse sides. Slice white beadboard foam 5/8 in. thick and glue it to the balsa front. Lay a tracing of the fuselage over the whole thing, line it up, and cut out the foam to the fuse shape. Taper the last 2 in. or so to fit the rudder/fin. Fit the fuselage sides to the foam and trim to outline. Glue in place with a very thin coat of epoxy or polyurethane(foamy glue). Make sure the balsa gets pulled down to the taper using clamps. Clamp the whole fuse while the glue is drying by laying it on a flat surface and cover it with bricks or magazines. It doesn't take a huge amount of pressure, but the sides need to be pressed against the foam all over.
Either cap the fuse top and bottom with 3 layers of 1/16 in. wood laminated, or round the edges and form a single piece of 1/16 in. over the edge. Glue in place.
Sand every thing smoothly to shape. For best results, taper the fuse sides with lots of sanding down to .03 or .02 at the tail post. This saves about half the weight of the balsa(as much as an ounce on a scale-like fuse), all behind the CG.
Doublers are two layers of 2 oz. glass cloth from the spars forward. Also apply one layer about 1.5 in. wide all the way back to the tailpost.
You can do this fuse and make a cutout for the wing, but the plane will be a LOT stiffer and stronger if you re-design it to be built like an I-beam stunter. Build the wings in right and left hand panels and join the wing spars, leading edge, and trailing edge, through the fuselage with joiners.
I usually mount the bellcrank on a T-shaped platform that goes through the fuse, just behind the spars. The ears of the T sticking through on the right side will key it into the front block or put a couple of pieces of 1/8 in. dowel up through the bottom.
Some folks build with sheet balsa strip formers instead of foam. It will be half an ounce lighter and MUCH more fragile. The foam fuse, if it breaks at all in a crash, will generally break into two pieces, usually right at the trailing edge of the wing. A built up slab fuse usually disintegrates into splinters.