Bird Dog
Ben,
I think you have a good application for the foam core wing. Will this wing be one piece? 90" inches is a lot to handle in one piece!
IMHO, if you use foam core, 1.5 oz. fiberglass between the foam and 1/16" balsa skin you won't need a spar. Reason being is the lift struts. Which will need special attention to the hard points you install for strut attachment. The fiberglass in the sandwitch makes the wing strong and stiff. I've used it on Pylon racers and never had a problem. As I recall a 50" wing (500sq. in) would come out of the press at about 11 oz. and finish up with servo at about 16 oz. Don't sell this construction short on strength and it will be light..... How you going to handle the flaps. ailerons? Good luck..... Jeff.......