ORIGINAL: iflynething
I found out they used foam because I have someone at my field that I asked why you didn't see the ribs coming out of the wing....so I asked the guy and he said that the stuff that they do, they use the foam, since that is "a little stonger than the balsa for the aerobatics they do"....and they make the covering look so good where you don't see any ribs or inperfections...because if they use the foam....they usually sheet the wing and that will give you the nice flat smooth finish that most people see when they see a giant scale plane......
The foam itself is not that strong. It is the balsa sheeting, and often carbon fiber reinforcement, that give it its strength and stiffness. A wing can be quite flexilble and still be strong (watch the wing of the 737 next time you ride one through turbulence). Wiglly wings are NOT good when you're trying to do precision aerobatics, so stiffness is important.