There are a lot of different methods of attaching balsa to foam wings. White glue works, (yellow carpenter's glue works better) but consider some of the other methods, also. I've used double-sided carpet tape to do a partially-sheeted wing, and yes, it was pretty strong. A lot of folks use laminating epoxy, which is strong, but it can be heavy if not applied in a very thin coat. Others use contact glues such as Dave Brown's Southern Sorghum. This works, but you have to get the sheeting in place right the first time! The method that's in vogue right now are the polyurethane glues, such as ProBond polyurethane or Gorilla Glue. These glues stick just about anything to anything, cure fairly slowly, and turn into a foam as they cure. This foaming action gets the glue into every nook and cranny, making for a very strong bond. You have to sheet the wing then put it back into the shucks and weight them heavily to keep the foaming glue from pushing the sheeting away from the core.
Morris Lee