This is similar to how I make ribbed foam wings. Its more robust and suited for larger aircraft. I've got a subject queuing up for after by Saab build to demonstrate this style of wing building. In my building experience using balsa sheeting is overkill and just plain expensive. Sheeting with foam is just as easy and cost effective. It has just as much strength and once finished properly you cannot tell the difference. I really wish I had taken pictures of my Funder and lightning build. That plane has flown so well this year and I am glad I took the time to learn the technique.