The airfoil is shape of the wing in cross section. The original wing has a flat bottom airfoil, good for slow flight because it produces lots of lift. The Eppler 197 is a semi-symmetrical airfoil, curved both top and bottom with the top having more of a curvature. Eppler is the name of the guy who "invented" this particular cross section and I guess it was his 197th design?. It allows the plane to fly faster because it produces a lot less drag. It's also better for aerobatics because a flat bottom airfoil always wants to fly right-side up, so you are constantly fighting it when trying aerobatic flying. The best case I can make for changing the airfoil is that a fellow member built this wing for his Eagle II, and flew the plane. He didn't notice a whole lot of difference, until one day he crashed and broke the updated wing. He put the original wing back on the plane and hated how it flew. He built a second updated wing and put it back on. I have written instructions for the wing build as well, hoping someday to send off a construction article to one of the big RC magazines but I haven't really thought too much about it recently. Let me know. By the way, the Eagle with the updated wing flys like on rails although if I were to build another, I'd probably chop a bay off each end of the wing. Like the original, it was tricky to fly in gusty conditions - big wing and a light plane!

Cheers!

Jollidude