ghost bear is right, on a 40 size trainer you only need one servo just go by the plan sheet, my eagle 2 had a 60 inch wingspan and one servo worked fine, you may need 2 on a warbird or aerobatic plane for more control, trainers dont do much more than float around, maybe a roll,loop or inverted for a few seconds before its fighting to right itself, .........Rog

flaps might be cool though