If I were to change from an 11x6 I'd go with an 11x7 before I'd use an 11x5. Again being a trainer the faster you pull it the quicker it produces lift and besides we never fly a trainer at full throttle so top end speed isn't what were looking for. I'd lay money that an LT40 will climb faster with an 11x7 over an 11x5. Why? Because a flat bottom trainer produces more lift at higher speeds. When instructing new pilots with a trainer such as the LT40 you should teach them that increasing the throttle makes it climb not giving it up elevator. In other words more speed and an 11x6, 11x7 will get it up to speed faster making it climb faster.