Selcuk; They don't have to be very big to be effective, and my tests indicate that low-speed lift is improved more with the plates below the wing. I know that several current commercial jets use turned-up winglets, which are useful at high speed, probably not very effective at model speeds.

Bruce; what you said is true. The down aileron is operating in a high-pressure area, and its effeciency is greatly improved with an end plate. Remember that the high-pressure air is trying to roll up from under the wing, so the plate would not be effective on the up aileron, even if it extended above the wing to "close the gap," as you mentioned.

Low-speed lift is improved in the same manner. Any time you can keep the high pressure air from moving to the "wrong" side of the airfoil, lift will be improved. This is especially true during ground effect.