Hi,

On single rotors, I have found a slight difference but the performance seems about the same. I have built and flown many dual rotors without any sort of wing. The rotors provided all of the lift. They seem to be more stable with the advancing blades in the center. I haven't tried with both rotors turning in the same direction, but I suspect the rotor with the advancing blade on the outside would provide more lift. However, if the rudder has enough authority, it probably would fly, at least until it hit the ground. Not sure?