OK, I have been avoiding this thread but I think something should be cleared up. The production of lift with an airfoil creates induced drag and its this drag that causes adverse yaw. The confusion seems to be in how and when lift is produced. Lets say for the sake of discussion that a symmetrical airfoil is at "1-degree" AOA, and we'll say the lift produced is equal to the weight of the plane allowing level flight. However with the application of aileron the camber of the airfoil is changed. The side with the aileron pointing to the wheels becomes more positive in camber generating more lift {and induced drag}, the other side becomes more negative generating less lift {and less drag}. Its the difference in the induced drag that causes the yaw. The amount of yaw varies with wing location, the center of balance {not gravity}, AOA, the size and location of the tail surfaces {as related to arm and moment} and a few others. In theory with a symmetrical airfoil with its camber center passing through the center of balance with "0" AOA will not need differential, this seldom happens. I know I'm not very good at explaining things, but I hope this helps.