ORIGINAL: brett65
Wouldn't the plane act strange if this was done to the ailerons? With one side lifting or lowering more rapidly than the other wouldn't this cause a problem? I was assuming that he was referring to the elevator.
The plane already acts strange or we wouldn't be trying to fix it's behavior with differential.
When an aileron goes down, the wing on that side creates more lift. Creating lift creates drag. Creating more lift creates more drag. So the wing on that side creates more lift and more drag. And slows down.
When the aileron on the other side goes up, the wing on that side creates less lift. Creating less lift creates less drag. That wing speeds up.
You wanted to turn left. The left wing goes down because it's producing less lift and forward because it's producing less drag. The right wing goes up and back. The airplane yaws to the right.