Hydro Junkie

What the rudder does, when used in a turn, is not what common sense says it does. Turn the front tires on a car, that vehicle changes direction. In the case of an airplane, however, it's the ailerons that actually turn the plane by changing the direction of lift. The problem comes in with the drag added by using the ailerons. An aileron that goes up adds little drag since it's partially masked by the thickness of the wing and the higher speed of the air over the top of the wing. An aileron that goes down, on the other hand, adds a considerable amount of drag since it drops further into the air flow under the wing. Since the aileron that drops is on the outside of the turn, the plane will try to turn in the opposite direction due to the drag. The rudder is used to offset that "inverse" drag to "help" the plane turn in the desired direction. To use the rudder alone, to try to change the plane's direction, will result in the plane sliding sideways through the air like a car trying to turn on a slick road