In a normal spin, the downgoing wing (the right wing in a spin to the right) is at a higher angle of attack than the other, but because both wings are stalled, the wing with the greater angle of attack has less lift. It is this difference that keeps the airplane spinning. If the center of gravity is not too far aft, the nose remains down and the flow of air over the airplane, and particularly over the rudder, has a significant front-to-back component. If the CG is too far aft, however, or the fuselage moment of inertia is too large, centrifugal force, pulling the CG away from the axis of the spin, tends to raise the nose and flatten the spin. In a fully-developed flat spin the airflow is predominantly from below, and the chordwise component of flow over the vertical tail is slight. The rudder, normally the primary spin recovery control, becomes ineffective. Depending on the airplane type-spinning behavior can be highly idiosyncratic-unexpected combinations and timing of control movements and engine power (used for its gyroscopic effects, as well as to blast the tail surfaces with slipstream) may be needed to recover. In many airplanes, the speed and stability of the spin increases with time, and it is more difficult to recover late in the spin than early.