In supersonic flight, the vertical tail becomes progressively less effective with increasing Mach number until the loss of stability may no longer be acceptable.[11] The stability is reduced because the lift, or side force, generated by the tail reduces with speed for each degree of sideslip angle (lift-curve slope). This results from the very different pressure distribution, with shock waves and expansion waves, compared to subsonic.[12] To achieve the required stability at the maximum operating speed of the aircraft the vertical tail may be enlarged, such as on the North American F-100 Super Sabre (the initial fin area requirement was underestimated). Extra area may be added by installing ventral fins (such as on higher-speed, later versions of the Vought F-8 Crusader), or folding-down wingtips (such as on the North American XB-70 Valkyrie).