Airplanes generally have a crossover airspeed (a minimum airspeed below which directional control of the airplane cannot be maintained in the event of a rudder hardover). This is because, as airspeed decreases, the amount of lift generated by an airplane’s wings also decreases, forcing the airplane to fly at a higher angle of attack in order to maintain steady flight; this places the airplane’s wings closer to their stall angle, reducing the wings’ lift reserve, and, thereby, reducing the amount by which a lowered aileron can increase the lift of its attached wing, resulting in a reduction in the airplane’s maximum roll authority.1
However, not all airplanes use ailerons for roll control. A famous example is the Mitsubishi MU-2, which, in order to achieve good STOL performance, has large flaps along the entire length of its wings, leaving no room for conventional ailerons. Instead, it uses spoilerons (spoilers used for roll control) on top of the wings; to roll the MU-2, the spoileron on the downgoing wing is raised (reducing the amount of lift produced by said wing, and causing it to drop), while nothing happens on the upgoing wing.
As the MU-2’s roll control system does not depend on increasing the amount of lift produced by either wing, it should be immune to the decrease in roll authority at high attack angles that curses an aileron-equipped airplane with a crossover airspeed; thus, one would, theoretically, expect the MU-2 (or any other airplane that does not use conventional ailerons for roll control) to have no crossover airspeed.
Is this true? If not, why?
1: This is aggravated by increasing the airplane’s vertical load factor beyond 1G, loading the airplane to its maximum allowable weight, and/or retracting the flaps (if any), all of which increase the angle of attack at which the airplane must fly in order to produce the amount of lift demanded of it, and, thus, increase the airplane’s crossover airspeed, or by retracting the leading-edge devices (if any) and/or flying with iced/insected wings, both of which decrease the wing’s stall angle, thereby increasing the airplane’s crossover airspeed; conversely, an aircraft’s crossover airspeed decreases if the nose is pushed over to maintain a vertical load factor below 1G, if the airplane is empty or very lightly loaded, if the flaps and slats/droops are fully extended, and the wings are clean of any contamination.