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, while simultaneously increasing the rate at which drag increases with increasing angle of attack, and, hence, increasing the severity of the adverse yaw generated by the ailerons.1
However, not all airplanes use ailerons for roll control; some rely solely on spoilerons (spoilers used for roll control) on top of the wings. To roll such an airplane, 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.2 Notable examples of spoileron-only aircraft are the B-52G/H, whose long, thin wings would twist to an unacceptable degree under the torsional loads imposed by conventional ailerons (shortening the wing's fatigue life and risking flight-control reversal and/or structural failure of the wing), and the 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.
As a spoileron-only airplane’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 airplanes that does not use conventional ailerons for roll control, such as the B-52G/H or MU-2, 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.
2: Most airplanes (other than very small ones) have, and use, spoilerons, due to their advantages at very low3 and very high4 airspeeds; however, the vast majority of those are also equipped with conventional ailerons.
3: As spoilerons do not increase the angle of attack of any part of either wing, they are unaffected by the reduction in lift reserve and increase in induced drag that come with higher attack angles. Additionally, unlike conventional ailerons, they produce proverse rather than adverse yaw (as spoilerons increase drag on the downgoing wing [the one with the raised spoiler], rather than on the upgoing wing, as ailerons do); thus, for spoilerons, the yawing moment produced by roll-control-surface deflections assists with the intended manoeuvre, rather than partially cancelling it out.
4: As the aerodynamic force produced by spoileron deflection is applied at near midchord (instead of at the trailing edge of the wing, as is the case with ailerons), spoileron deflection places much lower torsional loads on the wing structure at very high airspeeds than aileron deflection does, reducing the amount of wing twisting (which reduces the aircraft's roll-control authority, shortens the fatigue life of the wing structure, and, at high enough airspeeds, can cause full-on control reversal and/or structural failure of the wing, all of which are generally considered to be undesirable) produced by roll-control-surface deflections.