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.

  • $\begingroup$ I have never once heard of the term Crossover Airspeed. $\endgroup$ – Carlo Felicione Jul 4 '19 at 13:27
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    $\begingroup$ @CarloFelicione -- Crossover speed: "To further complicate the issue, the configuration and flight speed (flaps 1 and 190 knots) put the airplane in a flight regime where a fully deflected rudder would overpower the lateral control system. This phenomenon came to be identified as the "crossover speed," or the speed at which the lateral control system (ailerons and roll spoilers) could exactly balance the available full rudder deflection. " From FAA accident overview: lessonslearned.faa.gov/… . Google "crossover speed" for more. $\endgroup$ – quiet flyer Jul 6 '19 at 15:15
  • $\begingroup$ No, the Mitsubishi did not publish a crossover airspeed. $\endgroup$ – acpilot Jul 14 '19 at 20:40

The MU-2 flight manual makes no mention of any crossover airspeed. I suppose that's because of the excellent control authority of the spoilerons.

I think you mean Vmc/Vmca or Minimum Controllable Airspeed (I've seen it abbreviated both ways). I've never heard of the term "crossover airspeed."

All multi-engine airplanes (except those like the Cessna Skymaster where the engines arranged in a push-pull configuration along the center longitudinal axis) are tested to determine this airspeed during certification test flights. Vmc is designated on the airspeed indicator as a red radial line over the white arc that indicates when it is safe to extend and operate the flaps. (There is another red line that indicates Vne or Never Exceed airspeed).

This should never be confused with the blue radial line that makes Vyse, or best single engine climb airspeed.

Vmc is determined by the manufacturer as the minimum airspeed at which it's possible to recover directional control of the airplane within 20 degrees heading change and, thereafter, maintain straight flight, with not more than 5 degrees of bank, if one engine fails suddenly with:

  • Take-off power on both engines
  • Rearmost allowable center of gravity
  • Flaps in takeoff position
  • Landing gear retracted
  • Propeller windmilling in takeoff pitch configuration (or feathered if automatically featherable)

On the Mitsubishi MU-2 Vmc is between 89 and 100 (knots calibrated airspeed) depending on how the aircraft is configured at any given moment.

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    $\begingroup$ No, that's not what I'm referring to. The crossover airspeed is the airspeed below which the maximum roll authority from the airplane's lateral controls is less than the sideslip-induced rolling moment from a fully-deflected rudder, as explained in the question that my question links to the answer to. $\endgroup$ – Sean Jul 5 '19 at 1:36

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