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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 do 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.

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    $\begingroup$ 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
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    $\begingroup$ I don't think it can be answered in a general way, as edited. As you explain, there shouldn't be, but we can imagine a situation when it would be so (for example, spoilerons' actuators unable to drive at very high airspeed). But any sensible design would put such conditions outside of the flight envelope. How can we then assess if the aircraft is prone to such crossover if the conditions are limited by other factors (and thus probably not even tested for)? $\endgroup$ – Zeus Aug 10 '20 at 0:47
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    $\begingroup$ Have you seen the rudder on the B-52G/H? Won't take much to overcome a hard over of that tab. ;) $\endgroup$ – OSUZorba Apr 9 at 2:27
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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. $\endgroup$ – Vikki - formerly Sean Jul 5 '19 at 1:36
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    $\begingroup$ On a side note (it isn't what the question is about either way), the longer abbreviation Vmca means minimum control speed in the air speed, and is used in contrast to Vmcg, the minimum control speed on the ground. $\endgroup$ – Jan Hudec Oct 20 '20 at 6:33
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I can see what you're getting at, your point is essentially, if a pilot deflected the rudder to its fullest extent, is there a minimum airspeed at which it could be overcome by aileron or spoileron control? But I seriously doubt that aircraft manufacturers test this phenomena, in any event I have never come across the term in 38 years of aviation. A hard over rudder at any airspeed in anything other than a fully aerobatic airplane is going to cause a lot of grief. American Airlines flight 587 was an example of what would happen to an airliner if full rudder deflection is commanded. Airplanes are indeed tested for Vmca as mentioned above.

At the same time can mention that in many light aircraft one could perform a sideslip maneuver with full rudder and aileron input in order to lose height rapidly and indeed, this maneuver was used in the Air Canada Flight 143 emergency as well as TACA flight 110. However it is highly doubtful that manufacturers have ever published a "crossover airspeed".

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    $\begingroup$ AA587 is not a good example, since the overstressing of vertical stabilizer was a dynamic effect caused by quickly alternating between left and right command, not just keeping the rudder at the stop. A better examples would be United Airlines 585 and USAir 427, where full rudder deflection was a result of the actuator fault (both crashes) and Northwest Airlines 85, which landed safely, but a 747 has a split rudder and only half of it locked up in the extreme deflected position while pilots kept the other deflected the other way to compensate. $\endgroup$ – Jan Hudec Nov 8 '20 at 20:36
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It might not be good to trust an aileron or a spoileron at high AOA.

To start, "minumum cross over speed" is the minumum airspeed at which an aircraft has sufficient roll authority to create an opposite sideslip to a rudder hard over. Slipping in the opposite direction puts side force on the horizontal stabilizer, helping counter act the rudder force.

The difficulty with ailerons at higher angles of attack are as follows, the downward aileron creates drag on the same side as the rudder, creating an "anti-coordinated" condition (it makes the yaw worse). Also, at high AOA, the wing tip may stall, adding to the woes. On the other side, the upward raised aileron at high AOA is blocked from the airstream by the wing, reducing its effectiveness. A spoileron, though slightly safer, would have similar issues.

One solution, particularly with airliners blessed with plenty of available thrust, would be to use a clamshell on the opposite wing to hold straight. And, eventually, as on a hypothetical B-52 I, get rid of the vertical stabilizer/rudder and just use the clamshells.

With the current BUFF, differential thrust is also an option.

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