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I'm curious about the F/A-18C. I know it has ailerons, but they seem too small to be greatly effective at low speeds. Do the stabilators have a roll function as well? If so, is there a speed at which one or the other becomes inactive?

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Yes it has differential stabilators. Below is a description from the F/A-18's (A/B/C/D) NATOPS flight manual. Almost every surface including the flaps and leading edge devices are used in rolling the aircraft (fly-by-wire magic):

2.8.2.8 Control Augmentation System (CAS).

The lateral control system uses ailerons, differential trailing edge flaps, differential leading edge flaps, differential stabilator, and rudders to achieve the desired roll characteristics. Scheduled air data roll rate feedback is used to augment inherent airframe roll damping. At high airspeeds, aileron travel versus stick movement is reduced and the ailerons do not deflect above 600 knots. Differential stabilator and differential trailing edge flap travel is reduced at high speed to prevent exceeding structural limits. The leading edge flaps deflect differentially up to ±3° when below 30,000 feet and above Mach 0.7. Differential flaps are not used in the takeoff or land modes nor above 10° AOA in the auto flaps up mode. At low airspeeds, aileron and differential stabilator travel are reduced with increasing AOA to minimize adverse yaw. Differential stabilator may also be limited due to a pitch command which has priority. With lateral stick deflection, the rolling-surface-to-rudder interconnect (RSRI) schedules increasing rudder deflection as a function of decreasing airspeed and increasing AOA for roll control, coordination, and to reduce adverse yaw.

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