Flight Controls

An aircraft flight control system must be capable of providing precise maneuvering about each flight axis: pitch, yaw, and roll.

Conventionally, ailerons are used to provide roll moments (torque about the longitudinal axis). With the advent of advanced high speed aircraft, some issues arose. The thin wings needed for supersonic flight were more susceptible to bending as the ailerons were applied, leading to wing twisting. The effect of this was a torque opposite to the intended direction, a phenomenon called aileron reversal One solution to this was to position ailerons closer to the fuselage, where there was more wing chord to make for a stiffer wing section. This alleviated reversal, but of course the ailerons were less effective being closer to the fuselage.

North American took an entirely different approach: positioning the control surfaces mid-chord, a position which negated any wing twisting torque. Each wing contained six hydraulically actuated panels, in spoiler and deflector pairs. Two sets ducted air from the top wing surface to the bottom, increasing lift, while the other set performed in the opposite fashion. (See description from the RA-5C NATOPS nearby). This let to to a responsive but stable control feel, especially at high speed.

Low speed flight was another matter. The RA-5C was equipped with conventional trailing edge flaps, and leading edge flaps (or "droops") as well. Maintaining attached flow over the broad wing was a difficulty in high camber operation, with both droops and flaps extended. The solution was to blow high pressure air from the engine compressors over the wing from the leading edge. The high energy air served to keep the flow attached and maintain lift.

The spoiler/deflector control system did not work well in the high lift landing configuration. Raising either spoiler or deflector on the wing upper surface tended to detach flow and thus kill lift regardless, limiting the control authority. A solution was to use a completely different actuation approach with flaps down: a commanded roll would raise all spoiler/deflectors on one wing only; the surfaces on the other wing remained unactuated. This killed the lift on one wing only, resulting in good roll moment, though the aircraft tending to lose altitude as there was no compensatory increase in lift on the other wing.