The Jets at an Awkward Age

Performance of the first jet aircraft outstripped stability and control technology. Their high performance called for two stability and control technologies that were still quite crude – power controls and electronic stability augmentation.

The high transonic Mach numbers reached by the early jets, such as the McDonnell XF-88 and the North American FJ-1, led to large and generally unpredictable control sur­face hinge moments and the possibility of control surface flutter. Redundant, irreversible hydraulic control actuators on all surfaces were really needed. With irreversible controls, the normal aerodynamically generated control stick forces would be replaced by artifi­cial forces generated by such things as springs, weights, bellows, and closed-loop force generators.

Likewise, the operating altitudes in the 30,000- and 40,000-foot range that jet power had made possible required that the normal source of damping of oscillatory yaw, pitch, and roll motions be augmented. Satisfactory damping of the Dutch roll and short-period longitudinal oscillations comes naturally near sea level from forces generated on an aircraft’s wings and tail surfaces with control surfaces fixed. However, at the higher altitudes, control surfaces need to be driven by electronic stability augmentation systems in a series fashion, added to the pilot’s inputs and not especially apparent.