Stability and Control

Horner's questions lay at the very heart of aviation, poised to take center stage in space. What would be the role of the human pilot as aircraft entered ever faster, higher, and more demanding flight regimes? How would the pilot share the cockpit, and control of the aircraft, with electronics and computers?

Beginning with the Wright brothers and continuing to this day, control is an essential feature of aviation. How should the pilot command an airplane? How should the pilot be trained? What devices should aid the pilot? In the early twentieth century, new personalities accompanied the new technology: the aristocratic aviator, the fighter pilot, the ace, the barnstormer, the stunt pilot, to name but a few. Since then, the technology of aviation and the profession of the pilot have evolved together.

Yet within this history lay a paradox, or at least an irony. As aviation matured, aeronautical science became increasingly adept at measuring and modeling the airflow around an aircraft and designing structures and devices to accommodate it. But the core of the aircraft was still the pilot, a human being, a subject that engineering has never fully mastered. Hence the pilot's importance: performing tasks that are difficult to measure or model.

At heart, debates about control and automation in aircraft are debates about the relative importance of human and machine. They go back to the very origins of powered flight, and with a few modifications they pervaded the Apollo program. From the Wright brothers' flying machines to the jets of the 1950s, technologies of control have evolved in parallel with the people who did the controlling. In aviation, as with all technologies, technical change and social change are intertwined.

First, a few technical terms. In aviation, people and machines come together through stability and control. Stability is the tendency of an aircraft to remain in straight and level flight, at a given airspeed, even without the pilot's inputs, and to return to that state after external disturbances like wind gusts. Stability is a feature of the general design of an aircraft—the placement of the wings and center of gravity, their subtle angles and orientations. Most modern airplanes are inherently stable—they will return to straight and level flight if the pilot takes his or her hands and feet off the controls. Similarly, an automobile is stable: on a flat road it should drive in a straight line if the driver's hands are removed from the wheel. An unstable aircraft with no pilot inputs, by contrast, will depart from straight and level flight, and eventually crash. A bicycle is unstable—it will not go straight, or even go at all, without some active participation from the rider. Note that instability does not make a bicycle unrideable or an airplane unflyable—it just takes more work, attention, and skill on the part of the operator.

Today it might seem obvious that stability should be built into an aircraft, and many engineers indeed share this view. But consensus on this point took decades to develop within aeronautics. At the heart of the debate was a tension between stability and control, which to some degree operate at cross purposes. The more stable an aircraft is, the more effort will be required to move it off of its point of equilibrium. Hence it will be less controllable. The opposite is also true—the more controllable, or maneuverable, an aircraft, the less stable it will be. A fighter plane is more responsive than an airliner, but also more difficult to fly.

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