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Issue 20 - 2002 : Understanding Stability
Stability is a concept which is germane to fly-by-wire. Stability is defined as "the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed." Conventional issues of aerodynamic stability are restrictive to designers, however, in that they create an environment where the size of the wings, tail or fuselage of an aircraft should be in direct proportion to one another to ensure that the plane flies well. However, even the Wright brothers discovered that no matter how stable the aircraft itself, positive static stability (or the ability for an aircraft to right itself after a disturbance) also depends on a combination of man and machine. "The center of the problem thus shifted from determining and maintaining some form of inherent stability to that of allowing [positive] dynamic stability within controllable limits," say James Tomayko, author of Computers Take Flight, a study of NASA's fly-by-wire project on the F-8 fighter jet. Dynamic stability refers to the period of time it takes for a plane to return to its original position after a disturbance. Fly-by-wire changes the rules slightly by permitting a state of "relaxed stability," which leaves room for the designer to reduce the size and weight of various aerodynamic surfaces, creating new efficiencies. Tomayko further explains, "by actively controlling an aircraft's surfaces automatically, it is possible to stabilize it artificially, thus reducing the need for horizontal and vertical stabilizers with large surface areas." Fly-by-wire shifts the onus of control from the flight surfaces to the flight control system, opening up new possibilities for design. Restrictions on aircraft stability are drastically reduced, enabling aerodynamicists the freedom to design more efficient airplanes with higher thrust-to-weight ratios. Principles of Stability Static Stability The concept of static stability can be defined as the initial tendency of an object to diverge from or return to a state of equilibrium after having been disturbed.  An object can have positive static stability (fig. a)-its tendency is to return to its original state of equilibrium; negative static stability (fig. b)-the object does not return to a state of equilibrium; or neutral static stability (fig. c)-after a disturbance, the object remains neutral. Dynamic Stability Once an object has been disturbed from its position of stability, the behavior it exhibits in returning to equilibrium is known as dynamic stability. An object is considered dynamically stable if it eventually returns to and maintains equilibrium after a certain period of time. It is important to note than an object may be dynamically stable without being statically stable. As an airplane travels along its chosen flight path, it can be affected by disturbances such as an updraft. When that occurs, the aircraft will naturally diverge from its selected course. After a short period of time and some gradually decreasing oscillations, the aircraft will eventually return to its original course-exhibiting the characteristic of positive dynamic stability. The actual time it takes for the plane to return to that equilibrium is a measure of its degree of static stability. The Falcon 7X is designed to allow a state of relaxed static stability, enabled in large part by the fly-by-wire flight controls. The aircraft will meet all aviation rules and regulations regarding both dynamic and static stability. At the same time, the new system will deliver a safer, more efficient, better-performing airplane. By relaxing the margins of stability on the Falcon 7X, aerodynamicists were able to optimize the performance of the airplane within those margins. It also allowed them to design a more effective control system.
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