Deliberately unstable aircraft

Flying an aircraft in a neutrally or slightly unstable condition is not necessarily difficult or dangerous, but it involves hard work for the pilot, who cannot take his hands off the controls, and must make continuous control adjustments. The Wright brothers’ original aircraft was unstable, which made it more responsive and controllable than many of its contemporary rivals. There are, however, other more important advantages in moving the centre of gravity aft. By mov­ing it to the neutral point, the position where the aircraft is neutrally stable, the tail has to produce no trimming force, and hence there is no trim drag. By moving it even further aft to an unstable position (negative CG margin) we can arrive at a position where the wing and the tail are both producing lift at an efficient positive angle of attack. This considerably improves the lift-to-drag ratio of the aircraft, and can dramatically improve its performance. An un­stable aircraft will also respond more quickly to control inputs, making it highly manoeuvrable.

Aviation safety regulations traditionally took a dim view of flying in an unstable condition, but for military applications, the performance advantages are considerable. With the development of increasingly reliable electronic control systems it became practical to build aircraft that could be flown in a naturally unstable condition, relying entirely on automatic systems to maintain artificial stability. Most high performance military aircraft are in any case

totally unflyable in the event of a major electrical failure, so further depend­ence on electrical systems does not significantly reduce their safety. The X-29 (Fig. 9.20), and Typhoon (Fig. 10.8) are both designed to be inherently un­stable at subsonic speeds. For civil aircraft, some reduction in stability may be tolerated, if the overall system can be shown to be capable of coping safely with failures in individual elements. This normally entails duplicate or multiple components, and rapid automatic fault diagnosis.