Retrospective essay on nonlinearities in aircraft flight control

Preface and Introduction F years ago the structures of a linearized theory of servomechanisms and a linearized theory of aircraft dynamics were substantially complete. Combination and extension of the two subjects, accumulation of favorable experimental evidence, an increased general understanding, and the continuing challenge of complex and stringent requirements have made aircraft flight control a useful predictive science and a recognized engineering specialty. This much is no news to readers of the Journal of Guidance, Control, and Dynamics. Before we begin to congratulate ourselves, however, the editors have thought it appropriate to reflect on the perennial question from the floor: What about nonliriearities? The question is often facile and impertinent. A considered response is, of necessity, complicated, extensive but not comprehensive, and invariably controversial. No general response is possible^ Nevertheless, we may hope that reconsideration of specific topics will elucidate the matter and we have been persuaded to make the attempt. Our approach is historical. No new research results are presented, but our interpretation may disturb the conventional wisdom. The earliest, more less successful, automatic flight control systems were highly nonlinear in their sensing and actuating elements (see Fig. 1, taken from Ref. 1). Analysis, however, was at that time disdained, and performance was achieved by cut and try methods. We have told elsewhere the story of the synthesis of the art of the tinker er-in vent or and the science of the theoretician in connection with aircraft flight control. ' The confluence of techniques was well developed by about 1952. ' This included the experimental and theoretical study of significant nonlinearities. But the litera-