Flight control leverage on crack growth in a flexible aircraft. I

Abstract

The overall objective of this research is to investigate and assess the leverage generated by flight control system feedback loops for reducing fatigue damage and enhancing long-term integrity of aircraft structural components. In Part 1 of this paper, an isolated crack growth model was extensively investigated, and a combined flexible aircraft and crack growth model was developed. Here in Part 2 of this paper, design of an inner loop flight control system is considered. Objectives of the controller are to correct deficiencies in the vehicle motion behavior, not to directly augment the stress behavior. In spite of these objectives, comparison of short-term open-loop and closed-loop airframe stress responses indicate the control system has substantial affect on transient stress behavior. Long-term fatigue damage and structural integrity predictions, and the influence feedback control and loading variations have on such predictions, are made with a set of simulation runs covering the operational life of the vehicle. A mix of open-loop and closed-loop cases with nominal and variable feedback gains, vehicle motions including nominal maneuvers with superimposed gusts, deterministic and stochastic overloads, and mean stress levels are considered. For an existing control architecture, individual feedback gain numerical values have substantial influence on crack growth. This result implies significant extension of structural life may be possible by control gain adjustment within an existing architecture. Maneuver overload strength and frequency also have significant influence. By tailoring the overload maneuver with additional control logic not present in the existing architecture, this result also implies significant structural life enhancement may be possible.