Load and Control Effects on Crack Growth in Flexible Aircraft

Abstract

The fundamental leverage generated by load tailoring and flight control system feedback loops for reducing fatigue damage and enhancing long-term integrity of aircraft structural components is investigated. Nonlinear fatigue behavior indicates two significant factors that influence crack growth are overload magnitude ratio and application period. This behavior implies existence of nonintuitive optimal overload strength and frequency values that minimize crack growth. Such results have important implications for design of new control logic that exploits this nonlinear phenomenon to suppress crack growth and enhance structural life. A state-space crack growth model is briefly reviewed and linked to a flexible aircraft model. Long-term structural integrity predictions, and the influence feedback control and loading variations have on such predictions, are made with a set of simulations 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 gains 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, this result also implies significant structural life enhancement may be possible.