Studies on Aeroelastic Optimization of Constant and Variable Stiffness Laminates with Failure and Stability Constraints

Abstract

This paper presents studies on the aeroelastic optimization of three dimensional wings subject to strength and stability constraints. The design of aircraft wings usually demands that multiple requirements are met at once, among them, limiting stress and aeroelastic instabilities. They must be avoided inside the flight envelope. Nowadays, laminated composites are being widely employed as primary material for the wing skins. These materials offer multiple parameters to achieve all design goals, which are usually lightweight and compliance to safety requirements. The proposed optimization problem considers the minimization of total elastic energy, with a stress constraint expressed in terms of a failure criterion and minimum aeroelastic instability onset above certain prescribed speed. Multiple points of a flight envelope are considered for the static loads evaluation. The fiber orientations are the design variables, and both constant and variable stiffness are considered. In the first case, a single orientation is defined for each laminate ply, and in the latter, the fiber orientation varies constinuously on the wing surface. An heuristic method, Particle Swarm Optimization (PSO), is used as the search algorithm to find the best design. The aerodynamic loads are computed with the doublet-lattice method, and the structural normal modes and static response are obtained with a finite element code. The stability is verified with a classic PK method. A wing reinforced with internal ribs and spars is studied. It is observed that the requirements can be adequately achieved, specially using variable stiffness. The flutter speed might be found inside the flight envelope and thus it need to be evaluated during the optimization process. The methodology can be readily employed in the design of subsonic wings made of composite material.