Material grading for improved aeroelastic stability in composite wings

Abstract

A novel analytical formulation has been developed for the aeroelastic design of a class of solid nonuniform composite wings with improved aeroelastic torsional stability. Rectangular, unswept slender configurations made of unidirectional fibrous composites are considered, where the mechanical and physical properties can vary in the spanwise direction. Such a structural configuration yields to grading of the material properties along the wing span. The enhancement of the wing torsional stability can be attained, among others, by increasing the critical flight speed at which flutter or divergence instabilities occur. In this study, the latter problem is addressed, where the wing divergence speed is maximized while maintaining the total structural mass at a value equal to that of a known baseline design. Both continuous and discrete structural models have been examined, using classical elasticity and aerodynamic strip theories. The functional behavior of the divergence speed is comprehensively investigated by varying the volume fraction of the constituent materials in preassigned distributions. Exact solutions were obtained for different categories of unidirectionally reinforced composite wing structures: the linear volume fraction (L-VF), the parabolic volume fraction (PR-VF) and the piecewise volume fraction (PW-VF) wing models. Our results reveal that in general, the torsional stability of the wing can be substantially improved by using nonuniform, functionally graded composites instead of the traditional ones having uniform volume fractions of the constituent materials. Several solutions are given for determining the optimal in-plane fiber distributions, which maximize the divergence speed of a wing made of carbon/epoxy composites without violating the performance requirements imposed on the total structural weight of the aircraft.