Aeroelastic behaviour of composite wings
This research work presents a series of investigations into the structural, dynamic and aeroelastic behaviour of composite wings. The study begins with a literature review where the development of aeroelastic tailoring and specific applications of the technology are discussed in detail. A critique of methods for the determination of cross-sectional rigidity properties follows for beams constructed of laminated and thin-walled materials. Chordwise stiffness is shown to be an important parameter that must be considered as it has a significant effect on the amount of bending-torsion coupling present in a beam and, as a consequence, on the value of torsional rigidity. The free vibration characteristics of such beams are then examined using the dynamic stiffness matrix method. Natural frequencies and mode shapes of various beams are studied using the fibre angle, β, and the bending-torsion coupling which is measured (in this study) by the non-dimensional parameter ψ, as design variables. The results show that ψ has only a marginal effect on the natural frequencies of composite beams (wings) but can significantly modify the mode shapes of such beams. It can be used to decouple modes which are geometrically (inertially) coupled in the same way as mass balancing but without a weight penalty. It can also be used to abate the unfavourable coupling introduced by sweep angle. Classical flutter and divergence of swept and unswept uniform cantilever wings are investigated using laminated flat beams (plates) and thin-walled beams of rectangular and biconvex cross-sections. Various parameters, such as, the fibre angle, β, the coupling parameter, ψ, the angle of sweep, Λ, the static unbalance, Xα, and the non-dimensional ratio of the fundamental (uncoupled) bending to fundamental torsional frequency, ωh/ωα, are varied and their subsequent effects on aeroelastic stability are investigated. The importance of torsional rigidity GJ on the flutter of composite wings is shown to be substantial in contrast with ψ, which is generally the most important parameter to be considered when the objective is that of increasing the divergence speed. Modal interchanges in the free vibration and flutter of laminated composite wings are shown to be primarily responsible for behaviour not experienced with metallic wings, in particular the effect of wash-in and wash-out on flutter. The most intriguing features of these investigations, however, are those which show that models adequate for the analysis of composite wings may be based on two parameters, the frequency ratio ωh/ωα and the coupling parameter ψ. Some results are confirmed by independent optimisation studies. Finally, a preliminary investigation is carried out into the flutter suppression and gust alleviation of a laminated composite wing by the use of active controls. The results show that by using an active control in an optimum trailing edge position the gust response of a wing can be significantly alleviated without compromising the already optimised flutter speed by the use of aeroelastic tailoring.