On the Nonlinear Geometric Behaviour of Flared Folding Wingtips

Recent studies have considered the use of flared folding wingtips (FFWTs) to enable higher aspect ratios - reducing overall induced drag - whilst reducing gust loading and meeting airport operational requirements. The majority of these analyses have been conducted using linear assumptions despite the presence of large wingtip deformations. The aim of this work is to assess the effect of geometric nonlinearities introduced by an FFWT on the static and dynamic aeroelastic response of a wing. In this paper, a geometrically exact expression was formulated to describe the change in both the local Angle of Attack (AoA) and sideslip angle across all fold angles. This expression highlighted that the aerodynamic stiffness of an FFWT, and therefore quantities such as the linear flutter speed, are a function of the fold angle and therefore, the attitude of the wing. This effect was then verified using both: a wind tunnel model of a flexible semi-span wing incorporating an Flared FoldingWingtip (FFWT), and a newnumerical modelling technique, utilising MSC Nastran, which linearised the model about the equilibrium position of the wingtip. The results of these experiments show that the geometric nonlinearities introduced due to the large deformations of FFWTs can significantly affect the dynamics of the system, with flutter speeds varying by over 25%, simply by changing the root angle of attack of the model. Furthermore, good agreement was found between the experimental results and numerical predictions.