Experimental Effect of Sideslip Angle on the Dynamic Behaviour of Flared Folding Wingtips

A concept of growing interest in recent years is the Flared Folding Wingtip (FFWT), which can be used in-flight to reduce airframe loading due to gust encounters and augment the handling qualities of an aircraft. The performance of an FFWT is affected by the relative angle between the built-in hinge angle and the flow direction. Therefore, a critical concern is the behaviour of such a device at non-zero sideslip angles, such as experienced by aircraft in crosswind landings. In this paper, a specially designed wind tunnel model capable of large wingtip rotations, and a geometrically nonlinear numerical model, are utilised to explore how sideslip angle affects both the static and dynamic behaviour of such a system. It is shown that stable equilibrium positions exist up to and beyond a fold angle of 90 degrees, even when the effective flare angle is zero or switches sign. Additionally, to accurately capture the variation in the frequency of the wingtip with sideslip angle, it is shown factors such as the change in sweep angle of the wing must be accounted for. Furthermore, it is shown that these changes in frequency with sideslip angle can lead to a reduction in the flutter speed, but do not have a significant impact on the gust load alleviation of wings incorporating FFWTs, when exposed to one-minus-cosine gust encounters.