Two-stage gust load alleviating torsional stiffness device for hinged aircraft wingtips

This article introduces a novel mechanism exhibiting a two-stage, non-linear softening torsional stiffness designed to improve the aerodynamic performance of the Semi-Aeroelastic Hinged wingtip concept, while retaining its gust load alleviation capability. The device is comprised of simple mechanical components including gears, tendons, and springs, and is fully passive. Its kinematics are derived to analytically model its behaviour, establishing the two-stage stiffness behaviour: an initial stiffer region, followed by a secondary, more compliant region at higher deflection angles. The higher initial stiffness helps balance the steady aerodynamic loads on the wingtip, allowing for a more horizontal wingtip position during cruise, improving aerodynamic performance. During gusts, the lower stiffness mode allows for large wing tip rotations and alleviation of gust loads. A device is sized and designed for a notional UAV. The analytical model is validated against a prototype mechanism. Further studies explore the impact of the various design variables on performance. The model is shown to accurately predict the experimental response which is also shown to be tailorable for a chosen design point. In this way the basic mechanical viability of the concept is shown in preparation for future studies into its aerodynamic and loads transmission performance.