A Bendable Load Stiffened Wing for Small UAVs

Abstract

A bendable load stiffened wing, developed at the University of Florida, has the ability to load stiffen in the positive flight load direction while remaining compliant in the opposite direction, enabling UAV storage inside smaller packing volumes. The wing employs an under-cambered airfoil with a swept planform providing dissimilar stiffness in the flight load and the folding direction. A comparative experimental study is performed using two wing geometries; straight camber and swept camber. The load stiffening ability is tested by performing three point bend tests while monitoring the wing root airfoil shape change using a visual image correlation technique. For the wing utilizing a swept camber design, increase in the root airfoil camber with increased loading resulted in a load stiffening structure. Swept camber wing showed a higher load carrying capacity (7 g's load factor) over a straight camber wing design (2 g's load factor), still maintaining the compliant nature in the folding direction. Long term storage induced creep deformations are small in both of the wing geometries. By increasing the wing stiffness, sweepback helps in reducing spanwise residual creep strain. Wind tunnel tests at Re = 7x10 4 of both the straight camber and the swept camber wing show similar L/D ratios. The sweepback helps in improving the static stability of the wing. Thus the bendable load stiffened wing has a clear advantage of offering stiffness improvement and reducing storage induced creep residual strains while maintaining the aerodynamic efficiency and improving the static stability of the wing. NOMENCLATURE a CLmaxangle of attack at maximum lift coefficient