Preliminary study of an adaptive wing with shape memory alloy torsion actuators

This paper presents a preliminary design study aimed to verify the feasibility of an adaptive wing for a small unmanned aircraft (UAV), which is entirely actuated by shape memory alloy devices (SMA). The capability of the wing to bear the aerodynamic loads, the power required by the actuators and their force and torque during flight are assessed by finite element simulations. The wing consists of a sandwich box sub-structure with laminated faces, flexible ribs and a flexible skin. The adaptation capability to the changing flight conditions is obtained via airfoil shape change and local shape adjustments. Counter rotating, concentric torsion SMA tubes are employed for wing camber control, while levers powered by SMA wires are employed for local shape control. The external and the internal tubes control, respectively, the downward and the upward motions. They are connected to the flexible ribs through an electro-mechanical clutch and a positioning piezoelectric motor. Actuation occurs heating a tube at a time and making free the other by the clutch. With deformation limited to 4%, to allow a complete shape recovery by SMA, the wing appears capable to smoothly deform, with small stresses. A mean rotation of 21.7° of the flexible ribs and a rotation of 40° at the tip can be obtained, which are equivalent to a rotation of 30° of an aileron or flap of a conventional wing. A variable camber variation can be obtained at cruise speed, with a variation at least of 10° from the tip to the root. Also a local shape variation can be obtained, corresponding to a 4.5% increase of the thickness of the airfoil at 55% of the chord, while at 40% of the chord the thickness is reduced by 3.9%. The torque applied by the actuator tubes, which is up to 200 N m, and the peak power of 1223 W required for actuation appear compatible for an UAV.