Shape memory alloy induced wing warping for a small unmanned aerial vehicle

Abstract

The Wide Area Surveillance Projectile (WASP) is a gun-launched unmanned aerial vehicle designed to be carried as payload in an artillery shell. Due to the 15000 g shock sustained during gun launch, conventional ailerons are too fragile to be a reliable means of roll control for the aircraft. For this reason, the possibility of using shape memory alloys (SMA) to control the vehicle is investigated. A conceptual design is introduced in which pre-strained Nitinol wires are attached to the surface of the wing. When the resistively heated wires pass a transition temperature, a phase change occurs in the wires and they contract to recover the pre-strain. As the wires contract, they twist the wing in what is known as wing warping. This conceptual design is refined through extensive modeling and finite element analysis. Thermal analysis is used to determine how fast the wires heat and cool, which determines how fast the vehicle can be controlled. Structural analysis is used to determine the amount of twist induced in the wing when the wires contract. A preliminary performance analysis illustrates what bank angles and roll rates the WASP could achieve when the actuator is used. Tensile testing of the Nitinol wire is conducted to determine its modulus of elasticity in both its martensite and austenite phases. In addition, cycle tests are performed in which the wire is heated and cooled at constant stress to determine the transition temperatures of the material. Tests are conducted on prototype wings with Nitinol wires attached to determine the actual performance of the actuator. Using epoxy to attach the Nitinol to the wing is found to be problematic, since the epoxy degrades as the wires are heated. Using mechanical means to attach the wires is shown to be much more effective. This thesis shows that an SMA actuator can repeatedly twist the wing of a small UAV to angles in excess of one degree. Analytical results show that the wing can be actuated every 3.2 seconds. Performance analysis predicts that roll rates of over 25 degrees/second can be achieved. These results indicate that an SMA actuator has the ability to control the aircraft during slow, banking turns while the aircraft follows a racetrack pattern.