Spanwise Adaptive Wing: Coupled Shape Memory Alloy Tubes for High Torque Applications

Boeing and NASA have been developing lightweight and compact SMA actuators to support a growing need for improved aircraft efficiency and reduced emissions. We have successfully demonstrated SMA actuators with up to 500 in-lb (56 Nm) of torque although much higher loads will be required for many future applications. Simply increasing the size of the SMA components may not be a viable solution for larger torques. The cost of manufacturing and increased heating and cooling rates may make large tubes impractical. Boeing has recently demonstrated an SMA actuator design that combines multiple smaller tubes into a single assembly. Initially four ½ in diameter SMA tubes were coupled together using planetary gears around a single output shaft. The actuator generated an output of 5000 in-lbs (565 Nm) with a 60° deflection range. Negligible losses were measured through the gearing system. The device was able to meet or exceed the requirements for position control and actuation rate normally applied to smaller SMA actuators. Following the success of the initial coupled tube actuator a twelve tube 5,000 in-lb (565 Nm) actuator and an eight tube 20,000 in-lb (2259 Nm) actuator were tested as part of NASA’s Spanwise Adaptive Wing (SAW) program. The SAW multi-tube actuators used high temperature NiTiHf20 tubes. The SAW actuators were directly integrated into the wing fold structure of an F-18 wing section at NASA Glenn. These actuators demonstrate the feasibility of SMA actuation as a drop-in replacement for existing high torque aerospace applications.