Shape Memory Alloy Actuated Vortex Generators: Alloy Design

Aerodynamic devices, such as vortex generators, are often used to improve aerodynamic performance of aircraft control surfaces. Often the non-moving surfaces are designed for specific flight conditions and decrease performance, such as increasing drag, at other conditions. One example is vortex generators (VGs), small vanes located throughout the aircraft surfaces. VGs are typically not required for the entire flight profile, but are essential for conditions such as low speeds during take-off and landing. The static nature of standard VGs stems from the inability to adapt conventional actuators due to mass or complexity, given their small size and placement. SMAs present an opportunity to enable actuation of such devices with a minimal mass and dimension, while still providing high energy densities. Additionally, SMAs can be passively used as sensors if carefully “tuned” to respond to the altitude-temperature differential and passively actuate without the need for heaters or active controls. In this work, we report on the development of low temperature SMAs for passively actuating VGs based on temperature changes from ground to cruise altitudes. Starting with a binary NiTi, the addition of low levels of Hf and Zr (~2at.%) were critical in tuning the transformation temperatures to match a typical commercial flight profile with standard day temperatures, bound between a martensite and austenite finish of -50 and 0°C, respectively. Additionally, Hf/Zr helped stabilize the alloy’s response during training in torsion, resulting in low residual strains, while promoting transformation strains of >6%. The alloy formulation, microstructure and thermomechanical behavior are presented.