Shape memory alloy reconfigurable technology vortex generators (SMART-VG), part I: alloy design for environmentally actuated device

Abstract Vortex generators (VGs) are small vanes typically placed on aircraft aerodynamic surfaces (e.g. wings and vertical stabilizers) to delay flow separation and stalling and increase control surface authority (e.g. for ailerons, flaps, and rudder control). Although they are essential for certain scenarios and flight conditions, such as low-speed take-off and landing, VGs are typically not required for the entire flight profile. Despite this, VGs are traditionally static and always deployed, thus adding drag and fuel consumption over the entire flight profile. The static nature of standard VGs stems from the inability to integrate conventional actuators due to mass, complexity, or footprint constraints given the VGs’ small size and placement on outer surfaces of the aircraft. Shape memory alloys (SMAs) are capable of high energy density actuation and can enable reconfigurability of such devices with minimal added mass, volume and complexity. Additionally, SMAs can be passively used as sensors and actuators without the need for heaters, active controls, or additional instrumentation, if finely ‘tuned’ to respond to altitude temperature differentials. Recently, environmentally activated SMA reconfigurable technology vortex generators (SMART-VGs) were developed and successfully flight tested on the 2019 Boeing ecoDemonstrator airplane, a 777–200ER (extended range). This work is presented as a series of two manuscripts, covering the requirements, concept of operation, SMA development and characterization, device design, integration, and flight testing. Part I, presented here, is focused on the material development of low-temperature SMAs for environmentally activated VGs based on temperature changes between ground and cruise altitudes. For context, the background, concept of operation and some requirements will also be introduced. Starting with a binary NiTi alloy, the addition of low levels of Hf (∼2 at.%) 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 helped stabilize the alloy’s response during training in torsion, resulting in low accumulation of residual strains, while promoting very large shear strains of over 6%. The alloy formulation, microstructure, and resulting thermomechanical behavior are presented.