“Digital” SMA-Based Trailing Edge Tab Actuators for Aerospace Applications  Robert M. McKillip, Jr.*  Abstract  A novel approach for providing aerodynamic control using collections of discrete active tab devices was  completed recently under an Army-sponsored Phase II SBlR program, and is being developed further as  part of a NASA-sponsored effort on UAV control. Each tab incorporates a patented bistable design that  uses SMA wires for transitioning between one of two deflection positions, thereby providing a localized  perturbation to the sectional aerodynamics at the location of the lifting surface trailing edge. By arranging  several such tabs spanwise along a wing or rotor blade, incremental adjustments may be made using  these tabs to perform aerodynamic reconfiguration, tracking adjustments on rotor blades, or primary flight  control. The device has been prototyped as part of these SBlR research programs, and has been  designed so that it may be retrofit on existing rotor blade systems and UAV platforms to support further  application development in a variety of aeronautical applications. The underlying concept of using SMA-  based actuation for configuration control is directly extendable to additional aerospace systems, including  spacecraft applications.  Rotor Blade Tracking Application  Development of the underlying concept for SMA-based actuation of a bistable device originally grew from  the desire to provide in-flight adjustment of rotor blades to minimize one-per-revolution vibration. Blade  one-per-revolution vibration is often a result of blade aerodynamic and mass mismatch between blades of  a given rotorcraft’s “blade set”. This mismatch may be a consequence of assembly tolerance or from  unequal wear on in-service blades, and often can be alleviated through placement of balance weights and  aerodynamic perturbations via adjustments of both swashplate links and trailing edge tab deflection  angles. Aerodynamic adjustments that minimize vibration also typically minimize blade “tracking”  deviations, and thus the blade’s track is used to optimize the adjustment process. Traditional blade  tracking adjustments are performed by maintenance personnel using specialized hardware, coupled with a  series of flight test measurements, to attempt to optimize blade aerodynamics across the helicopter flight  envelope. This process is often expensive, as it tends to be iterative in nature. In fact, some current  operational military helicopters must devote nearly 10% of their flight hours to track and balance activities.  Clearly, in-flight tracking capability would significantly reduce this cost burden, as all tab tracking changes  could be made in one flight without the requirement for periodic landings to stop the rotor and perform  manual adjustments. In addition, since tracking adjustments would be easier, they could be performed  more often, on an as-needed basis, thereby providing “trickle-down” benefits of reduced vibration  exposure and hence longer service lives for dynamic components. This motivation spawned the  development of the SMA-based actuation system described here, in that the tab deflection adjustments  would be performed using SMA wires as the prime mover for an electrically-actuated tracking tab device.  “Diaital” Tab lmolementation  The system initially developed under U.S. Army Phase I SBlR funding initially replaced the maintenance  specialist’s adjustment tools with a pair of agonist-antagonist SMA wires, so-that the imposed moments  generated from the SMA wire’s phase change would provide plastic deformation of a metal trailing edge  tracking tab mounted on the rotor blade (Figure 1). While successful at the conceptual demonstration  stage, this design required a means of sensing the blade tab angle in order to properly control the SMA  wire material properties to position the tab deflection to a desired angular orientation. In addition, the  hysteresis inherent in the SMA wire’s thermal properties made closed-loop control challenging for this  application implementation. Under Phase II, this concept was replaced with a “digital” tab device, whereby  each discrete tab uses SMA wires to move between one of two positions, thereby significantly reducing  * Continuum Dynamics, Inc., Ewing, NJ  Proceedings of the 38‘” Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  373