Conventional electric servo actuators incorporate electric motors that drive a gear train that ultimately is  connected to a control arm, on which is also mounted a potentiometer for sensing actuator displacement.  Thus, the positioning control and the actuator impedance are provided only while the electric motor draws  power from its energy source. Use of the SMA-based actuator described for the rotor blade tracking  application would be an ideal alternative, in that it would permit spanwise distributed load modification with  minimal electrical power expended during the reconfiguration process. NASA is presently sponsoring CDI  in an SBlR effort directed at investigating the use of these snap-through SMA-based actuators in support  of HALE-type UAV aircraft control. Results to date in this investigation are described below.  UAV Application Desican Challenqes  Since the detailed design specifications for a future NASA HALE RONUAV are yet to be completed, it was  felt prudent to consider HALE vehicle operating features in the actuator design, but size the actual device  to be more immediately applicable to potential test platforms, such as CDl’s 2m span electric radio control  (FUC) model aircraft, and the two NASA Dryden’s APV-3 test aircraft. Fortunately, these two platforms are  roughly the same size, and thus it was decided to base the actuation sizing on a compromise that would  accommodate each aircraft type. Fundamental to each actuation concept is its application in a distributed  fashion, so that several (possibly many) of them would be arrayed along the trailing edge of the UAVs  lifting surfaces. Sizing of each individual actuator was selected to provide:  Desired flight control forces/moments, dictating a minimum ratio of actuator to surface chord  length;  Appropriate speed of actuation, thus impacting maximum wire diameter used;  Sufficient stiffness to support operational aerodynamic loads, fixing a minimum thickness for the  base material. 0  0  Key factors in the design considerations applied included operational environment, endurance,  robustness, actuator weight and power consumption. HALE RONUAV systems operate at altitudes in  which the ambient air temperature is approximately -48”C, and thus the actuator may require insulation on  its wires to mitigate the power lost through ambient cooling to the atmosphere. Endurance requirements  for these actuators would flow from possible HALE missions lasting from several days to several weeks.  While use of multiple, distributed actuators provides some robustness relief in that single actuator failures  represent graceful degradation of control capability (vs. catastrophic), the actuators nonetheless should be  capable of accommodating multiple flights/missions without replacement. Standard practice for SMA wire-  based actuators has shown that as long as the wire strains are kept below 3% in actuation cycles, millions  of repetitions are possible. For the actuation systems here, actual wire stress changes in the discrete  actuation system varied from 0.5% to 1.0% depending upon end termination of the wire that stretches  across the base tab material.  Robustness design goals imply that all material stresses are kept well below limiting values, and the  opportunity for failure be mitigated as much as possible. The actual stresses on the base plate assemblies  are well within linear bounds below yield, and failure modes were reduced in the actuation elements by  reducing the number of parts required in assembly. Toward this end, use was made of custom-fabricated  SMA wires having metal balls welded onto their ends. By using a welded ball with a diameter twice that of  the wire, the wire could be “dropped into place” on the base tab assembly through insertion into a slot  created in the base manufacturing process. This approach provided a convenient mechanical means for  coupling the SMA wire to the base material (Le., the actuator armature), but continued experimentation  with this approach revealed that the stress imposed at the junction of the weld could be past yield if the  wire were tensioned at an angle nearly parallel to the surface of the base tab. Put another way, the ball  end fitting was not adequate if the wire had significant non-axial stresses at the weld point. This  observation resulted in a design having an offset mount for the actuating SMA wires.  Weight considerations on the actuation schemes, both continuous and discrete, were actually a secondary  issue, as it was a fall-out effect from material choice based upon actuator performance requirements.  Basic stiffness requirements dictate a minimum actuator thickness, and the fact that the buckling-  enhanced actuation concept routinely induces stresses on the base material showed in previous actuator  implementations that plastic base structures are inadequate due to their creep properties. Thus, the  design of these actuation systems has routinely used tempered steel (spring sheet stock) for the base  plate material. Despite the use of steel components, the net weight of these actuators, when compared  against conventional servo-system components, is almost insignificant.  38 1