* Design of the SMA wire is simplified to specifications on maximum stressktrain required for  transition  Maximum power required is that required to heat onlv one SMA wire, since the tabs are  transitioned sequentially  The basic actuation concept is described referring to Figure 3. Figure 3a shows a notched flat plate  element that deflects out-of-plane to the plate when points A-B are forced together. The surface buckles  elastically into one of two stable positions, determined by the manner in which the points are drawn  together. Figure 3b illustrates the plate prior to installation of the actuation wires but installed on the trailing  edge of an airfoil. The deflection results from the pre-stress introduced in the plate, and studies to date  have shown that trailing edge deflection angles of +25” are achievable for acceptable levels of plate  stress. Figure 3c schematically shows the pre-strained SMA wires attached to the upper and lower  surfaces which, when heated, result in shortening of the wire and a “snap-through” and locking of the plate  into the “mirror” position. The heating is directly accomplished by running a current through the wire, since  SMA wires have relatively high resistivity. The innovations in this actuator are two-fold:  The structural or load-bearing component of the actuator is a very simple machine (a notched flat  plate). This component is readily fabricated and encapsulated to form a useful aerodynamic  surface, and has no bearing surfaces.  Power is only required to transition the actuator from one position to the mirror position. Once the  transition is made, the power to the actuating wire is cut-off and the actuator is “locked” into  position as a consequence of its geometric compound curvature.  Figure 3. Schematic of “SNAP” trailing edge tab:  (a) undeformed base plate (b) stressed plate on airfoil t.e. (c) tab with SMA wires  Rotor Blade Tracking Testing Activities  Testing of this device to support the rotor blade tracking application included benchtop functional tests,  high dynamic pressure tests, high centrifugal field tests, and actual on-rotor testing. These data were used  both to support analytical model development for design, and to validate configuration choices for  construction and wire attachment.  Tab Actuator Benchtop Testing  While an analytical model for tab design was under development, a series of tests were performed on an  oversize representation of the tab configuration of Figure 3. These tests were undertaken with the goal of  generating a database for analytical model correlation, as well as validating a scaling analysis for  performing ”nomogram”-like design calculations for similar tab geometries. Aluminum sheets of  approximately 4 cm by 8 cm were constructed with a center stress-relief hole, and an offset stud was  mounted near the center of the tab, from which a wire cable was mounted in series with a strain gauge  load cell. Tests were performed to measure the tab deflection angle with applied cable load, much like an  SMA wire would generate on the actuator tab, and notes were made of the displacement and force levels  just prior to tab snap-through. These data, taken over a range of tab length to width ratios, thicknesses,  offset stud locations, and initial wire pre-strains were ultimately used to validate both in-house CDI finite  element (FE) code predictions and commercial FE simulations.  Whereas the super-scale tab testing only included one nominal wire orientation for applying pop-through  forces on the base tab material, several different arrangements were fabricated and evaluated at design  scale on the benchtop. Two of these configurations, termed pre-prototype configurations, are shown in  Figures 2 above and 4 below. Figure 2 oriented the SMA wires in a nominal spanwise orientation, while  Figure 4 shows the SMA actuator wires aligned with the blade chord direction. Both configurations could  be made to operate successfully, but it was found through assembly of several units that wire pre-  tensioning was easiest to perform for the chordwise orientation. Thus, the ultimate configuration, th&t of a  375