## 3 Working Principle of the Concept and Main Features  
The architecture is made of different cell elements serially connected along the span. Figure 1 shows, on the top, two cells connected to each other, one covered by the skin and the other naked. The main parts of a cell are:  
* The main structure: this is a solid element made of aluminum alloy; two lateral blocks reproduce the leading edge shape; one of them presents a square hole, while the other one has a lateral circular opening and four radial holes. The blocks are linked by two transversal spar elements whose shape was set to minimize the stress due to the external loads and to transmit the twist; holes were made in the aft spar; these were used to pass through the supply cable for the heating coil, and a thermocouple was used to measure the temperature of the SMA (see detail on the scheme on the left of Figure 1). * A pre-twisted SMA rod actuator is mounted in the main structure between the two spars; one edge is clamped into a side block. * A heating coil is wrapped around the SMA rod; when required, this thermo electrical component is switched on to increase the temperature of the alloy and enforce phase transformation, with the macroscopic effect of recovering angular strain. * A ring-like element made of steel alloy (see detail on the scheme on the right of Figure 1), where the element is fixed on the other edge of the SMA rod. The side face of the ring is suitably shaped to apply a torsion through a dedicated tool, while the outer cylindrical surface presents four radial holes. During the pre-twist operation, these holes are aligned to the corresponding ones on the main block and finally filled in by pins to avoid any relative rotation between the parts. * A couple of truss-like elements made of aluminum alloy reproducing the lateral ribs. These elements are connected to the main structure and transmit the twist to the external skin. * A plate element between the two ribs at the trailing edge zone, made of aluminum alloy, integrated with strain sensors. This element is clamped at one rib and can slide within the opposite one; external actions are transmitted by the lateral ribs to the plate; the twist and/or the bending of the cell are then estimated through a dedicated logic that elaborates the strain measures. FBG fiber optic sensors were selected after a trade-off among different strain sensors: resistive strain gages, piezoceramics, and fiber optics. Piezoelectrics are characterized by a wider operational frequency band and a higher sensitivity compared to the other types of sensors considered. Their fragility and, above all, the routing of the connections within the blade narrow space, however, discouraged their use. The same cabling issue limited the use of resistive strain gages, adopted for comparison in tests in the wind tunnel and in substitution of the FBGs during the whirl tower campaign in compliance with specific requirements of the interface of the facility. On the contrary, fiberoptics improved, offering the attractive possibility of accommodating more FBG onto a unique line. Epoxy resin was used to finalize the bonding on the plate's metallic surface. * An inclinometer sensor mounted on a supporting plate is used as a reference for the estimate performed through the strain sensors installed on the plate element. The use of this type of sensor was considered for verification purposes. As clarified below, the strain information provided by the fiberoptics will be elaborated by a dedicated logic of shape reconstruction to get a view of the blade's current configuration. In line with this, the inclination around the three axes will be used to tune the system for assigned types of solicitations and validate the logic of reconstruction under the combined action of different loads. * A couple of masses are made of lead: these masses, with the same shape as the leading edge, are mounted on the main structure's lateral blocks and contribute to keeping the center of gravity in the first quarter of the chord.