## 3 Working Principle of the Concept and Main Features
To correlate the strain measures with the plate deflection, an inclinometer was foreseen on each cell to detect the relative rotation around the chord and twist axes. Different types of inclinometers were considered, having as a constraint the range of measure, the accuracy, and the overall dimensions. The IS40 two-axial inclinometer was selected for this purpose, fitting the requirements in terms of the range of measure and accuracy in a compact volume of 60 \(\times\) 30 \(\times\) 20 mm [51]. Another important feature of this sensor is represented by the high shock resistance of 30 g, making it particularly suited to the harsh WhT environment. Although the sensor network design is out of this work's scope, the strain caused during the operational tests was estimated in the next section to develop an idea of the expected sensitivity. After having considered the interior parts, the skin was investigated. It has to meet different requirements. First, it must withstand the most severe load conditions, determined by the combination of the SMA transmitted twist and the different forces occurring during WT (aerodynamic) and WhT tests (aerodynamic + centrifugal). Second, it has to maintain the prescribed aerodynamic shape to guarantee acceptable performance of the system. Third, it must be flexible enough to permit the morphing twist. Different types of materials were considered during the preliminary design: aluminum alloy, glass and carbon fiber laminates, and plastic materials. Metallic and especially composite skins represent an optimal solution for conventional applications, assuring a flexibility level adequate for the load application scenario but not enough for a morphing application that would be thus penalized. Moreover, reduction of the thickness of the sheet to mitigate these losses would have been prone to instability events. Thus, attention was paid to a family of plastic materials with relatively high mechanical properties. It is worth noting that also the manufacturing process drove the choice of the material. In fact, to mitigate any surface discontinuity, a glove-like skin solution was taken into account. Additive manufacturing was exploited to this purpose. Two materials were considered: the ULTEM(tm) 9085 Resin [52] and the ALFANYLON CF [53]. Due to the higher heat temperature deflection, the former of these materials was finally selected for the realization of a 1.8 mm thick skin. The logic of control is aimed at guaranteeing the achievement of the target temperature and maintaining it within an acceptable range of +/\(-\)5 \({}^{\circ}\)C. This choice and relevant tolerance derive, first, from safety considerations. The heating coil, in fact, is included in the cell between the two spars shown in the scheme on the left of Figure 1 and between the top and bottom parts of the skin, the most critical element from the thermal point of view. The maximum allowable temperature of 180 \({}^{\circ}\)C and the imposed tolerance are well compliant with the heat temperature of deflection of the ULTEM material (215 \({}^{\circ}\)C). However, to take into account, even any fault event of the logic, current, and temperature fuses were included in the supply circuit. Another important aspect concurring in the definition of allowed tolerance is strictly related to the impact that this deviation could have on the transmitted twist. As will be shown in the next section, this impact is negligible. The information provided by the sensors is supposed to be collected and elaborated using dedicated equipment. In line with the main scope of the WT tests, that is to say, to estimate the impact of the twist on the aerodynamic performance, the onboard sensors have to provide information useful for the reconstruction of the current shape, which in turn is related to the aerodynamic measures. In this sense, a dedicated optical interrogator and a multi-purpose acquisition system will be necessary.