## 1 Introduction
The present work focuses on the part of the activities addressed by the Italian Aerospace Research Centre, CIRA, within the above-mentioned SABRE project. CIRA developed a blade morphing system belonging to the family of compliant architectures capable of producing an adaptive twist using compact and solid actuators based on SMA technology. The adaptive twist offers advantages in terms of [37]:  
* Reduction in the power required by the main rotor and improvement in its performance; in refs. [38; 39], a reduction in power between 5 and 15% was estimated by implementing the adaptive twist in synergy with other morphing strategies. This dramatically mitigates the environmental impact of the rotorcraft, with a net reduction of NOx emissions of about 5-8%;
* Improvement of the maneuverability at different flight regimes; this aspect, as discussed in ref. [40], directly derives from the capability of reducing the power consumption and optimizing the load distribution on the blade;
* Abatement of noise and vibrations up to 65%, as discussed in refs. [41; 42];
* Enhancement of the safety and stability of the rotorcraft in case of failure and gust. This aspect is strictly related to the better distribution of the loads on the blade, obtainable by altering the twist, and to the mitigation of vibration at the hub. Despite these noteworthy advantages, some relevant challenges have to be outlined; among the others:  
* The cross-section of the blade is generally characterized by a small area integrated with a cell-like structure covering the leading-edge zone up to the maximum thickness of the airfoil [43]; this architecture is suited to increase the stiffness but represents an obstacle for the implementation of potential morphing architectures, as a droop nose or the twist itself. Even the aft part of the airfoil is stiffened by the skin and a hard foam or honeycomb that should be accurately considered before implementing a chamber morphing. * The just mentioned narrow space dramatically limits the use of the actuators: solutions characterized by compactness and high power and force densities play, in this case, a crucial role. Also, the flexibility of the blade and the centrifugal forces represent a challenge for the actuation. The large displacements of the blade due to the inertial forces determine the relative movement between the internal parts of the actuators, favoring friction and, thus, loss of actuation or, in the worst case, jamming. SMA elements, in the case of steady applications, can be considered for this type of actuation, even if their non-linearity, fatigue behavior, and performance should be carefully considered because of their dependence on the cycles of activation. * The implementation of morphing cannot be separated by the development and integration of dedicated logic and sensor networks. It is important for structural health monitoring and for shape reconstruction purposes that the logic can determine the level of solicitation of the structure, separating the contribution of twist and bending in the flap and lag directions from that due to inertial, aerodynamic, and internal actuation. The sensor layout has to be accurately investigated to adequately support the logic and fit the best narrow space; moreover, the sensors should withstand the harsh environment represented by the large displacements and the centrifugal actions. The scope of the architecture ideated by CIRA is to improve the performance of the rotorcraft for some specific flight regimes, such as hover and vertical flight. These regimes are generally penalized by the conventional design due to their requirements, which are in conflict with more extended regimes. The structural design needs to consider requirements of a different nature: first, the safety in terms of structural integrity and thermal and electrical protections during test execution; second, the achievement of the prescribed morphed (twisted) shape under the most severe load condition; third, the exploitability of the measurements.