## 2 Specifications  
The architecture presented in the next part of the work was conceived to produce an additional pitch-ward twist on a blade segment in a steady state condition, independently from the azimuthal position of the blade. The rotor of the Bo105 was assumed as a reference. The required additional twist must follow a linear trend along the span, achieving a maximum pitch-ward of 8 deg at the tip on a span of 5 m and considering a useful distance between 20 and 90% of the radius, which means an additional twist gradient of 2.3 deg/m. This twist meets the necessity of getting an induced drag as uniform as possible along the span to increase the figure of merit [44]. This approach is effective for hover conditions, and conventional blades are manufactured with an original twist, tending to reduce the lift at the tip because the flow pattern makes more uniform the induced drag. This has a relevant impact on the figure of merit, which may be increased by 5%, with a net rise of up to 20% of the payload [44]. However, conventional twist laws are limited by the undesired impact that they could have in forward flight. Here, the back-and-forth excursion of the center of lift of the disc rotor against the azimuthal rotation of the blade makes difficult the achievement of a trimmed condition [44]. Investigations on the specific architecture proved a reduced needed power of about 10% in hover and some benefit, 2% on average, up to a forward speed of 30 m/s [45]. Other important aspects drove the assessment of the final layout: first, the narrow room within the blade to host an actuation system; second, the flexibility of the structure that undergoes large displacements; third, the magnitude of the centrifugal actions. The narrow space led to the consideration of compact actuation solutions characterized by high energy densities. Friction among contiguous parts, gaps, and free-play angles, which is problematic in terms of loss of effectiveness, made the designers converge towards solid, monolithic solutions. SMA materials seemed a good candidate for the application, also offering adequate stiffness and, thus, being able to absorb loads together with the pre-existing blade structure. However, the use of an SMA actuator, even for a prototype, poses some specific issues. Achievable performance, differently from conventional actuation, depends on the level of prestress. In practice, a certain pre-load (in the specific case, pre-twist) must be provided to enforce the transformation of the austenite phase into martensite. In fact, the more martensite is initially present, and the more austenite can be potentially produced by heating the material, with the macroscopic effect of larger displacement recovery. The drawback is that the structure hosting the SMA will have to bear conventional loads (aerodynamics, inertial) plus the ones due to the pre-twist of the SMA actuator. Another critical aspect is the thermal management. The temperature of activation of the SMA must be, in fact, adequately higher than the maximum level foreseen by the flight envelope of the rotorcraft to avoid undesired activation. This poses a number of problems: the protection of the structural and electronic components from the heat source, the control of the temperature level involving dedicated sensing and logic, and the effectiveness of heating, which must minimize losses and guarantee a uniform temperature within the SMA material. Finally, last but not least, the de-activation performance is a cooling time that is adequately short to fit operational requirements. In addition to the requirements discussed above, there are others strictly related to the demonstration in the wind tunnel and whirl tower plants. First, the structural integrity imposes a safety factor greater than 2 on all the parts of the main structure. This limitation was not applied to the SMA being pre-loaded for activation purposes; however, to mitigate any risk, the SMA was hosted within the main structure, avoiding any fragment projection.