## 9 Critical Analysis of the Layout Adopted for the Demonstrators  
Even if the same cell architecture was adopted to build the two demonstrators, the operational scenario they were conceived for and the different constraint conditions determine different performances, as discussed in the previous section. That said, it is important to highlight the advantages and drawbacks presented by the architecture for these different test environments.  
A first consideration concerns the way the prototypes absorb the twist generated by the SMAs. The higher twist obtained for the WhT model, not constrained at the tip, gives a perception of the role of the relative movement among the rib planes. A coupling among twist and flap and lagging dofs can be assumed. In fact, the layout, characterized by two spars connecting the ribs on the forward part, favors the relative movement of the aft parts of the airfoil, in practice enabling load absorption and redistribution along the span direction.  
The just mentioned aspect, representing an advantage for the WhT configuration and, thus, for the real working modality of the blade, has to be accurately considered for its impact on the strain measures. The sensing system proposed in this work is based on a plate element clamped at one rib and sliding within a rectangular-shaped hole in the opposite rib. This specific constraint condition allows the transmission to the plate of the relative motion of the ribs; however, the distance of the plate from the torsional actuator, from one side, enhances the strain measured and, from the other side, stiffens the structure and, thus, hinders the twist actuation. To mitigate this last drawback, a weaker constraint was considered for one edge of the plate, allowing its sliding. However, when the relative movement of the ribs along the span is allowed, jamming and/or gaps can occur in the sliding guide, altering the measure. This problem is further emphasized when centrifugal forces are present. In this sense, the proposed sensor layout is more suited for the WT model.  
Another important aspect that differentiates the two demonstrators is represented by the interface with the supply and the measurement equipment. The unavoidable presence of sliding connections for the WhT model leads to issues that must be accurately considered, even in view of a real application. As already discussed, noise can be generated by the  
Figure 27: WT and WhT model performance comparison.  
floating nature of the connection. This has an immediate impact on the quality of the measure and may require off-line filtering operations. However, the most relevant impact is in terms of safety. The system is equipped with a logic of feedback on the temperature, and any error on the signal may alter or make, in the worst case, ineffective control. The most severe consequence is the trespassing of the allowable thermal limits of the skin material. Current and temperature fuses must be inserted in the supply circuit to prevent this issue.  
Given temperature problems, the different behavior of the two models is worth noting. Due to the narrow available room, no refrigerating systems were considered; cooling down is obtainable only through the control logic and the intrinsic capacity of the system to disperse heat. As shown by the thermal analyses, the higher speed makes the WT model once again safer.