A shape memory alloy torsion actuator for static blade twist

Abstract

Active blade twist is an option to increase helicopter performance, for instance moving its condition from hovering to cruise. Shape memory alloys give the possibility of realizing compact devices, with high energy density. Several devices have been proposed in literature, showing limitations in terms of effectiveness and necessary room. In this article, the capability of a shape memory alloy torque tube to induce a certain twist law along the blade, while preserving its integrability within the structure, has been exploited. The study refers to a complex theoretical model, made of different specialized modules. In detail, transmitted twist action by the shape memory alloy actuators, aerodynamic effects caused by the induced geometrical change, inertial impact following the motor system integration, and system layout influence on the blade response have been taken into account. Through this model, a parametric investigation has been organized to highlight the importance of selected design variables. Tube thickness, mass, and length have been considered. Two different configurations have been initially taken into account, distinguished for the twist transmission mode and their outline. In the first hypothesis, a pre-stressed wire system converts tensile stress into a rotary action. In the second sketch, a pre-twisted solid tube connects two different stations of the blade, transmitting relative rotation. After the first trade-off, the second architecture has been selected for further analysis, focusing on its performance in terms of net transmitted twist, aerodynamic effects, while paying attention to a proper mass balance. In the chosen approach, the actuator has been installed at the torsion center. A finite element model has been used to validate the assessed analytical representation and has permitted establishing the applicability domain. Apart elastic forces, acting both in the shape memory alloys and the blade components, centrifugal forces have been taken into account by considering an increased stiffness of the reference structural element. Aerodynamic forces have been evaluated after the target configuration has been reached; helicopter trim has been considered to this purpose. The researchers aim at developing this concept by integrating the reverse action of the aerodynamic field and evaluating the importance of the actuator position along the chord. The research herein presented has been carried out within the SABRE project, project ID 723491, gratefully funded by the European Union within the Horizon 2020 program.