Flutter analysis of a large civil aircraft in case of free-plays and internal failures of morphing wing flaps mechanical systems

The adoption of mechanical systems represents a very promising solution to realistically enable wing-camber morphing for large civil aircraft. These systems implement the change of shape through the relative motion of parts usually interconnected by means of hinges, and therefore, without any morphing-induced elastic deformation of the load carrying structure; conversely to what happens in compliant structures, the energy provided by the actuation system is here spent to counteract only the external aerodynamic loads which are in turn dependant on shape, speed and flight altitude. Apart of the more effective use of the available power, the mechanical systems show a higher level of technological maturity and readiness for flight thanks to their higher robustness, reliability and maintainability, as well as in force of their similarity with conventional airworthy architectures already in flight. On the other hand, the use of multiple-hinges connections imposes a careful analysis of the effects induced by any degradation of their mechanical performance leading to overall system malfunction or local failures. In the framework of the CleanSky2, a research program in aeronautics among the largest ever founded by the European Union, the authors focused on the design and validation of a camber-morphing flap specifically tailored for EASA CS-25 category aircraft. The shape transition is obtained through a smart architecture based on segmented (finger-like) ribs with embedded electromechanical actuators. Three large tabs were located at the flap trailing edge to actively control the shape of the wing in cruise and to optimize the aerodynamic load distribution along the span. Aeroelastic phenomena related to these flap components were duly addressed since the very preliminary design stage in order to avoid the maturation of a potentially unstable architecture; rational approaches compliant with applicable airworthiness requirements were implemented to properly model and investigate the aeroelastic behaviour of the flap tabs in nominal working conditions. Finally, free plays and internal failures were accurately simulated and their effects on the aeroelastic stability of the aircraft were duly investigated in order to assess the robustness of the conceived tabs as well as of the embedded mechanical subsystems driving their motion.