Preliminary assessment of morphing winglet and flap tabs influence on the aeroelastic stability of next-generation regional aircraft

Future aircraft wing technology is rapidly moving toward flexible and morphing wing concepts capable to enhance aircraft wing performance in off-design conditions and to reduce operative maneuver and gust loads. However, dealing with reduced stiffness, increased mass, and increased degree of freedom (DOF), this kind of mechanical systems typically require dedicated aeroelastic assessments since the early design phases; this to mitigate the impact at aircraft level of any unconventional arrangement adopted for the conceptual design of the morphing mechanisms especially when they are installed in sensible regions such as the winglets and the wing trailing edge. Speaking about the latter, preliminary investigations have shown that the combined use with adaptive flap tabs allows a global improvement of aerodynamic performance of regional aircraft wings in climb and cruise conditions by the order of 3%. Additionally, by adapting span-wise lift distributions to reduce gust solicitations and to alleviate wing root bending moment at critical flight conditions, significant weight savings can also be achieved during aircraft wing design. Within the scope of Clean Sky 2 Airgreen 2 project, flutter and divergence characteristics of a morphing wing design are discussed, with specific reference to a configuration involving adaptive winglets and flap tabs. Multi-parametric flutter analyses are carried out in compliance with CS-25 airworthiness requirements (paragraph 25.629, parts (a), (b), (c) and (d)) to investigate static and dynamic aeroelastic stability behavior of the aircraft. The proposed kinematic systems are characterized by movable surfaces, each with its own domain authority, sustained by a structural skeleton and completely integrated with EMA-based actuation systems. For that purpose, a sensitivity analysis was performed taking into account variations of the stiffness and inertial properties of the referred architectures. Such layouts were reduced to a stick-equivalent model which properties were evaluated through MSC-NASTRAN-based computations. The proprietary code SANDY 4.0 was used to generate the aero-structural model and to solve the aeroelastic stability equations by means of theoretical modes association in frequency domain. Analyses showed the presence of critical modal coupling mechanisms in nominal operative conditions as well as in case of systems malfunctioning or failure. Design solutions to assure clearance form instabilities were then investigated. Trade-off flutter and divergence analyses were finally carried out to assess the robustness of the morphing architectures in terms of movable parts layout, mass balancing and actuators damping.