Phenomenology of Nonlinear Aeroelastic Responses of Highly Deformable Joined-wings Configurations Rauno Cavallaro, * Andrea Iannelli † , Luciano Demasi ‡ , AlanM´arquezRaz´on § Dynamic aeroelastic behaviour of structurally nonlinear Joined Wings is presented. Three configurations, two characterized by a different location of the joint and one pre- senting a direct connection between the two wings (Sensorcraft-like layout) are investigated. As a first step, the snap-divergence is studied from a dynamic perspective in order to assess the real response of the configuration. Later on, the investigations focus on the flutter occurrence (critical state) and go further in analyzing postcritical phenomena: Limit Cycle Oscillations (LCOs) are observed followed by a lost of periodicity of the solution as speed is further increased. In some cases, it is also possible to ascertain the presence of period doubling (flip-) bifurcation. Differences between flutter (Hopf’s bifurcation) speed evaluated with linear and nonlin- ear analyses are discussed in depth, in order to understand if a less computationally intense approach may be used with confidence. Both frequency and time-domain approaches are compared. Moreover, aerodynamic solvers based on the potential flow are critically examined and discussed. In particular, it is assessed in what measure the use of more sophisticated (and computationally more intensive) aerodynamic and interface models impacts the aeroelastic predictions. These differences range from the methods adopted for load transferring to the models employed to describe the wake. When the use of the tools gives different results, a physical interpretation of the lead- ing mechanism generating the mismatch is provided. In particular, for PrandtlPlane-like configurations the aeroelastic response is very sensitive to the wake’s shape. As a conse- quence, it is suggested that a more sophisticated modelling of the wake positively impacts the reliability of aerodynamic and aeroelastic analysis. For Sensorcraft-like configurations some LCOs are characterized by a non-synchronous motion of the inner and outer portion of the lower wing: the wings’ tip exhibits a small os- cillation during the descending or ascending phase, whereas the mid-span station describes a sinusoidal-like trajectory in the time domain. I. Introduction T here is currently great interest in innovative aircraft configurations. Among them, joined-wing concept [1, 2] has captured the attention as a possible candidate for the airplane of the future. However, it was argued and also demonstrated that a reasonably accurate conceptual/preliminary design is hard to be pursued due to inherent structural nonlinearities that may invalidate the results obtained with fast lower- fidelity tools, which are a necessity when exploring the large parameter’s space typical of the early design stages. For example, references [3–5] showed non-negligible differences when comparing structural responses obtained with linear and nonlinear capabilities. As a consequence, consolidated design strategies and tools developed in decades, and effectively used by the industry, have to be reviewed. This represents one of the major barriers to the development of Joined Wings [6]. ∗ PhD Candidate, Department of Aerospace Engineering, San Diego State University and Department of Structural Engi- neering, University of California San Diego, AIAA Member † Visiting Graduate Student, San Diego State University, MS Candidate at the Dipartimento di Ingegneria Aerospaziale, Universit`a di Pisa. ‡ Associate Professor, Department of Aerospace Engineering, San Diego State University, Lifelong AIAA Member § Undergraduate Student, Department of Aerospace Engineering, San Diego State University 1 of 63 American Institute of Aeronautics and Astronautics 55th AIAA/ASMe/ASCE/AHS/SC Structures, Structural Dynamics, and Materials Conference 13-17 January 2014, National Harbor, Maryland AIAA 2014-1199 Copyright © 2014 by Rauno Cavallaro; Luciano Demasi; Andrea Iannelli; Alan Marquez Razon. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. AIAA SciTech