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Structural geometrical nonlinearities strongly affect the response of PrandtlPlane Joined Wings: it has been shown that linear buckling evaluations are unreliable and only a fully nonlinear stability analysis can safely identify the unstable state. This work focuses on the understanding of the main physical mechanisms driving the wing system's response and the snap-buckling instability. Several counterintuitive effects typical of unconventional non-planar wing systems are discussed and explained. In particular, an appropriate design of the joint-to-wing connection may reduce the amount of bending moment transferred, and this is shown to dramatically improve the stability properties. It is also demonstrated that the lower-to-upper-wing stiffness ratio and the torsional-bending coupling, due to both the geometrical layout and anisotropy of the composite laminates, present a major impact on the nonlinear response. How the material anisotropy modifies the Snap Buckling Region and the response is also discussed. The findings of this work could provide useful indications to develop effective aeroelastic reduced order models tailored for airplanes experiencing important geometric nonlinearities such as PrandtlPlane aircraft, Truss-braced and Strut-Braced wings and sensorcrafts.

Structural geometrical nonlinearities strongly affect the response of Joined Wings: it has been shown that buckling evaluations using linear methods are unreliable and only a fully nonlinear stability analysis can safely identify the unstable state. This work focuses on the understanding of the main physical mechanisms driving the wing system's response and the snap-buckling instability. Several counterintuitive effects typical of unconventional non-planar wing systems are discussed and explained. In particular, an appropriate design of the joint-to-wing connection may reduce the amount of bending moment transferred, and this is shown to eventually improve the stability properties although at price of a reduced stiffness. It is also demonstrated that the lower-to-upper-wing stiffness ratio and the torsional-bending coupling, due to both the geometrical layout and anisotropy of the composite laminates, present a major impact on the nonlinear response. The findings of this work could provide useful indications to develop effective aeroelastic reduced order models tailored for airplane configurations experiencing important geometric nonlinearities such as PrandtlPlane, Truss-braced and Strut-Braced wings, and Sensorcrafts.

Past work on Joined Wings pointed out the importance of including structural geometric nonlinearities since the early stages of the design. However, the attention was mainly focused on conservative loadings and several open questions needed an answer. In a effort to simulate aerodynamic-pressure-like loads, this effort focuses on non-conservative follower forces. Several numerical evaluations demonstrated that follower loadings exacerbate the risks of snap-instability in Joined Wings.Dynamic response analysis of vanishing perturbations showed that for certain combination of geometry and stiffness distributions, the phenomenon of branch-jumping is possible. This is directly linked to the existence of a bi-stable region. The presence of follower loads increases these risks with respect to conservative forces.For some configurations, a small change of a parameter (e.g. the lower-to-upper-wing bending stiffness ratio) can produce a sudden appearance of a bi-stable region and the risk of branch jumping becomes serious.This paper demonstrates that even if the design of a joined-wing system is well below the critical point (snap-buckling state) and the response appears to be quasi linear, there is a potential risk that a dynamic disturbance (perturbation) may move the system to a relatively far equilibrium state on a post-critical branch. Thus, a post-critical analysis need to be included even if a linear design point for the Joined Wing is sought.Results of this effort suggest that preliminary multidisciplinary optimizations technique can not confidently rely on linear analyses augmented with eigenvalue instability investigations. On the contrary, a post-critical study is strongly recommended for a safe design of innovative wing configurations based on the joined-wing concept.