Propeller whirl flutter analysis of the nasa all-electric X-57 through multibody dynamics simulations
With ridesharing companies researching into intracity air vehicles, aircraft efficiency has a significant impact on cost and resources. To increase efficiency, thinner and higher aspect ratio wings are used to reduce drag along with propellers due to their high propulsion efficiency. This research presents a whirl flutter analysis of the NASA all-electric X-57 and how it influences the design of the vehicle. Multibody dynamics analysis (MBD) is used for this study, Dymore, with correlating data from CAMRAD II given by NASA. Along with numerous studies of the X-57 using these MBDs, two cases are presented for validating the use of multibody dynamics: a Goland wing undergoing torsion bending flutter and an isolated propeller in whirl flutter. The whirl flutter stability of the X-57 is conducted in stages, starting with an isolated propeller and closing with a full free-flying aircraft. The wing is input into Dymore as an equivalent beam derived from the full NASTRAN FEM of the X-57. This dissertation is divided into four parts for the whirl flutter analysis: isolated propeller, semi-span, full-span, and a free-flying model. The isolated propeller has a large margin of safety with the pylon pitch and yaw stiffness values having to be reduced by two orders of magnitude for whirl flutter to occur. The semi-span undergoes three design revisions to ensure whirl flutter is not encountered for any of the symmetric wing modes. The full-span does not experience whirl flutter for the wing symmetric or anti-symmetric modes, and a parametric study is performed on various design variables. The free-flying model is stable for all the wing modes and shows the longitudinal flight dynamics modes interact with the flexible wing modes. When the stiffness in one of the pylon mounts is reduced to simulate damage, the system becomes unstable when the short period mode crosses the wing symmetric out-of-plane bending mode. This shows the need to include the flight dynamic modes when considering the whirl flutter stability of future aircraft.
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