Linearized CFD-and CSM based flutter process for very flexible aircraft

ifasd2024 Tracking Number 169

New aircraft developments might include higher aspect ratio wings compared to the current commercial transport aircraft. Their aspect ratio is currently around nine, whereas new studies, like truss-braced wings, envisage an aspect ratio between 15 and 19. To reduce the mass penalty for such designs, these wings will usually become more flexible. The modal mass and stiffness matrix as well as the mode shapes themselves become dependent on the static aeroelastic equilibrium state and therefore, the challenges for flutter stability analysis are increased compared to a purely linear analysis. The application case for this paper is an aircraft with aspect ratio 17 and a wing deformation of up to 11% relative to half-wing-span. It is designed for transonic Mach numbers. Figure 1 shows the differences of flutter results when considering a jig shape or a flight-shape structural model. The aerodynamics for both cases have been computed in flight shape. Updated Flutter Process The challenge for the p-k-based, linear flutter process is that matrices for mass, stiffness and aerodynamics can vary during each velocity step. The two main influence parameters on the wing deformation are the lift coefficient and the dynamic pressure, which are spanning up the interpolation space I. For each sample point in I, a static CFD-CSM solution, the structural eigenmodes in the deformed shape and the CFD-LFD aerodynamics are computed. Afterwards, for a certain production mass cases the eigenmode analysis must be repeated for the same sample points in I and the aerodynamics must be approximated accordingly from the LFD database. Afterwards, a mode-mapping between the structural sample point mode matrices must be performed to gain structural and aerodynamic matrices which allow direct interpolation. During the p-k solver run these matrices are interpolated in each velocity step. Finally, the paper will compare differences of the jig-based approach and the approach based on interpolation between different CSM deformation conditions.