Study of fast aeroelastic solvers for gust load computation
Résumé
Aeroelasticity plays an important role in the design and certification of aircrafts. Whether steady aerodynamics is key for static methods, unsteady phenomenon is key for dynamic approaches. Nevertheless, in both aeroelastic problematics, it is basically aerodynamics who monopolizes the CPU cost. Thus, fast methods are interesting as alternative to the high cost of CFDCSM simulations.
The aim of this study is to present an unsteady method that predicts the aerodynamics of aircrafts encountering a wind gust. First of all, the process that brought to the formulation of the static aeroelasticity from Wind Tunnel Tests (WTT) is refreshed. It is a rapid and robust method based on the principle of the local incidence shift due to flexible effects. This difference in the angle of attack is used to interpolate the pressure coefficient from aerodynamics lookup tables.
Secondly, the static method (called Fast Nonlinear Static Aeroelasticity –FNSA–) is validated when analyzing a high aspect ratio wing which assures a flexible structure. The method deals with nonlinear, flexible and compressible effects in steady aerodynamics.
Finally, an algorithm is proposed to integrate the FNSA formulation to the unsteady problem of gust load computation. The methodology uses a quasi-steady approach of the static solver together with Wagner delay function modeling the unsteadiness of the flow. Restricting our aeroelastic equation of motion to a simple mass-stiffness system, the airstream directed downward due to the gust is added to the downwash of each section. The formulation allows, then, a reaction to the gust at each time step that yields to flexible results with an accuracy comparable to the Unsteady Vortex Lattice Method but around 3600 times faster.
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