טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLevin Dani
SubjectAeroelastic Instability of Fins with Non-linear Stiffness
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Mordechay Karpel
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

An efficient method for time simulation of aeroelastic system with structural nonlinearities is described in this research and applied to LCO analysis of plate-type fins. The method is based on well known linear models, structural and aerodynamic. The method is applied for plate-like metallic fins that are common in many flight vehicles.  When flown above the linear flutter velocity, the fins start fluttering with diverging amplitudes, but then they often reach steady amplitudes and continue in limit-cycle oscillations.  The structural model of a typical fin is constructed with rectangular plate elements. The structural non-linearity is due to non-linear membrane forces that arise during plate bending. First-order approximations of the nonlinear strains in von Karman’s plate equations are used to express this non-linearity. The resulting equations, combined with the Co-Rotational method for distinguishing between rigid-body displacements and elastic deformations, are used to construct the nonlinear feedback loops in the simulation.  The main linear block is modeled by common generalized coordinate techniques with a finite-element model that is based on linear plate elements and with a linear panel aerodynamic model.  The resulting application is very efficient, robust, suit any fin geometry and can be integrated easily in the process of the vehicle design.  The method is applied with a model of a cropped delta wing that was previously tested in the wind tunnel and used for numerical applications with fully nonlinear models.  The results exhibit good agreement with the test results, which verifies the modeling approach and the dominance of the nonlinear terms taken into account.