טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRoizner Federico
SubjectAeroservoelastic Stability Analysis using Response-Based
Parametric Flutter Margins
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Mordechay Karpel


Abstract

At linear flutter onset conditions, an aircraft undergoes self-excited harmonic oscillations in response to any initial trigger, leading to a homogenous frequency-domain flutter equation. Accordingly, common-flutter methods search for the conditions at which the aeroservoelastic (ASE) matrix determinant is zero. The main difficulty stems from the fact that the aerodynamic force coefficients and control laws depend on the vibration frequency. While being well established and widely used, the applicability of these methods is limited. Being based on the system matrix properties rather than on response simulations, they are not based on standard response solvers and their results are difficult to be compared with test results. The solution reflects an actual physical situation only at the flutter point because it consists of adding an artificial term that is canceled only at this point. Hence, it is difficult to obtain flutter margins with respect to practical design parameters. Due to their non-direct nature, the common solvers cannot be extended directly to the investigation of nonlinear effects.


This research developed the Parametric Flutter Margin (PFM) method that uses a different flutter search strategy. It is based on calculating flutter margins with respect to a stabilizing parameter, which is added to the nominal system, via frequency response functions of the stabilized system. Efficient sensitivity studies can be performed with this method, and the combination with the Increased-Order Modeling approach facilitates the application of PFM to nonlinear flutter that yields limit-cycle oscillations. A considerable advantage of the PFM method is that the frequency-response functions at and beyond the flutter boundary of the nominal system are calculated with a stable system. This allows us to perform safe flutter tests in which the ASE system is stabilized, and the flutter-onset conditions of the nominal system are positively identified. The idea of performing safe flutter tests was validated by performing a wind-tunnel flutter test using a realistic 3D aeroelastic model experiencing bending-torsion flutter.


The Thesis is based on a collection of papers that have been published in the AIAA Journal and the Journal of Aircraft.