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 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.
In this seminar, the recently developed Parametric Flutter Margin (PFM) method that uses a different flutter search strategy will be presented. 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. The combination of the PFM method with the Increased-Order Modeling approach facilitates the application of PFM to nonlinear flutter that yields LCO. It will be also shown that the PFM method can be applied for performing efficient sensitivity analysis. 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 test in which the aeroelastic model is stabilized, and the flutter-onset conditions of the nominal system are positively identified. Two PFM-wind-tunnel flutter tests were performed, a proof-of-concept one in TU-Delft using a 2D-aeroelastic wing, and one in the Technion subsonic wind tunnel using a more realistic-3D aeroelastic model.
