Development and Design Optimization of High Fidelity Reduced Order Models for Dynamic Aeroelasticity Loads Analyses of Complex Airframes

Abstract

Identification of aircraft critical loads envelope requires a lengthy and rigorous analysis procedure that includes simulating the aircraft at thousands of load cases identified in the certification requirements. Imposing a Global Finite Element Model (GFEM) in this process is computationally very expensive. Hence, Reduced Order Models (ROM) of airframes are commonly employed in the static and dynamic aeroelasticity analyses. ROMs must be simple enough to be analyzed thousands of times during the iterative aeroelastic simulation but sufficiently accurate to have their dynamic characteristics closely matching those of the GFEM within a frequency range of interest. Several Model Order Reduction (MOR) methodologies are available in the literature with the Stick Model (SM) being the preferred methodology adopted by the aerospace industry. A SM is a series of beam elements extending along the airframe elastic axis that offers an intuitive spatial representation of the airframe mass and stiffness distributions, a feature of paramount importance to the development engineers in the aerospace industry. However, due to several approximations and simplifications in the current development process, it is evidently found that SM's are not sufficiently appropriate for dynamic simulations. To overcome such limitation, this thesis presents two approaches for the development of high fidelity stick models. The first approach is based on a Hybrid Stick Model (HSM) representation in which the conventional SM is augmented by a set of structural matrices to account for inaccuracies that might be encountered in the modal performance of the base SM as compared to the GFEM. In the second approach, we solve a design optimization problem in which we optimize the stiffness parameters of the conventional SM to minimize errors in its modal pairs with reference to the GFEM. The final product of the optimization iii problem is an Optimized Stick Model (OSM) with dynamic characteristics that closely matching those of the GFEM within a specified frequency range of interest. Case studies are presented where the HSM and the OSM along with the conventional SM are employed in the dynamic aeroelasticity loads analyses of a Bombardier aircraft platform. The extracted aeroelastic loads are compared against those generated employing the aircraft GFEM. The dynamic characteristics of the ROMs are also assessed based on their modal characteristics using metrics of Modal Assurance Criteria (MAC) and Modal Participation Factors (MPF). Results obtained show that the developed HSM and OSM have superior dynamic characteristics compared to the conventional SM.