Experimental Identification of Concentrated Nonlinearity in Aeroelastic System

Identification of concentrated nonlinearity in the torsional spring of an aeroelastic system is per- formed. This system consists of a rigid airfoil that is supported by a linear spring in the plunge motion and a nonlinear spring in the pitch motion. Quadratic and cubic nonlinearities in the pitch moment are introduced to model the concentrated nonlinearity. The representation of the aerodynamic loads by the Duhamel formulation yielded accurate values for the flutter speed and frequency. The results show that the use of the Duhamel for- mulation to represent the aerodynamic loads yields excellent agreement between the experimental data and the numerical predictions. Nonlinearities in aeroelastic systems can a ect the response of aerospace vehicles and lead to complex phenomena such as flutter, limit-cycle oscillations (LCOs), chaos and bifur- cations (1-3). Such phenomena are associated with non- linear aerodynamic or structural conditions that character- ize aeroelastic systems. Regarding structural nonlineari- ties, they can arise from large structural deflections and/or partial loss of structural integrity. Furthermore, the e ects of aging, loose attachments, and material features could lead to undesirable and dangerous responses (4). The com- bination of the structural and aerodynamic nonlinearities can lead to changes in the basic response of the system such as changing it from supercritical to subcritical re- sponses and then catastrophic behavior can occur. As such, assessing the evolution of these inevitable behaviors through modeling and analysis of these nonlinearities is important to avoid or control these dangerous responses. In this work, we perform a nonlinear analysis to iden- tify the concentrated nonlinearity in the torsional spring of an airfoil section. The objective is to show how the use of a cubic nonlinearity to model the concentrated nonlinearity and the Duhamel formulation to represent the aerodynamic loads can yield relevant characterization of the aeroelastic system.