Increased-Order Modeling (IOM) is a practical and efficient approach to the modeling ofdynamic systems that are mostly linear, but their behavior may be significantly affectedby separable nonlinearities. The approach is based on the augmentation of a main linearblock with nonlinear feedback loops that represent the important system nonlinearities. Anew IOM-based framework for nonlinear aeroservoelastic simulations is discussed. Theframework was designed to serve two purposes, efficient dynamic-loads calculations forindustrial applications, and investigation of the effects of structural, aerodynamic andcontrol-system nonlinearities on aircraft stability and response characteristics. Thesolution sequence starts with the calculation of frequency response functions of the linearsystem with the nonlinear elements disconnected. Time-domain responses of the linearblock to specific gust, maneuver or direct-force excitations, and to control input impulses,are then calculated using Fast-Fourier-Transform (FFT/IFFT) techniques. The responseis then corrected by incremental nonlinear effects in a process that combines time domainsolutions of the nonlinear elements and convolution integrals for the linear parts withimpulse response functions. Three nonlinear aeroelastic studies are posed and solved in aunified and systematic manner within the IOM framework. The cases are industrialdynamic design loads with nonlinear control, limit-cycle oscillations (LCO) of plate-typefins with nonlinear plate elements, and dynamic gust loads with nonlinear aerodynamics.
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