Reduced Order Analysis of Aeroelastic Systems with Freeplay using an Augmented Modal Basis

Freeplay in aircraft control surface actuators is a common source of nonlinearity that can cause undesirable aeroelastic phenomena and, as a consequence, certification authorities place strict limits on the size of the deadband gaps of actuators. In recent years many authors have worked on determining the effects of freeplay on the dynamic behavior of aeroelastic systems; these efforts have yielded significant improvements in the understanding of the phenomena, but have concentrated on simple models. Investigations of industry-relevant aeroelastic models of complete aircraft with freeplay in the actuators have been much rarer, the main difficulty being the selection of an appropriate modal basis that can represent the full range of possible dynamic phenomena. This work presents a novel implementation of the residual vectors approach to create consistent and numerically stable reduced order models suitable for industrial-standard aeroelastic models of aircraft. The methodology relies on the piecewise linear nature of freeplay to create a single reduction basis that represents the dynamics of the system both inside and outside the freeplay deadband gap. The method is demonstrated on an Embraer generic test bench aircraft, showing that the resulting reduced order model is efficient and effective. Nonlinear analysis is carried out using equivalent linearization as well as time integrations of the full nonlinear system. It is shown that, while equivalent linearization is an indispensable tool for a preliminary mapping of the main aeroelastic instabilities, the time integration-based nonlinear analysis is an essential complementary tool to confirm the characteristics of the system behavior.