Interaction of torque link freeplay and Coulomb friction nonlinearities in nose landing gear shimmy scenarios

Abstract

Aircraft nose landing gears are known to be prone to shimmy oscillations, rooted mainly in inadequate design and maintenance. Being a highly nonlinear dynamical system, stable limit-cycles can arise for certain values of input parameters, which are both a nuisance to the occupants and a threat to the safety and integrity of the aircraft. Influence of different landing gear nonlinearities are discussed in differing depth in the literature, leaving the interaction and simultaneous effect of these nonlinearities mostly unexplored. In particular, Coulomb friction in the shock absorber is largely neglected or oversimplified, while it was shown in the authors’ previous work to have a substantial effect on the stability boundaries. In the present novel investigation, we further explore the significance of state-dependent Coulomb friction in the shock absorber in presence of rotational freeplay, which is another key nonlinearity of the landing gear systems. Using a representative high-fidelity Multi-Body Dynamic model and numerical solution scheme in the time domain, 3D shimmy maps and 2D stability boundaries are obtained and employed to understand the nonlinear phenomena of interest, contributing to more realistic performance estimation of nose landing gears in early design stages. Together with time histories of the limit-cycles, phase portraits, and frequency spectra for selected inputs, the stability maps showed that presence of freeplay leads to emergence of a new area in the stability map, characterized with low-amplitude rotational oscillations and zero steady-state lateral amplitude. Furthermore, with Coulomb friction and freeplay both being part of the model, the freeplay-induced area of the stability map vanishes, while the freeplay effect is still manifested in the frequency spectra and expansion of the rotational-dominant area. The sensitivity of the freeplay-induced changes to the magnitude of the torque link freeplay is also depicted and discussed, for cases with and without Coulomb friction.