Aeromechanical instabilities arising due to frequency coalescence of different modes of a helicopter rotor and fuselage system is investigated. The focus is on removing the small angle approximation typically used for ground resonance analysis and perform simulation of the resulting nonlinear model. The results reveal significantly different characteristics (existence of limit cycles) for the nonlinear model compared to linear model that is commonly used. These differences are primarily (though not necessarily) restricted to regions of frequency coalescence of more than one degrees of freedom present in the system. The paper does numerical simulations of different simplified models of coupled-rotor-fuselage systems used in the past by researchers and show that the nonlinear behavior is fundamental to the system. The results in the paper stand out because, in the past, limit cycle oscillations for coupled-rotor-fuselage systems were shown to exist only with nonlinearity associated with stiffness or damping parameter. The paper clearly shows that nonlinearity inherent in the lead-lag and flap equations will result in limit cycle oscillations and this is also demonstrated through experimental studies. © 2020, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.