Investigation Of Coupled Rotor/Airframe Coaxial Helicopter Dynamic Stability

The intention of this paper is to investigate the rotor/fuselage coupled dynamic stability of coaxial helicopter working on the ground. Firstly, the rotor/fuselage coupled dynamic analytical model was established, the rigid blade with root spring of the rotor model is of flap and lag degree of freedom, aerodynamic interference factor was induced to describe the interference of upper and lower rotor, the disc induced velocity was modeled by uniform inflow. The fuselage model is of lateral, longitudinal, pitch and roll degree of freedom, governing equation sets of the model was deduced by Newton method, and linearized based on the hypothesis of small disturbance then the perturbation equations were obtained. Secondly, the analysis of rotor/fuselage natural characteristics and stability was conducted by eigenvalue calculation of coaxial helicopter, time histories of the system were researched with given lag motion as the initial condition. At last, the validation of previous theoretical analysis was conducted by mechanical dynamics simulation software Adams. The influence of rotation speed and distance between the upper and lower rotor on stability of the system was gained by the same parameterized model of the coaxial helicopter in Adams/View. The simulation showed good agreement with the theoretical research in this work, and the validity and accuracy of the theoretical modeling were validated.The result of investigation reveal that the couplings of backwards lag motion of upper rotor and rolling motion of the fuselage is main source of instability, even the lower rotor make things worse. The gear stiffness, distance of the two rotors and the mass of the fuselage are all count for instability of coaxial helicopter. The researches of this paper provide theoretical support for rotor/fuselage coupled dynamic design of coaxial helicopter.