Modeling and control of a twin-lift helicopter system

A twin-lift system offers an efficient and economically attractive solution to the heavy lift problem. The open and closed loop stability and control characteristics of a twin-lift helicopter system are studied in this work. A comprehensive three dimensional nonlinear dynamical model of the system is developed. The model contains rigid body dynamics and aerodynamics of each system component. The open loop study is based upon linear, two dimensional models of two twin-lift configurations. The linear models are obtained by an analytical linearization of the system model about symmetric hover equilibrium conditions. Using the three dimensional nonlinear model, a numerical simulation capability is developed for the twin-lift system. A nonlinear flight controller is designed for the system using an extension of the application of the theory of input-output feedback linearization to non-square systems. The resulting closed loop input-output behavior is linear in the least square sense. The nonlinear feedback law forms an outer loop for the twin-lift flight control system and is used in conjunction with the existing Stability Augmentation Systems of the helicopters, which constitute the inner loop for the controller. The controller is validated using closed loop simulation results for a typical twin-lift mission. Consideration is given to the fact that the model developed may contain parametric uncertainty, mainly due to inexact knowledge about the load characteristics. A parameter adaptive version of the nonlinear control law is formulated for a two dimensional twin-lift system. The adaptive nonlinear control scheme does not require any knowledge of the bound of uncertainties present and drives the output tracking error to zero asymptotically. Simulation results are presented with and without adaptation for cases involving parametric uncertainty.