Modeling, simulation, and non-linear control of a rotorcraft multi-lift system

A simulation model of a rotorcraft multi-lift system is developed and implemented in the MATLAB/Simulink environment. The modeling tool is designed to be modular and generic, capable of modeling various configurations of rotorcraft, spreader bars, and external loads. This paper focuses on simulation and control of a formation of four helicopters carrying a single external load. Rather than treating the aircraft and slung load as a single dynamic system, they are treated as separate rigid bodies each with its own independent equations of motion. All objects in the four-helicopter system are coupled exclusively through elastic cable forces that act on each of the bodies. Analytical checks of the model are performed for the trim and stability characteristics of each helicopter, and results show reasonable and expected behavior due to the physical coupling from the cables. A non-linear control scheme is developed that using an aerodynamic inverse approach. The control calculates desired control inputs based on an inversion of the quasi-steady aerodynamics and the desired trajectory of each helicopter. In order to accurately track the commanded trajectory, the control law makes direct use of the measured cable force acting on each helicopter. The controller's performance is demonstrated in non-linear simulation of a four-helicopter multi-lift system and compared to a standard dynamic inversion controller. Simulation results show that the controller should make explicit use of measured cable forces and moments in order to ensure constant formation and safe separation of the helicopters.