Active stabilization of slung loads in high-speed flight using cable angle feedback

This paper addresses the problem of active stabilization of slung loads up to 130 kt. To demonstrate the method, simulations of a utility helicopter with a dynamic inversion controller (as its automatic flight control system) and a CONEX cargo container were used. An airspeed-scheduled controller utilizing cable angle feedback was designed for the nonlinear-coupled system by the classic root locus technique. Nonlinear simulations of straight and level flight at different airspeeds were used to validate the controller performance in stabilizing the load pendulum motions. Controller performance was also evaluated in complex maneuvers with different levels of turbulence. The results show that using cable angle feedback provides or improves system stability when turbulence is not included in the simulation. When light/moderate turbulence is present, sustained limit cycle oscillations are avoided by the use of the controller. For severe turbulence levels, the controller did not provide any significant improvement.