Heavy helicopter-slung-load coupling system flying qualities in closed-loop state

Flying qualities of a heavy helicopter augmented with an automatic flight control system (AFCS) and flying with externally slung loads are investigated using the mathematical model of the CH-53A/D helicopter. A nonlinear dynamical model of helicopter-slung-load in closed-loop state was built based on single mass-point hypothesis and a control system model. Under this hypothesis, extra degrees of freedom and constraints were brought in by the consideration of the slung-load, which made the set of equations of motion increased to a 13-order one. The coupling system in closed-loop state was linearized under small-perturbed conditions. After this, the slung-load flight control response characteristics in both time and frequency domain were analyzed. The effect of slung-load mass, slung-load rope length and helicopter velocity on coupling system's flight quality was analyzed according to the handling qualities requirements for military rotorcraft indicated by ADS-33E. The final results show that the slung-load mass, slung-load rope length and helicopter velocity have varying degrees of impact on the time domain features and handling qualities of the coupling system.