Equilibrium characteristics and stability analysis of helicopter slung-load system

Based on rigid-body slung-load hypothesis, a nonlinear dynamical model of the helicopter and slung-load system is presented. As for helicopter and slung-load system, the introduction of the rigid-body slung-load results in several extra degrees of freedom and constraints, and this makes the nonlinear equations of helicopter and slung-load motion transfer from 9 orders to 19 orders. The nonlinear equations are linearized by small perturbation hypothesis for stability analysis and they are trimmed by the continuation method. First, the simulation trimmed results are compared with the helicopter flight test data without slung-load and the calculation results of helicopter with mass-point slung-load in the literature. Then, the differences among trimmed states of the helicopter with a rigid-body slung-load, a mass-point slung-load, and without slung-load are studied. The motion modes of helicopter with rigid-body slung-load are calculated in different cases, and the effects of extra slung-load to the helicopter stability are analyzed at the same time. Results showed that the rigid-body slung-load added five new motion modes to the whole helicopter and slung-load system, two of which concerning cable swinging motion are stable, while the others concerning slung-load attitude motion are unstable. At the same time, the introduction of rigid-body slung-load makes the short-period mode and the roll mode of helicopter unstable, which has a strong impact on the flight quality of helicopter.