Equilibrium characteristics and stability analysis of helicopter-slung-load coupling system

A nonlinear dynamical model of the helicopter-slung-load coupling system was presented, based on single mass-point hypothesis. Under this hypothesis, the consideration of the slung-load brought in extra degrees of freedom and constraints, which made the equations of motion increased by 4 orders, and being a set of implicit differential algebraic equations. By distinguishing the generalized acceleration terms from the quadratic generalized speed terms in the inertia forces, the equations of motion can be turned into a set of explicit ordinary differential equations. For the helicopter model and the helicopter-slung-load model, trim states for zero-side-slipping forward flight were first computed, using direct numerical approach, and the results were compared with flight test data. Then, both of the two models were linearized under small-perturbed conditions, and modal decomposition was performed on these linearized models. Results show that 2 new dynamical modes would be introduced by the single-mass-point slung-load. Motions of the slung-load's 2 degrees of freedom would be coupled with the helicopter's altitude response, and modify the characteristics of helicopter's inherence modes, make some of them worse.