On the Role of Aerodynamic Non-Linearities in Helicopter Sling Load Operations

Analytical predictions of the dynamic response of a helicopter carrying a prismatic suspended load of rectangular cross-section are presented in this paper. A simplified stability-derivative model of the helicopter is considered, with helicopter roll, lateral translation and lateral load swing. Particular attention is paid to the aerodynamic side forces acting on the sling load. The quasi-steady and unsteady (periodic) forces on these shapes are represented using models developed by the Civil Engineering community, since bluff body forces are representative of wind loading on buildings. The steady-state lateral swing of sling loads at various airspeeds are obtained and a simulation study is carried out to ascertain the dominant parameters. Due to strong non-linearities in the aerodynamic side forces, the load swing exhibits a hysteresis-like dependence on airspeed. There exists a "jump speed", where the load swing increases sharply by about 20° for a small increase in airspeed due to the non-linearity, and this jump may be perceived by the pilot as an instability. Incorporation of unsteady forcing leads to a reduction in the jump speed but does not significantly affect the load swing at higher airspeeds. Finally, Bode plots of (open-loop) roll attitude response to lateral cyclic stick inputs were generated in the vicinity of the jump speeds. The features of these plots do not indicate the presence of the jump.