Dynamic coupling of nonlinear motion and unsteady aerodynamic load of helicopter bluff-body slung load

To analyze the dynamic coupling mechanisms between the nonlinear motion of a helicopter's bluff-body slung load and its unsteady aerodynamic load, and to further improve the prediction accuracy of coupling effects during the helicopter' slung load operations, this paper establishes the numerical simulation method based on computational fluid dynamics (CFD), which is applicable to the bluff-body slung load. Then, the mathematical model of the sling constraining the motion of the slung load is developed, and its rigid-body dynamics model of the slung load is also established. By coupling the CFD model of the slung load with the six-degree-of-freedom rigid-body dynamics model, the dynamic coupling mechanisms between the nonlinear motion of the bluff-body slung load and its unsteady aerodynamic loads are analyzed. The paper clarifies the influence of the transition from narrow-side to broad-side orientations of the slung load on the aerodynamic motion coupling characteristics. The analysis results indicate that the transition, which is triggered by the continuous increase in yaw angle oscillation amplitude, is the primary cause of instability during high-speed flight. During the transition, the drag, the side force and the roll and pitch moments of the slung load rapidly exchange and redistribute. This leads to the rapid attenuation of longitudinal oscillation, while the lateral oscillation amplitude increases significantly. Simultaneously, the yaw angle enters into a stable "spinning" mode. When the speed increases from 40 km/h to 64 km/h, the instability time occurs 40% earlier, and the oscillation amplitude increases by 150%.