AERODYNAMIC INSTABILITY MODES FOR A LOAD SLUNG FROM A HELICOPTER

The aerodynamic-dynamic interaction of a load slung from a helicopter in flight can lead to divergent oscillations. In this paper, interactions between degrees of freedom are explored through fundamental simulation and wind tunnel experiments of generic shapes. Video analysis of the behavior of loads in a wind tunnel provides insights into dynamic behavior, and reveals the difference in behavior of the same model between wind tunnels of different sizes. A compound pendulum motion in a plane orthogonal to a steady freestream is first studied. Even with proximate walls, a purely symmetric single degree of freedom oscillation does not amplify. The addition of an out-of-phase force as a simplified model for yaw effects, leads to amplification or damping under some conditions. Wall effects amplify this interaction further. This level of simulation enables guidance to develop metrics for unsteady wall interference in dynamic testing. Results to-date set the context for future work in refining the aerodynamic and dynamic models, including tether and vehicle dynamics. Incorporation of quasi-steady and unsteady loads data from wind tunnel measurements and computational fluid dynamics would enable prediction of the linear superposition stage leading towards divergence. Comparison of simulaiton results to measured parameters from wind tunnel experiments shows relative agreement.

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