Aerodynamic Instability Modes for a Load Slung From a Helicopter

Abstract

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.

Testing-Based Approach to Determining the Divergence Speed of Slung Loads

“…This is probably due to tunnel-wall effects. Sharma et al [57,58] studied this problem with a simple mathematical formulation using doublets to represent the forces acting on the model. They showed that spin and roll disturbances reinforced each other when the phase difference between them approached 90 degrees, and the amplification depended strongly on wall effects.…”

“…The reason for the difference was unambiguously traced to unsteady wall effects in the small tunnels. Sharma et al [57,58] presented a simple potential flow method to model dynamic wall effects that could amplify the roll oscillations of models suspended in a wind tunnel. Extending this work, Liberi found more modes of yaw-roll instability that would amplify, even in free air.…”

“…The reasons why these excursions occur are still being studied, per the authors. Sharma [57,58] and Liberi [117] showed, using simple potential flow aerodynamics and mathematical models, how roll oscillations (pendulum motion) could amplify when coupled with yaw, due to wall effects. They then realized that amplification also occurred in the absence of walls, but much less so.…”