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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When the Magnus effects are completely absent the oscillatory motion of a dirigible body is said to be plane-yawing. Such bodies are stabilised by attaching fins or control vanes at their rear ends. Initial choice of the fin-size is made, depending on a static stability condition: JM < O, where JM is the normalised overturning aerodynamic moment coefficient. A proper fin-size requirement should normally be found from an appropriate dynamic stability condition. Under the very severe aerodynamic restriction stipulated above, one would expect, if the static stability condition is liberally satisfied by attaching over calibre fins, perhaps dynamic stability requirements could be met. This, however, may considerably reduce the ballistic range of the body. One would, therefore, need some sort of an upper majorant for JM consistent with the dynamic stability of the body and this should be a function of the other associated aerodynamic forces. In the present note a suitable majorant function for JM has been worked out. For this purpose, the aerodynamic coefficients are assumed to be slowly ranging functions of the path length as is usually stipulated. The stability problem has been solved using certain known results in the oscillation theory of differential equations.