Nonlinear Dynamical Characteristics of Fin-Stabilized Underslung Loads

Abstract

The maximum safe speed of a helicopter carrying an underslung load is frequently limited due to load instability problems. Rear-mounted fins were previously demonstrated as an efficient means for stabilization of a problematic load. Past stability analyses of fin-stabilized loads were achieved mainly through the use of classic linearized small-perturbation techniques and simplified aerodynamic models. The nonlinear nature of the system dynamics was overlooked during most analyses, thus leading to an incomplete description of its main characteristics. The present paper describes the theoretical prediction of the nonlinear dynamical characteristics of a boxlike underslung load, passively stabilized by rear-mounted fins. Predictions are shown to compare favorably with dynamic wind-tunnel test data both qualitatively and quantitatively. The nonlinear nature of the dynamic system is first demonstrated using time-history data from test results and simulation predictions for specific airspeeds. Bifurcation and continuation analysis is then performed to provide a more complete description of system stability at the airspeed range of interest. It is shown that, although the fins are successful in stabilizing the load motions above threshold airspeed, significant instabilities may still reappear at higher airspeeds. Therefore, dynamic tests should be continued beyond the first indication of load stability. The developed simulation tool allows a broader understanding of the inherently nonlinear behavior of the system’s dynamics and can be used for the study of the combined helicopter–slung-load dynamics.