Analysis of Helicopter Slung-Load Instabilities with AFCS Feedback using Coupled Linearised Models

During conduct of clearance testing for the carriage of a heavy external load under a CH-47F aircraft, the Australian Army experienced an incident involving high frequency divergent oscillations in hover. Modelling and simulation was conducted in order to identify the likely source of the oscillations. In this paper, a high order 3 degree of freedom linear CH-47F helicopter model is coupled with a linearised model of an externally slung load in the dual point configuration. This is combined with a linearised version of the longitudinal flight control system to estimate the closed loop coupled helicopter/load dynamics. Analysis of predicted stability margins using the coupled model indicated that interactions between the load and the closed loop dynamics were likely in this configuration, with low airframe gross weight configurations contributing to the destabilisation of the system. The coupled linearised model approach is extended to facilitate parametric studies, allowing for analysis of the impact of configuration parameters and rigging geometry on the overall stability using root locus techniques. This paper presents the methods for generating the coupled linearised model and parametric analysis. It also highlights the importance of conducting stability margin analysis for external load configurations, particularly for high load-mass ratios and dual point configurations.