A method to define a scalable turbulence response model for the ship-helicopter dynamic interface

Abstract

Maritime helicopters are often required to operate in the turbulent flow or air wake generated in the lee of a ship’s superstructure. This turbulent air wake disturbs the aircraft motion and therefore compensatory control inputs need to either be provided by a pilot or a control system for accurate station keeping. This paper presents the method used to understand how aircraft size, expressed as either maximum take-off weight or rotor disc loading, affects the turbulent response of an aircraft operating in a ship’s air wake. Details of the simulation implementation and investigation are given. The turbulence responses of the aircraft are analyzed using frequency domain system identification techniques and low order equivalent systems are identified. The results of this analysis were then captured using a simple scaling law to provide an approximation of an aircraft’s turbulence response using the ambient wind-speed and aircraft disc loading. The derived scalable turbulence model consists of a transfer function whose standard deviation and break frequency relate to the ambient wind velocity and the rotor disc loading. ‘Conservative’, ‘Standard’ and ‘Optimistic’ versions of the model were created. For the standard deviation model parameter, the ‘Standard’ version of the model appears to be the best fit, except for the pitch and roll axes of stiffer hingeless or teetering rotor head vehicles, where the ‘Conservative’ fit appears to be the better model. The ‘Standard’ model appears to be the best fit for all rotorcraft types for the break frequency parameter of the model.