Investigating the impact of ship superstructure aerodynamics on maritime helicopter operations

The work reported in this thesis has investigated the impact of ship superstructure geometry on helicopter operations to the flight decks of naval warships. Ship-Helicopter operating limits for military frigates and destroyers are often restricted in difficult weather conditions because of excessive pilot workload caused by the unsteady ship airwake. Experiments have been conducted in a water tunnel using a specially designed Airwake Dynamometer (AirDyn) to characterise the steady and unsteady aerodynamic loading of a helicopter immersed in the airwake of a generic ship that has been called the Shortened Research Frigate (SRF). The AirDyn is a 1:54 model-scale helicopter, mounted on a six-component force block; it has a simplified spinning main rotor and fuselage based on a Merlin AW-101. The AirDyn has been shown to be an effective tool for characterising the steady and unsteady aerodynamic loading of a helicopter model in a ship’s airwake and the aerodynamic loads measured by the AirDyn were found to correlate with at-sea and simulation flying experience for a range of Wind-Over-Deck (WOD) conditions and ship geometry configurations. The airwakes of the SRF without the presence of a helicopter model have also been investigated using unsteady Computational Fluid Dynamics (CFD). A Detached-Eddy Simulation (DES) approach was used for the turbulence modelling as it has been shown to be capable of capturing the bluff-body type flow features typical of ship airwakes. Analysis of the CFD data revealed the underlying aerodynamic causes of the observed loading characteristics of the AirDyn. A range of ship geometry modifications were made to the SRF to determine the feasibility of mitigating the adverse effects of the airwake by modifying existing ships or by improving the designs of future ships. A range of modifications to the windward hangar side-face of the SRF were tested using the AirDyn and were found to reduce the severity of unsteady loads by up to 55% for oblique WOD angles (Green 30°-45°) at important locations through the flight path of a standard Royal Navy deck-landing manoeuvre. Unsteady CFD analysis showed that the modifications controlled the flow separation from the top edge of the windward hangar side-face in such a way as to reduce the height and the angle of the separated flow and thus the severity of the unsteady flow structures being drawn into the main rotor of the AirDyn. The unsteady CFD data computed for the baseline and modified SRF ship geometries was also integrated into the University of Liverpool’s motion-base flight simulator and piloted flight simulation trials were conducted to determine the impact of the ship modifications on pilot workload. The results of the simulation flight trials confirmed the usefulness of the AirDyn as a tool for predicting pilot experience and showed that it is feasible to modify ship superstructures to the extent that tangible reductions in pilot workload are achieved