Coupling flight mechanics and CFD - numerical simulation of shipborne rotors

This thesis demonstrates the use of Computational Fluid Dynamics (CFD) for the simulation of manoeuvring helicopters. Results are presented for the problem of shipborne operations, for which a literature survey showed that little work has been carried out. The CFD solver HMB2 was first validated using available experimental data for isolated ship wakes and helicopter loads at low advance ratios. A rotorcraft flight mechanics model was then developed and integrated into HMB2. The model includes a trimming method and a linearisation routine based on finite differences. The linear model of the aircraft can be used to estimate the controls applied by the pilot during a prescribed manoeuvre via the use of the SYCOS inverse-simulation method or via an LQR auto-pilot. The methods developed in the framework of this thesis include a general multi-body grid motion and an alternative formulation for earth-fixed frame of reference in the CFD. A study of the ship/rotor wake interaction was carried out using the actuator disc method that approximated the effect of the rotor, in a steady fashion and without resolving the flow around the blades. Various positions and thrust of the rotor were tested and the flowfield obtained via coupled simulations were compared with those obtained by super-imposing isolated rotor and ship flowfields. The results show that the superposition principle is not valid and leads to flowfields that have little to do with the real flow that is dominated by the interaction of helicopter and ship airwakes. The case of a rotor hovering in close proximity to a frigate deck was reproduced with fullyresolved blades, and the results shows a significant reduction of thrust due to the flow topology behind the hangar. The Helicopter Flight Mechanics (HFM) method was tested by simulating the aircraft response to a collective pilot input, using simplified models and coupled with CFD. Then, the coupled HFM/HMB2 method was used, in conjunction with the LQR auto-pilot, to simulate the phase of landing of a Sea King helicopter. Simulations were carried-out in free-air and above the frigate deck and the specified trajectories were followed adequately. Results for the ship landing show that the wake of the ship alters the obtained landing trajectory and that the current method captures some of the effects of the wake interaction