Active Control of Turbulence-induced Helicopter Vibration

Helicopter vibration signatures induced by severe atmospheric turbulence ha\ e been shown to differ considerably from nominal, still air vibration. The perturbations of the transmission frequency have significant implications for the design of passive and active vibration alleviation devices, which are generally tuned to the nominal vibration frequency. This thesis investigates the existence of the phenomena in several realistic atmospheric turbulence environments, generated using Computational Fluid Dynamic (CFD) engineering software and assimilated within a high-fidelity rotorcraft simulation, RASCAL. The RASCAL simulation is modified to calculate blade element sampling of the gust, enabling thorough, high frequency analyses of the rotor response. In a final modification, a numerical, integration-based inverse simulation algorithm, GENISA is incorporated and the augmented simulation is henceforth referred to as HISAT. Several implementation issues arise from the symbiosis, principally because of the modelling of variable rotorspeed and lead-lag motion. However, a novel technique for increasing the numerical stability margins is proposed and tested successfully. Two active vibration control schemes, higher harmonic control 'HHC' and individual blade control 'IBC', are then evaluated against a `worst-case' sharp-edged gust field. The higher harmonic controller demonstrates a worrying lack of robustness, and actually begins to contribute to the vibration levels. Several intuitive modifications to the algorithm are proposed but only disturbance estimation is successful. A new simulation model of coupled blade motion is derived and implemented using MATLAB and is used to design a simple IBC compensator. Following bandwidth problems, a redesign is proposed using Hý theory which improves the controller performance. Disturbance prediction/estimation is attempted using artificial neural networks to limited success. Overall, the aims and objectives of the research are met. Anderson. D. 4ctiý e Control of Turbulence /,, doted Helicopter l ihraii ': Acknowledgements I'd like to express my deepest thanks to Dr Stewart Houston for his exceptional guidance, support and supervision throughout the course of this research and also for his motivation and encouragement in getting me to write this dissertation. I would also like to thank Dr Douglas Thomson for fielding my endless questions on inverse simulation and for having the restraint to not throw me out of his office! Special mention must also go to Dr Steve Rutherford, Dave Ewing and Garry Leacock for participating in those incredibly beneficial brainstorming sessions, even if we did get a little side-tracked at times. Dr Chris O'Neill also deserves many thanks for his Monday morning motivation classes, as do Dr John Maclean and Dr Nick Brignall from Marconi. There are many others within the Aerospace Department and beyond who contributed in helping to make this possible, you know who you are, many thanks. Many thanks must also go to the Centre for Systems and Control at the University of Glasgow for providing me with the research grant and to DERA Bedford for volunteering additional sponsorship. My gratitude to these two bodies for their financial and intellectual contributions cannot be understated. I'd like to thank Charles and Rae Anderson, my parents, for their loving support throughout some painful times and for their unending belief that one day this thesis would be submitted, even though at times I doubted their judgement. Most of all, I'd like to thank my beautiful and beloved wife Theresa, who has stoically supported me throughout my academic career and continues to do so to this day. She has been my rock of faith throughout all of the ups and downs, successes and failures I've experienced with this work. The love and gratitude I feel for her cannot be expressed in mere words, so perhaps dedicating this work to her will in some small way make up for my linguistic inadequacy. David Anderson March, 1999.