1. Engineering Design and Mathematics, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK e-mail:
2. University Technology Centre for Gas Turbine Transmission Systems, University of Nottingham, University Park, Nottingham NG7 2RD, UK e-mail:
3. Ricardo UK Ltd., Shoreham Technical Centre, Shoreham-by-Sea, West Sussex BN43 5FG, UK e-mail: