As a consequence of increased stringent engine emission regulations, in a highly competitive market, it has become necessary to explore innovative, economic and environmentally friendly cycles to sustain competitive advantages. Among these innovative cycles, both the geared and the direct drive counter-rotating open rotors, due to their relatively higher propulsive efficiency, have the potential to significantly reduce fuel consumption and emissions relative to conventional high bypass ratio turbofans. A detailed TERA (Technoeconomic Environmental Risk Analysis), multidisciplinary optimisation framework, can be used to optimise both engines and thereby assess their potential as well as quantify their risks on a formal and consistent basis. This technique is based on detailed and rigorous engine performance, aircraft performance, engine geometry, engine weight, noise, gaseous emissions and environmental impact simulation models. No specific performance simulation methodology for counter rotating open rotors is available in the public domain. An innovative technique is introduced, comprising novel models of: • Counter-rotating propellers (including their interaction); • Counter-rotating turbines; • Planetary differential gearboxes. A thorough description of the modelling methodology (with a justification of the main assumptions) of each of these three components is presented and an indication of work in progress is provided. These components are then used to develop direct drive and geared open rotor performance models. The results of steady state design point and off design performance simulations of these two engine models are subsequently presented via two case studies. Some of the differences in the performance of the low pressure system of geared and direct drive open rotors are highlighted. It was observed that the impact of the key OR performance DP parameters is different for the two engines. Consequently the optimal design and control strategies of theses two configurations will differ. The flexibility of the new simulation technique makes it a suitable candidate to perform multi-disciplinary TERA design space exploration and optimisation studies assess and optimise open rotor designs and control strategies in a multidisciplinary framework.

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