Optimization of an engine with a gear driven counter rotating fan Part II: Cycle selection and performance

Abstract

Aircraft fuel efficiency becomes more and more important because of increasing fuel prices and environmental concerns. Engine efficiency contributes substantially to this, but further improvements are very demanding due to the already mature technology. To achieve further enhancements, the introduction of new engine concepts is currently in discussion. Most of these concepts focus on improving propulsive efficiency. In this two-part study, the concept of the counter rotating fan (CR-fan) is assessed. In the first part of this study, the fan concept is analysed considering aerodynamical aspects. The focus of the present part of the study is to assess the CR-fan concept on engine level. To assess the results of the aerodynamic optimization on an overall engine level, a cycle design study is performed. For a civil long range application, the aircraft characteristics are modelled and engine requirements are derived. Then a cycle is developed, taking both the constraints from the thermodynamic limitations and the thrust requirements from the flight mission into account. Besides the thermodynamic modelling, weight and drag analyses for the engine are performed. The fan aerodynamics and weight is directly taken from the first part of this study. The technology assumptions for the other engine components are taken from literature. The close link between fan design and performance study allows for a detailed description of the propulsor within the thermodynamic model. As a result the optimal aerodynamic fan design is identified by finding the best compromise between cycle demands, fan efficiency, weight and drag in order to minimize fuel consumption. Finally, an engine cycle for a CR-fan is defined taking into account the gearbox influenced off-design behaviour of the fan rotors.