Simplified thrust and fuel consumption models for modern two-shaft turbofan engines

Performance data of two-shaft turbofan engines have been used to investigate the validity of previously published models (empirical equations) that describe an engine's thrust variation as a function of Mach number and altitude during takeoff and climb. Where necessary, the constants and the format of the models have been revised. An updated model is presented that describes the takeoff thrust to within ±1% of that of the reference engines, for flight speeds up to Mach 0.4. This takeoff thrust model has been adapted to account for the impact of bleed air extraction and altitude effects. To describe the maximum climb thrust, a new approach has been devised in which the climb path has been divided into three segments and an equation developed for each segment, based on a reference thrust at 30,000 ft and typical climb/speed schedules. A widely used thrust specific fuel consumption power law model for cruise has been investigated, and new empirical constants determined. These results will allow a more realistic prediction of engine performance for the purpose of preliminary aircraft design or initial performance analysis.