A New Approach to the Design of the Large Turbofan Power Plant

Abstract

The lower direct operating costs of the Big Twin subsonic transports encourage the building of ever larger turbofan engines installed on the wings. The steadily improving reliability of the turbofans and the good safety statistics of twin-engined aircraft over many years encourages this trend. Fuel economy is still the dominant factor in determining the design layout of turbofan engines. It requires the combination of the highest possible thermal efficiency of the gas generator core of the engine with optimum propulsion efficiency of the power plant as a whole in cruise flight, allowing for engine nacelle drag and nacelle to wing interference drag. High thermal efficiency and high propulsion efficiency together, lead to relatively small volume flow rate gas generators and high volume flow rate propulsion fans. The resulting geometrical mismatch between the compressors and turbines of the principal turbomachinery components within the engine, introduces losses that penalize the performance gains expected from theoretical considerations of thermodynamics cycle and component efficiencies alone. The paper presents two possible turbofan design layouts intended to overcome the limitation of current turbofan power plant designs. The aim is to design a power plant with the highest thrust per unit frontal area combined with the highest air miles per gallon in cruise flight.