A NEW APPROACH TO THE DESIGN OF THE LARGE TURBOFAN POWER-PLANT

The lower direct operating costs of the Big Twin subsonic transports e ncourage the building of ever larger turbofan engines installed on the wings. The steadily improving reliability of the turbofans and the go od safety statistics of twin-engined aircraft over many years encourag es this trend. Fuel economy is still the dominant factor in determinin g the design layout of turbofan engines. It requires the combination o f 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 ge nerators and high volume flow rate propulsion fans. The resulting geom etrical mismatch between the compressors and turbines of the principal turbomachinery components within the engine, introduces losses that p enalize the performance gains expected from theoretical considerations of thermodynamics cycle and component efficiencies alone. The paper p resents two possible turbofan design layouts intended to overcome the limitation of current turbofan power plant designs. The aim is to desi gn a power plant with the highest thrust per unit frontal area combine d with the highest air miles per gallon in cruise fright.