High-bypass turbofan engines

Schematic diagram illustrating a 2-spooI, high-bypass turbofan engine with an

unmixed exhaust. The low-pressure spool is colored green and the high-pressure one purple. The fan is driven by the low-pressure spool.

The introduction of variable compressor stators enabled high pressure ratio com­pressors to work surge-free at all throttle settings. This innovation made its debut in the General Electric J79, a single-shaft turbojet for supersonic military aircraft. When variable stators were combined with multiple compressors, dramatic increases in over­all pressure ratio became possible. Coupling this with significant increases in fan mass flow, made the high-bypass turbofan' engine feasible. Bypass ratios of 5 or more are now common.

The tremendously higher thrust provided by high-bypass turbofan engines also

made civil wide-body aircraft practical and economical. In addition to the vastly increased thrust, these engines are also generally quieter. This is not so much due to the higher bypass ratio, but as to the use of low pressure ratio, single stage, fans, which significantly reduce specific thrust and, thereby, jet velocity. The combination of a overall pressure ratio and turbine inlet temperature improves thermal efficient together with a lower specific thrust (better propulsive efficiency), leads to a lower specific fuel consumption.

For reasons of fuel economy, and also of reduced noise, almost all of today’s jet airliners are powered by high-bypass turbofans. Although modern military aircraft tend to use low bypass ratio turbofans, military transport aircraft (e.g. С17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency.

Efficiently done, the resulting turbofan would probably operate at a higher nozzel pressure ratio than the turbojet, but with a lower exhaust temperature to retain net thrust. Since the temperature rise across the whole engine (intake to nozzle) would be lower, the (dry power) fuel flow would also be reduced, resulting in a better specific fuel consumption (SFC).

The Yakovlev Yak-42, a medium-range, rear-engined a ire raft seating up to 120 passengers was the first Soviet aircraft to use high-bypass engines.

The turbine blades in a turbofan engine are subject to high heat and stress, and require special fabrication. New material construction methods and material science have allowed blades, which were originally polycrystalline (regular metal), to be made fir lined up metallic crystals and more recently mono-crystalline (i.e. single crystal) blades which can operate at higher temperatures with less distortion.

Although turbine blade (and vane) materials have improved over the years, much of the increase in turbine inlet temperatures is due to improvements in blade/vane cooling technology. Relatively cool air is bled from the compression system, bypassing the