Science Fair Project Encyclopedia
High-bypass turbofan engine
History of turbofans
Early turbojet engines were very fuel-inefficient, as their compression ratio was limited. The Pratt & Whitney JT3C engine increased the compression ratio by introducing twin shafts, while the General Electric J79, a single-shaft turbojet, introduced variable compressor stators, which enabled the compressor to work at full efficiency at all aircraft speeds. All high-bypass turbofans use both multiple shafts and variable stators.
These improvements to compressor efficiency improved the thermodynamic efficiency of engines, but the propulsive efficiency was still poor. This results from the fact that pure turbojets have a low-mass, high velocity exhaust. The original low-bypass turbofan engines were designed to improve propulsive efficiency. As compressor design improved, more energy could be extracted from the turbines, making larger fans (and thus higher bypass ratios) practicable.
The first high-bypass turbofan engine was the General Electric TF39, built to power the Lockheed C-5 Galaxy military transport aircraft. The civilian General Electric CF6-50 engine used a related design. The tremendously higher thrust provided by high-bypass turbofan engines also made wide-body aircraft practical and economical, although the Soviet Union did build a low-bypass powered wide-body aircraft, the Ilyushin Il-86. In addition to the vastly increased thrust, these engines also generally quieter, and more fuel efficient. Other high-bypass turbofans are the Pratt & Whitney JT9D, the Rolls-Royce RB211 and the CFM International CFM56.
Developments in blade technology
The turbine blades are subject to high heat and stress, and require special fabrication. New material construction methods and material science have allowed blade designs to be made from polycrystalline (regular metal) to lined up crystals, so that when the blades heat up or cool, the crystals don't have to deform, to mono-crystalline blades.
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