Bypass ratio is the ratio of bypass air to air that flows through the HP compressor. A high bypass engine produces most of its thrust via the large fan at the front, accelerating a large mass of air around the side of the engine core, with a small proportion of the air flowing into the core to provide the power to the gas generator section. The air that flows through the core is counted at the HP compressor section.
Bypass air mass flow = inlet mass flow – HP compressor mass flow
Bypass ratio = bypass air mass flow / HP compressor mass flow
Bypass mass flow = 400 kg/s – 80 kg/s = 320 kg/s
Bypass ratio = 320 / 80 = 4
The bypass ratio of this engine is therefore 4:1, also written as 4/1 or just the value 4.
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