bypass ratio
Quick Summary
The bypass ratio in an aircraft engine, such as a turbofan, is the measurement of how much air is directed around the engine's core compared to how much air passes through it. This ratio is a key factor in classifying turbofan engines.12
In a turbofan, air first passes through a fan, which partially compresses it. Most of this air then bypasses the engine core, creating "cold thrust," while a smaller portion enters the core for combustion, producing "hot thrust". The bypass ratio is the comparison of the mass flow rate of this bypassed air to the mass flow rate entering the core.12
Engines with higher bypass ratios are generally more fuel-efficient. However, very high bypass ratios can lead to larger, heavier fans, posing installation challenges and increasing complexity for features like thrust reversal. Despite these challenges, the trend in engine design is towards increasingly higher bypass ratios. Variable-cycle engines are being developed to adjust the bypass ratio for optimal performance across different flight speeds, from subsonic to supersonic.123
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