In a turbofan engine, the bypass ratio is defined as the ratio of the mass airflow through which paths?

• A. Ratio of thrust to weight.
• B. The ratio of the mass airflow through the gas generator to that through the fan.
• C. The ratio of the mass airflow through the fan to the mass airflow through the gas generator.
• D. The ratio of fuel flow to air flow.

Answer: (C)

Bypass ratio tells you how much air goes around the engine core versus through it. In a turbofan, air enters the engine and splits: part flows through the core where fuel is burned, and part flows around the core via the bypass duct. The bypass ratio is the mass flow rate of the bypass air divided by the mass flow rate of the core air (gas generator). So the correct description is the mass airflow through the fan (bypass path) to the mass airflow through the gas generator (core path). The other ideas don’t describe these two distinct air paths: thrust-to-weight is a different performance metric, fuel-to-air is a combustion ratio, and using the inverse would describe core air relative to bypass air, not bypass relative to core.