Bypass air from the N1 fan produces approximately what percent of the engine thrust?

• A. 60%
• B. 80%
• C. 40%
• D. 20%

Answer: (B)

In a turbofan, most of the engine’s thrust comes from the bypass air that the fan pushes around the core. This large mass of air is accelerated by the fan to a moderate exit velocity, and even though it isn’t as hot or fast as the core exhaust, the sheer mass flow makes its momentum change contribute the majority of thrust. The core exhaust, while hot and fast, carries less mass, so its overall thrust share is smaller. For modern high-bypass turbofans, the bypass stream typically provides about 80% of total thrust, with the core contributing the remaining ~20%. So the bypass air from the N1 fan is responsible for roughly 80% of engine thrust.