Bypass air accounts for _____% of total thrust?

• A. 60%
• B. 70%
• C. 80%
• D. 90%

Answer: (C)

In a turbofan engine, the majority of thrust comes from the bypass air—the air that flows around the engine core through the fan duct and exits the nozzle. Although the core gases leave at a higher speed and contribute significant thrust, the mass flow through the bypass is much larger, so the momentum change of that large stream dominates the total thrust.

Because of this, modern high-bypass turbofans typically derive roughly four-fifths of their thrust from the bypass air and the remaining from the core exhaust. That makes 80% a common, practical figure for many airliner engines, including ones used on planes like the CRJ-550. Keep in mind the exact percentage can vary with engine design and operating conditions, but 80% is a solid, representative value.