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SESK: Stuff Everyone Should Know · sesk.info/aero-turbofan-bypass

In a turbofan engine, what does the bypass air do?

It makes extra thrust, cools the engine and makes it quieter.

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  1. It feeds the fire in the combustion chamber
  2. It makes extra thrust and helps quiet the engine ✔
  3. It powers the cabin lights

Why: A big fan pushes extra air around the engine's core. That bypass air adds thrust, cools the engine and lowers noise, so turbofans are quieter and use less fuel than turbojets.

Huh, didn't know that: The bypass ratio compares air going around the core with air going through it.

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Why are turbofans so cool? They have tons of fans.
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References

Answer: It makes extra thrust and helps quiet the engine. A big fan pushes extra air around the engine's core. That bypass air adds thrust, cools the engine and lowers noise, so turbofans are quieter and use less fuel than turbojets.

  1. FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C) www.faa.gov/regulations_policies/handbooks_manuals/aviation/faa-h-8083-25c.pdf
    The turbofan bypass air generates increased thrust, cools the engine, and aids in exhaust noise suppression.
    Checked 2026-10-10.

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