Fan Air or Bypass Air
The larger air stream exiting the fan, representing 85 percent of the total, is called fan air or
bypass air, because it bypasses this entire process.
The engine itself is shrouded in a metal casing called the nacelle, shaped roughly like a
sideways ice cream cone with the bottom cut off. Bypass air is forced through the ever narrower
space between the nacelle wall and the engine, picking up speed along the way.
Because of its huge volume, bypass air needs only to accelerate a small amount to produce
an enormous kick of thrust. In the PW4084 engine, bypass air accounts for 90 percent of the
thrust, and has the added benefits of keeping the engine cooler, quieter and more fuel efficient.580 Chapter 17 Engineering Materials
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