The majority of the air going through the bypass is producing most of the thrust and providing the air that cools the engine.

Does is also provide the pressurization and cooling/heating for the cabins?

What is the purpose of the air passing through the core? Is the objective to spin the fan rotor even though it still produces thrust? Does it provide air for any other purpose such as cooling?

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    $\begingroup$ Does this answer your question? How do jet engines work? $\endgroup$ – CatchAsCatchCan Nov 22 '20 at 21:44
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    $\begingroup$ A turbofan is a turboprop with a direct drive fixed pitch prop with lots of extra blades and a shroud around it. The core on the turbofan and the gas generator on the turboprop are more or less the same thing. The fan does the same thing the propeller does. $\endgroup$ – John K Nov 22 '20 at 22:15
  • $\begingroup$ @John K - Technically with the GTF, they're no longer Direct Drive. On the whole this is the case though. $\endgroup$ – Kaddyuk Nov 24 '20 at 22:06
  • $\begingroup$ Yes but a small number of the total so far. But actually, even more like a turboprop. $\endgroup$ – John K Nov 24 '20 at 22:17

The core is the Internal Combustion Engine - it has intake (of air), compression (of air), combustion (of the fuel/air mix), and exhaust (water vapor, CO2, air) that provides the motive force to rotate the turbine (N2) and provide perhaps 25% of the actual thrust of the whole engine when it exhausts out of the tailpipe.

From How Do Jet Engines Work? on this same forum, we get this diagram to aid us:


This core may also provide the motive force for electrical generators, hydraulic pumps, bleed air, ant-ice etc. (it would be inefficient and make no sense to get these from the fan air).

The thus driven turbine via it's shaft rotates the compressor blades at the front/intake of the core thereby completing a loop and making this cycle self sustaining as long as there's fuel to feed the engine.

The turbine rotation and the compressor rotation and the flow of air so induced also make the fan (bypass) part of the engine rotate (N1) through aerodynamic coupling. Thus the fan cannot function without the core, but the core can function without the fan.

So, yes, the objective is to drive the fan rotor, because the fan rotor by itself is not an engine and cannot self-rotate with purpose of producing thrust, but it is a device that can be driven by an engine to produce thrust, around 75% of the total thrust of a typical combination.

  • $\begingroup$ a yes would have sufficed $\endgroup$ – Crazy MANE41 Nov 26 '20 at 0:11
  • $\begingroup$ " . . . . even though it still produces thrust?" - this is what prompted the lengthier answer. I haven't understood this point, I guess. $\endgroup$ – skipper44 Nov 26 '20 at 5:17

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