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I want to start out by saying these questions will make me look a fool but i simply haven't found answers for them anywhere so i'm hoping you could help me!

  1. What is the purpose of bypass ducts? If the air is going around the turbine then what even is the point? Why not just make jet engines simple consisting only of the turbine and skip out on the bypass duct?

  2. What makes the fan blades in a jet engine keep spinning? I know how they are started but what keeps the blades constantly spinning with enough inertia to keep compressing air? Is there like a small motor connected to the fan blades via a crankshaft or is a portion of bleed air used to keep the blades in motion?

Thanks in advance for answering my questions :)

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    $\begingroup$ for 1. did you have a look here aviation.stackexchange.com/q/24536/1467 ? for 2. you can start here: aviation.stackexchange.com/q/25916/1467 $\endgroup$
    – Federico
    Commented Sep 11, 2017 at 16:54
  • $\begingroup$ 1. Yeah then it's a turbojet not a turbofan or jetfan. Turbojet has bad fuel economy in the sub sonic range compared with turbofan and jet fan. Essentially the nozzle only is not the most efficient device to convert thermal energy to thrust. 2) Not a portion bleed air, but all of the exhaust gas. $\endgroup$
    – user3528438
    Commented Sep 11, 2017 at 18:00
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    $\begingroup$ My lord, I didn't know you were interested in aviation! :P $\endgroup$
    – TomMcW
    Commented Sep 11, 2017 at 21:51

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Well, let's tackle the second question first: What keeps the fan spinning? It is connected by a shaft to the low pressure turbine and the gasses expanding from the high pressure turbine towards the outside keep it spinning. Look at a good diagram and you'll see two (or three) concentric shafts that connect the low pressure turbine and the fan and the high pressure turbine and the compressor.

As for why bypass, pure jet engines where all the air goes through the combustors are really fuel hungry. You have to have a certain air/fuel ratio to get combustion. You can't lean it beyond a certain point or the fire goes out. Using all the air would use really a lot of fuel. The high-bypass ratio turbofan only takes some of the air through the combustors and just sucks the rest in with the fan and blows is out the back. The difference between that and a turbo-prop is mostly the cowling around the fan.

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