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I have been understanding more about engines lately, but I can't figure out what the section in the middle of a turbofan is for:

enter image description here

What benefit does this part provide to rotating the air? Is it some kind of separate fan or something?

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    $\begingroup$ By "the weird silver thing", do you mean the hub that all the blades are connected to? $\endgroup$ – egid Sep 3 '15 at 2:21
  • $\begingroup$ @egid yes I do mean that $\endgroup$ – Ethan Sep 3 '15 at 2:23
  • $\begingroup$ @Ethan. Before you edited the question, I thought you were referring to the spirals. $\endgroup$ – aeroalias Sep 3 '15 at 4:01
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    $\begingroup$ And, before anyone asks why there's a spiral painted on it. $\endgroup$ – FreeMan Sep 3 '15 at 12:40
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egid is right. It is called spinner.

The spinner basically covers the part of the hub where the compressor blades are attached and reduces drag.

CFM56 Spinner

Source: b737.org.uk

It also serves other purposes:

  • Diverting the airflow efficiently in the center of the engine so that the airflow is smooth over the compressor blade hub

enter image description here

  • More importantly, the spinner serves to divert the rain and (more importantly) hail impinging on it towards the bypass fan and away from the 'core' of the engine, reducing chances of engine flameout.

The second point an important design parameter for the spinner. According to AGARD-AR-332, Recommended Practices for the Assessment of the Effects of Atmospheric Water Ingestion on the Performance and Operability of Gas Turbine Engines,

An effective way to remove a sizable amount of hail from the core is to optimize the spinner shape to deflect hail upon impact directly towards the fan duct.

The spinner of the CFM56 engine was modified in response to the engine flameout experienced in Garuda Indonesia Flight 421. The shape was changed into a conelliptical profile.

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  • $\begingroup$ You can also add that the spiral mark on the spinner is there to produce a spinning white visible circle that scares birds away. $\endgroup$ – Alexus Sep 3 '15 at 17:18
  • $\begingroup$ @ Alexus See aviation.stackexchange.com/questions/12806/… $\endgroup$ – aeroalias Sep 3 '15 at 17:44
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That's called a spinner, and it's not that weird! You see them on propellers and turbofans alike.

Spinners are a streamlined shape, and provide aerodynamic drag reduction to what would otherwise be a very draggy, complex part of the powerplant. Here's how a propeller looks underneath:

Cutaway of controllable pitch prop

Perhaps that can explain why streamlining the middle of a propeller (whether a turbofan disc or not) might be beneficial to performance. PeterKämpf would better be able to address the aerodynamics, but I am under the impression that cleaning up the airflow at the center of the propeller or fan improves efficiency of the blades as they are encountering less turbulent airflow.

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