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I have read somewhere regarding turbofan engine 2 spool that says

  1. "LP or HP turbine will drive the LP or HP compressor"

  2. "During engine starting, the starter will drive the HP compressor via the gearbox"

So I'm confused on whether the turbine or starter drives the compressor. Is it only during starting the starter will drive the HP compressor and after that turbine will take over?

Thanks

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    $\begingroup$ Your question is how (or if) the turbine takes over, hopefully the linked post, "How is bleed air used to start a jet engine?" answers that, if not please edit the question to clarify what is unclear. $\endgroup$
    – user14897
    Feb 7, 2019 at 16:07
  • $\begingroup$ @ymb1 - OP: "I'm confused on whether the turbine or starter drives the compressor", so isn't your proposal for a duplicate far fetched? The OP is asking about the mechanical link between the accessory box and the N2 spool, as answered by John K. $\endgroup$
    – mins
    Feb 7, 2019 at 17:32
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    $\begingroup$ @mins - I'm not sure what good would an extended discussion do. Anyway, on the contrary no, the post I proposed, unlike the other one or the answer, explains the disconnect mechanism of the starter. OP's confusion is confusing (as evident by the different interpretations), hence my proposal to revise the question if OP is still confused, as is standard procedure here. $\endgroup$
    – user14897
    Feb 7, 2019 at 17:37

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The starter drives the entire HP spool (HP compressor, interconnecting shaft, and HP turbine). The gearbox in question is the accessory gearbox that runs the various pumps and is driven off the HP spool shaft, which by happy coincidence is on the outside of the LP shaft. The starter is mounted on that gearbox and is able to back drive the HP spool through the gear train for engine starting.

The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start.

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  • $\begingroup$ Question 1 "The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start. " - does it mean that the LP shaft is connected to the HP shaft? So once HP shaft turns, LP shaft will turns too? Question 2 So once starter cuts off, everything will run on its own? Thanks $\endgroup$
    – sherline lin
    Feb 7, 2019 at 14:24
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    $\begingroup$ The LP and HP shafts have no mechanical connection. But the air displaced by the LP compressor will start turning the HP compressor. $\endgroup$
    – Hobbes
    Feb 7, 2019 at 16:07
  • $\begingroup$ That's what I meant by "going along for the ride". The LP shaft has a set of bearings that suspend it within the HP shaft and it is free to spin independently. Replace the fan at the front end with a gearbox and propeller, and you have a free-turbine turboprop. In a TP, the HP rotor (comp/shaft/turbine) and the burner can between them is called the "gas generator". On a turbofan it's called the "core". The starter system winds up the core to start the engine. The fan only starts to turn when the airflow through the engine starts spinning its turbine. $\endgroup$
    – John K
    Feb 7, 2019 at 17:03
  • $\begingroup$ So during starting, the HP spool will be turn by the starter then the air displaced by the HP compressor will start turning the LP spool? $\endgroup$
    – sherline lin
    Feb 8, 2019 at 1:11
  • $\begingroup$ To the extent that there is enough air moving past the LP turbine blades to make the disc start to spin, yes. Depending on the engine, you might see the fan start to rotate slowly on the flow past the LP turbine being created by the starter alone, but not really start to move until light off. $\endgroup$
    – John K
    Feb 8, 2019 at 1:37

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