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Any further information is welcome, this is what I have learned so far:

Engines are designed to operate at a certain RPM (N1). The compressor consists of small 'wings' which are fixed in position, so the angle of attack will vary only by engine speed and forward speed. The engine design speed should be 85-95% N1 for most turbofan engines, this results in an optimum airflow through the compressor. Above or below a higher risk exists for an engine to stall.

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    $\begingroup$ The "wings" inside the engine are called "vanes". They are not as "fixed" as you may think, they float in the hub to allow for thermal expansion/contraction, but they do not vary the angle. $\endgroup$
    – Ron Beyer
    Jan 30, 2020 at 13:38
  • $\begingroup$ correct thanks, term fixed was referring to the angle of attack. $\endgroup$
    – Joren S
    Jan 30, 2020 at 14:16
  • $\begingroup$ @RonBeyer: Well, variable stators are fixed radially and also vary the angle of attack ;) $\endgroup$
    – user14897
    Jan 30, 2020 at 14:47
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    $\begingroup$ @RonBeyer: Many civilian turbofans have them, the CFM56 for instance, the #1 most selling turbofan. $\endgroup$
    – user14897
    Jan 30, 2020 at 17:22
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    $\begingroup$ @Vikki-formerlySean: But I didn't say "modern" :D $\endgroup$
    – user14897
    Mar 20, 2021 at 0:51

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Engine design speed is the shaft RPM and vane pitch which yields the rated thrust that the engine was originally intended to furnish. This will typically be quoted for takeoff at or near sea level at a standard ambient temperature (including a time limit to control maximum temperatures inside the engine) and for optimum cruise conditions at altitude (which will include specification of nominal fuel burn rate in pounds per hour).

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