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I read that the bleed flow negatively affects the thrust levels for a turbofan engine. However, I was wondering if vice-versa is true as well? Whenever the thrust levels for a turbofan are lowered at the same conditions, does the bleed flow value also decrease? What practically happens in an engine?

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  • $\begingroup$ It looks to be more of an on-demand supply than a fixed portion of the engine airflow, otherwise the passengers would not be ccomfortable during approacha md landing. Some quantifications in this answer and this one $\endgroup$
    – Koyovis
    Mar 22 at 13:42
  • $\begingroup$ Okay, so what I understand from this is that bleed flow (customer bleed) is dependent on the requirement and not a fixed value, and may change depending on the aircraft/engine operating thrust or power level (e.g. during LTO cycle). Have I got it right? $\endgroup$ Mar 22 at 14:32
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    $\begingroup$ When they say it negatively affects thrust they mean that taking bleed air results in less thrust being generated just like running the AC in your car “robs” the engine of power that could otherwise be delivered to the wheels. $\endgroup$
    – Jim
    Mar 22 at 16:39
  • $\begingroup$ Related $\endgroup$
    – Pondlife
    Mar 22 at 17:06

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Bleed air pressure is not directly linked to the passenger cabin.

Bleed air is used to power air cycle machines which provide cabin pressurization and temperature control. The bleed air supply is regulated so that a minimum constant supply is maintained over a wide variety of engine operating parameters.

Bleed air is also often used in other aircraft systems. Wing anti-ice, engine anti-ice, and engine starting are 3 more common uses.

On some aircraft, selecting engine anti-ice will automatically increase the flight idle speed to insure an adequate minimum bleed air pressure reaches the engine inlets.

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  • $\begingroup$ It is also sometimes run to the leading edges for anti-ice. $\endgroup$
    – Jim
    Mar 22 at 16:41
  • $\begingroup$ Certainly you would know your aircraft systems better than I, but it seems to me that in the planes I ride in most often (CRJ2) the cabin airflow is very much dependent on the rate at which the turbines are rotating. I feel the airflow increase and decrease as the engines are spooled up and down when taxiing, when the engines are brought back to idle for a descent, etc. $\endgroup$
    – randomhead
    Mar 22 at 20:10
  • $\begingroup$ I edited my answer to say “minimum constant supply”. Different aircraft behave differently at different power settings, and I can’t speak for all aircraft. The amount of air flow will change at different power settings, but even at idle power, there must be enough flow to keep the aircraft pressurized. $\endgroup$ Mar 22 at 21:32
  • $\begingroup$ Down at idle range you will feel the flow changes because the pressure regulating shutoff valve at the engine will eventually be full open to keep the duct pressure up, so any change in 10th stage bleed flow (what supplies air cond on the CRJ200) will be noticed in the cabin. But when it's modulating at higher thrust settings you don't have major flow changes as the valve keeps the bleed duct pressure at about 80 psi. regardless of the upstream pressure which will be well above that. $\endgroup$
    – John K
    Mar 23 at 17:29

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