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I've read that bleeding air for the pneumatic system from the engine core can produce important losses in thrust, and also rising the fuel consumption too. Today this problem is worse, with the more uses of high-bypass turbofans engines, which have less air passing through the engine core.

How much engine thrust is lost due to bleed air?

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    $\begingroup$ Are you asking about energy loss to bleed air in general, or only about energy loss to bleed air routed to pneumatic systems? Compressor bleed air is used for many purposes including bearing systems, engine cooling, engine control, pneumatic systems. $\endgroup$ – J Walters May 11 '17 at 19:25
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For the Boeing 737-800:

Bleeds off:

enter image description here

If bleed air is turned off, it gives ~1% N1 increase. This higher N1 also means a 1 or 2 knot faster $V_1$ and/or $V_{mcg}$. Bleeds on adds about a 2.5-tonne correction to the gross weight.


For the various anti-ice:

enter image description here

The penalty can be as high as 5% N1. Note the single- and dual-sources figures. Dual means APU is running and is providing bleed air.

Exact loss in thrust (force)

That will be very hard to calculate without test bed data from the engine maker. Not to mention it varies with the different elements: pressure altitude, temperature, forward speed, etc. That's why I included the V1 and weight impacts.

Based on the graph below, 1% N1 at high takeoff power equates to about 2.4% of total thrust. That should be treated as a ballpark figure.

enter image description here
(Source)

In cruise however, having all anti-ice on might warrant flying lower due to the decreased thrust.

Other uses and alternatives

Bleed air used for cooling the turbine blades for example, helps increase thrust, not decrease, as the burners can run hotter without damaging the turbines.

The "all-electric" Boeing 787 uses bleed air for engine anti-ice. Using the generator to run the other anti-ice elements also reduces the thrust due to the shaft off-take. But, there is the benefit of weight reduction.

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    $\begingroup$ How much thrust is lost due to ~1% of N1 at takeoff power though? (since the question is how much thrust is lost...) $\endgroup$ – Lnafziger May 11 '17 at 21:38
  • $\begingroup$ I could be wrong, but I thought that 1% N1 at high power settings would be more than 1% of overall thrust from the engine since most turbine engines produce very little thrust at low power settings (it isn't linear). While it would be hard to calculate, I would assume that a rough percentage of thrust could be determined (we need one of the engineers to chime in! :) ) $\endgroup$ – Lnafziger May 12 '17 at 9:23
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    $\begingroup$ @Lnafziger - check now :) $\endgroup$ – ymb1 May 12 '17 at 10:41
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It's on the order of a few percent. Not huge but not negligible either.

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    $\begingroup$ This answer could be improved by providing more detail and sources. $\endgroup$ – fooot May 11 '17 at 16:32
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    $\begingroup$ It seems to be a more significant factor when one considers turboshaft powered aircraft. At least in the case of the old UH-60A Blackhawk helicopters which were (or are in the case of 1 remaining aircraft in the PR NG) powered by the GE T-700 engine. The activation of the engine inlet anti-ice, a bleed air pneumatic system, results in a loss of 16% available torque output. The UH-60L's GE T-701Cs addressed this issue but the mitigation only yielded a 2-3% available torque improvement. I suspect the greater realized loss of torque vs. thrust is explained by some insanely complex algorithm! $\endgroup$ – BigNutz May 12 '17 at 0:12

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