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How many kg of air enters an average CFM56 turbofan engine cruising per minute?

I have looked everywhere and cannot find it or figure out how to find it, if you could either tell me or give advice on where or how to find it I would appreciate it. Thanks

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  • $\begingroup$ A reasonable first approximation would be the mass of air that the engine flies through in a minute - i.e. frontal area of the fan blades $\times$ aircraft velocity $\times$ air density. As a sanity check on your estimate, though at different conditions where the inlet velocity is not equal to the aircraft speed, at takeoff each engine on a B747 generates about 200kN of thrust, with an air mass flow of about 1 tonne (1000kg) per second. $\endgroup$
    – alephzero
    Jan 10, 2017 at 23:34
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    $\begingroup$ Aviation.SE is the right place to ask this. $\endgroup$
    – Mostafa
    Jan 10, 2017 at 23:44
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    $\begingroup$ @alephzero Please don't post answers in comments. $\endgroup$
    – rob
    Jan 10, 2017 at 23:55
  • $\begingroup$ When you say "how much" do you mean by mass or by volume? $\endgroup$
    – TomMcW
    Jan 11, 2017 at 1:47
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    $\begingroup$ @SRawes Put that back in :) $\endgroup$
    – TomMcW
    Jan 11, 2017 at 16:04

1 Answer 1

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Airflow at takeoff

The CFM56-5C, the largest CFM56, powers A340. Its thrust is about 145 kN at takeoff. It has an air mass flow of 485 kg/s (source). A A340 has four engines.

enter image description here
CFM56-5C, source.

Fortunately this thrust is not needed after takeoff and the engine airflow is considerably lower in cruise. I don't have the actual figure but we can do a quick estimate using the fuel consumption. It can be assumed relatively safely that air is in proportion of fuel.

Fuel consumption

According to this document, for the CFM56-5C:

  • T/O thrust is 150 kN, fuel consumption 2.3 kg/s
  • Cruise thrust is 40 kN, fuel 0.5 kg/s.

(meaning cruise at 27% T/O thrust requires 22% of the T/O fuel, the engine is more efficient in altitude.)

Airflow in cruise

So we can roughly estimate air mass flow in cruise to be about 485 * 0.22 = 107 kg/s.

This is supported by figures found online in non authoritative sources, like page 245 of this book:

enter image description here

Bypass flow vs core flow

After the fan, air is separated in two flows: core (hot/primary) and bypass (cold/secondary) according to the bypass ratio.

The CFM56-5C bypass ratio is 6.4:1. So:

  • The part used in the core for combustion, bleed air and core cooling is 485/7.4 = 66 kg/s

  • The part bypassing the core, and providing only thrust by fan acceleration is 419 kg/s.

For comparison the Trent XWB (A350 XWB), a larger engine developing 430 kN at takeoff, has an air mass flow of 1,440 kg/s (source).

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    $\begingroup$ will that not be different at a cruising altitude @mins $\endgroup$
    – SRawes
    Jan 11, 2017 at 16:00
  • $\begingroup$ Is this figure the amount passing through the intake, or the amount passing through the combustion chamber? Can you break down the two for a feel for how much air is bypassed? $\endgroup$
    – PJNoes
    Jan 11, 2017 at 16:54
  • $\begingroup$ @SRawes: Yes, this is the maximum thrust/airflow. I have updated the answer. $\endgroup$
    – mins
    Jan 11, 2017 at 19:33
  • $\begingroup$ The specific fuel consumption at cruise you give is suspect. The thrust-specific fuel consumption of high-bypass turbofans should increase with speed. This table gives $33.7\ \frac{\mathrm{g}}{\mathrm{Nh}}$ static and $55.6\ \frac{\mathrm{g}}{\mathrm{Nh}}$ at cruise. The power-specific fuel consumption still decreases, so propulsive efficiency increases, but 27% thrust for 22% fuel seems wrong. $\endgroup$
    – Jan Hudec
    Jun 21, 2021 at 23:19
  • $\begingroup$ @JanHudec: Thanks for the remark. The different sources I use are obviously not related to the same CFM56-5C customer version. However, the variations are not either extremely large, and the approximation seems to be still correct. So except for the 27% vs. 22% non essential comment, I believe air quantities shown 485/107 kg/s are correct (I'll update them if someone can point me to complete CFM56-5C specifications). $\endgroup$
    – mins
    Jun 22, 2021 at 11:24

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