7
$\begingroup$

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

$\endgroup$
  • $\begingroup$ 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 '17 at 23:34
  • 3
    $\begingroup$ Aviation.SE is the right place to ask this. $\endgroup$ – Mo_ Jan 10 '17 at 23:44
  • 2
    $\begingroup$ @alephzero Please don't post answers in comments. $\endgroup$ – rob Jan 10 '17 at 23:55
  • $\begingroup$ When you say "how much" do you mean by mass or by volume? $\endgroup$ – TomMcW Jan 11 '17 at 1:47
  • 1
    $\begingroup$ @SRawes Put that back in :) $\endgroup$ – TomMcW Jan 11 '17 at 16:04
5
$\begingroup$

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).

enter image description here
CFM56-5C, source.

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

T/O vs cruise power and fuel consumption

According to this document, the CFM56-5C T/O thrust is 150 kN and the cruise thrust is 40 kN (other sources states 30 kN), 27% of the T/O thrust. Related fuel consumption are 2.3 and 0.5 kg/s, meaning cruise fuel consumption is 22% of the T/O fuel consumption.

Air mass vs fuel mass

For a fuel consumption of 22% of the T/O consumption, it can be assumed relatively safely that the air flow is in proportion, 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

$\endgroup$
  • 1
    $\begingroup$ will that not be different at a cruising altitude @mins $\endgroup$ – SRawes Jan 11 '17 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 '17 at 16:54
  • 2
    $\begingroup$ @PJNoes: This is the total air mass thru the fan. After the fan, air is separated in two flows: core (hot/primary) and bypass (cold/secondary) according to the bypass ratio, here 6.4:1. So the part used in the core for combustion, bleed air and core cooling is 485/7.4 = 66 kg/s and 419 kg/s are bypassing the core. $\endgroup$ – mins Jan 11 '17 at 18:49
  • $\begingroup$ @SRawes: Yes, this is the maximum thrust/airflow. I have updated the answer. $\endgroup$ – mins Jan 11 '17 at 19:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.