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I just wanted to know how much air PER METER a GE90 turbofan engine sucks in considering the same thrust output at different altitudes to better understand how the engine can travel further while burning the same amount of fuel at cruise level.

As we all know, air gets thinner as we go higher. The engine operates at a certain air-fuel ratio, and that means that at ground level where air is thicker the engine needs to travel less metres to ingest enough air to combust a unit of fuel then when it is at, say, 35.000 feet.

I found elsewhere that the GE90 operates at an air-fuel ratio of 33,3:1 meaning that it needs 33,3kgs of air to combust 1kg of fuel.

What I'd like to know is how many metres it has to travel to ingest said mass of air at ground (sea)level with a density of 1,2985 kg/m3 at 19C° and at 11.000 metres with a density of 0,3639 kg/m3 at -56,5C° (values taken from the table at this page)

Can anybody provide any help?

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  • $\begingroup$ This has to be something to do with a university assignment. That data is so specialized you need to contact someone at GE tech support or their engineering organization that can help you out. $\endgroup$
    – John K
    Commented Jun 4, 2018 at 17:15
  • $\begingroup$ Can you expand on why you think an engine needs to travel to ingest air? $\endgroup$
    – user14897
    Commented Jun 4, 2018 at 17:59
  • $\begingroup$ The reason it can travel further with the same amount of fuel has to do with the drag of the aircraft being lower, it can go faster with the same thrust. $\endgroup$
    – TomMcW
    Commented Jun 4, 2018 at 18:53
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    $\begingroup$ I'm voting to close this question as off-topic because the premise of the engine needing to travel a certain distance to be able to consume air for combustion is incorrect, therefore any answer would have to rewrite the question entirely. $\endgroup$
    – Ron Beyer
    Commented Jun 5, 2018 at 1:04
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    $\begingroup$ The question is about the operation of an aircraft engine. It doesn't turn into another topic just because OP's assumptions are incorrect. It does make it unclear what is being asked, and that needs clarification. If he asked about mass flow instead of meters it might work. $\endgroup$
    – Pilothead
    Commented Jun 5, 2018 at 5:42

1 Answer 1

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From this answer a GE90:

has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350).

It doesn't need to move at all to have air flowing through it. At stand still, the rotating compressor blades will suck air in. The air pressure will decrease as it accelerates towards the engine, then as the compressor increases its pressure as it flows through the compressor. So, its pressure will rise, and increase significantly higher than the ambient air pressure.

When the engine is moving, the air isn't sucked in, rather, it is "force fed", or "rammed" into the engine (speaking generally). This means it usually actually slows down as it enters the engine, and so it's pressure rises in the intake even before it's encountered the fan blades. (Thus known as a ram pressure rise).

Thus, the question "how much air per meter does a GE90 consume" is not a valid question because the amount of air does not depend on the forward speed of the engine, it depends very largely on the rotational speed of the compressor (determined by the throttle position), and to a much lesser effect, the speed of the aircraft (also determined by throttle and other aspects such as its weight, if it's climbing or not).

Basically, an engine takes the mass flow it wants, not what it's provided by the forward speed of the aircraft. If it's not moving forward, it's provided with nothing, so it sucks in the air it wants. If the aircraft is moving forward, such as in cruise, the intake area mutiplied by the forward speed is likely providing more air than the engine needs, so the excess air is spilt out of the intake.

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