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General Electric GE90 Takeoff: 0.278 lb/lbf/h (28.3 kg/kN/h) Cruise: 0.545 lb/lbf/h (55.6 kg/kN/h) Source: https://en.wikipedia.org/wiki/General_Electric_GE90#Specifications

I assume this is typical for a modern HBPR turbofan.

But why?

Of course, as altitude rises, the reduced air density means the fan has less airflow, causing less thrust to be created. But then again, the bypass ratio does not change, so the core airflow also decreases by the same amount as fan airflow. So fuel flow should be decreasing too! So why is the TSFC TWICE as bad for cruise as takeoff? Is it because of losses in the fan duct increasing rapidly with speed?

(By the way, I know the higher speed helps reduce the seat-mile consumption, but still.)

EDIT: Wait what the? RB211, another HBPR turbofan, has almost no difference between cruise and "sea level" https://en.wikipedia.org/wiki/Thrust-specific_fuel_consumption

Abdullah is not an Amalekite
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1 Answers1

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Yes, that is typical for high bypass ratio (HBPR) turbofans.

But why?

Because the entry impulse goes up while the exit impulse stays roughly constant. Thrust is the difference between both, derived over time. The moving engine needs to slow down the airflow for combustion to take place, and then needs to accelerate the air by more than it has been slowed down to have positive thrust. Hence, SFC goes up in parallel with speed.

While the internal process inside the engine in flight is very similar to that at rest (only the pressure levels are increased by the ram effect of the moving engine and density drops with increasing altitude), thrust is reduced due to the smaller impulse difference in flight. Since a HBPR engine accelerates most of the air volume streaming through it by only a little, the average exit speed is relatively low compared to a jet engine with no bypass flow. This is similar (but less severe) to a propeller where thrust drops with the inverse of speed.

SFC gets even worse in supersonic flight. For a meaningful SFC comparison speed needs to be the same.

SFC of a turbofan with BPR 3

Thrust-specific fuel consumption in g/kN·s of a turbofan engine with bypass ratio 3. Y axis shows altitude in Meters. Isolines wer made with R contour function and should be more rounded in reality.

Peter Kämpf
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  • But why does the exit velocity remain constant? it would seem that as speed increases, the energy being added to the air stays constant, so the exit velocity relative to the ground should be static exhaust velocity + airspeed. – Abdullah is not an Amalekite May 09 '20 at 09:03
  • If the fuel flow remains same, the energy released in the engine should be same, but as you say, the airflow is being accelerated less at cruise, therefore the energy consumed by that process would seem to be decreasing, giving a lot of engine power, which to me would seem to make the engine want to increase RPM – Abdullah is not an Amalekite May 09 '20 at 09:06
  • Or put another way, at a stanstill, the angle of attack to the fan blades is maximum. At cruise, it decreases at constant rpm. – Abdullah is not an Amalekite May 09 '20 at 09:07
  • And if you kept the RPM same and increased airspeed further, the fan becomes a turbine! And yet the engine continues to burn takeoff amounts of fuel? – Abdullah is not an Amalekite May 09 '20 at 09:10
  • @ABJX: The intake ensures that the angle of attack of the fan blades does not change much. Only the pressure level does. The exit speed is limited by the maximum turbine temperature. Precompression heats the air entering the combustion chamber and raising the temperature to the maximum needs a bit less fuel but keeps the exit speed constant. – Peter Kämpf May 09 '20 at 09:12
  • Oh yes I forgot. The fan has a ram intake. – Abdullah is not an Amalekite May 09 '20 at 09:14
  • But still, you're saying that the fuel flow decreases a "bit". But the velocity at the fan face remains constant, so the acceleration by the fan.........only decreases a bit. And this acceleration is added to the velocity the air gains from ram pressure release when it is expanded through the fan nozzle at cruise. – Abdullah is not an Amalekite May 09 '20 at 09:18
  • @ABJX: Flow speed is reduced to almost zero in the diffusor. The compressor does not accelerate the air, it only compresses it, hence the name. In order for combustion to take place, fuel and air need time to mix and a higher velocity would simply blow out the flame. Remember that typical flame front speeds for air-hydrocarbon mixtures is only a few meters per second. – Peter Kämpf May 09 '20 at 09:38
  • I think you mistook the core compressor for the fan, which is what I was referring to. Though I must say that the core also seems to experience the same effect. – Abdullah is not an Amalekite May 09 '20 at 09:40
  • @ABJX: Ah, you mean the outer flow. Precompresison is drag – what is taken away is returned to the flow in the nozzle. Speed increase from the fan is roughly constant over flight speed. – Peter Kämpf May 09 '20 at 09:44
  • Let us continue this discussion in chat. – Abdullah is not an Amalekite May 09 '20 at 09:44