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Faro
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Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 9:20 am

On a bypass engine the fan has a net forward force acting on it due to all the bypass air passing unimpeded into the atmosphere. What about the compressor(s)? Do they also experience a net forward force or is the energy they extract from the turbines just sufficient to compress the air to the desired pressure and overcome friction losses?

Faro
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keta
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 11:23 am

The compressor does produce a forward force indeed. The thrust an engine provides is distributed across the different components, the following image shows this:



[Edited 2009-12-31 03:27:04 by keta]
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Faro
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 12:17 pm

Quoting Keta (Reply 1):
The compressor does produce a forward force indeed. The thrust an engine provides is distributed across the different components, the following image shows this:

Wow, thanx for the super diagram! There is a gross forward thrust generated by the compressor, yes, but the compressor/shaft/turbine assembly has a net rearward force acting on it from what I see. Therefore the compressor/shaft/turbine assembly does not contribute positively to the net overall forward thrust of the engine.

Also very interesting to see that for a nominal 11k lb thrust engine, the forces at play inside the engine are much greater, up to 58k lb gross forward force in this case. I wonder with beasts like the GE90-115B how high these gross forces can go...

Faro

[Edited 2009-12-31 04:21:39]
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dl757md
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 2:15 pm



Quoting Faro (Reply 2):
There is a gross forward thrust generated by the compressor, yes, but the compressor/shaft/turbine assembly has a net rearward force acting on it from what I see.

According to the diagram the forward component is 57,836 pounds while the rearward component is 46,678 pounds. That gives a net forward force (thrust) of 11,158 pounds.

On a turbofan the ratio of the volume of air that flows through the bypass too the volume of air that flows through the core is called the bypass ratio. Modern transport category turbofans are high bypass ratio which starts at a bypass ratio of 4:1. That means that for every 1000 ft³ of air that enters the inlet 800 ft³ will go through the fan only and 200 ft³ will go through the fan and the core (compressor, combustor, turbine). It is true we get a lot of thrust from the fan. In our 4:1 bypass ratio example we might get 67% of our thrust from the fan and the other 33% from the core. Therefor the thrust ratio would actually only be 2:1. We get more thrust in proportion to the amount of airflow from the core because we are adding heat (energy) to that airflow and accelerating it much faster than we are the bypass air.

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jetlife2
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 3:22 pm



Quoting Faro (Reply 2):
I wonder with beasts like the GE90-115B how high these gross forces can go...

You are correct that they get very high. These loads (amongst others) are used to size such things as the shaft/rotor wall thicknesses, the bearing housings, and consequently the frames that hold it all in place. Although, for each given component, the load that sizes it may be different : for example the limit load case in a bladeout is usually the thing that sizes frames.
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baroque
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 4:37 pm

Whittle makes the claim in his autobiog that the direction of thrust that the turbine folk at B T-H assumed for the turbines was incorrect. He does not elaborate, but if he was correct, how in heck did the turbine folk get their bearings to work?
 
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Faro
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 4:46 pm



Quoting DL757Md (Reply 3):
Quoting Faro (Reply 2):
There is a gross forward thrust generated by the compressor, yes, but the compressor/shaft/turbine assembly has a net rearward force acting on it from what I see.

According to the diagram the forward component is 57,836 pounds while the rearward component is 46,678 pounds. That gives a net forward force (thrust) of 11,158 pounds.

If you look closely at the diagram, the compressor only provides 19,049 lbs of forward thrust against 41,091 lbs of turbine drag at the end of the driveshaft. The remaining components of total forward thrust are provided by the diffuser (2,186 lbs) and the combustion chamber (34,182 lbs). The compressor/shaft/turbine assembly is therefore a net drag on the engine as a whole.

The proportion of forward thrust provided by these assemblies will conceivably vary greatly in turbofans and 2/3 spoolers.

Faro
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tdscanuck
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 5:54 pm



Quoting Baroque (Reply 5):
Whittle makes the claim in his autobiog that the direction of thrust that the turbine folk at B T-H assumed for the turbines was incorrect. He does not elaborate, but if he was correct, how in heck did the turbine folk get their bearings to work?

I suspect they used bearings that take thrust in both directions...you could get transient forces in either direction and a symmetric bearing seems like the easiest way to deal with it, especially in the early days when you're not 100% sure what's going on.

Quoting Faro (Reply 6):
The proportion of forward thrust provided by these assemblies will conceivably vary greatly in turbofans and 2/3 spoolers.

2/3 spool shouldn't make much difference (it's just pressure ratios that matter)...turbofans are best thought of as just having a really large first stage compressor for these types of discussions. All the compressor discs are fans that pull forward, all the turbines pull rearwards.

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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 6:26 pm

Thank you, all...until now I had read too much into the bypass ratio. Now, with the fairly high bypass ratio of the GE90 (9:1, was it?), what would the net thrust ratio be?

Taking DL757Md's example of a 4:1 bypass-ratio engine providing only a roughly 2:1 net thrust ratio, I'm assuming a roughly 4:1 or 5:1 net thrust ratio exists for 9:1 bypass-ratio engines, unless I'm mistaken?
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tdscanuck
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RE: Does The Compressor Provide Net Forward Thrust?

Thu Dec 31, 2009 7:53 pm



Quoting Western727 (Reply 8):
Thank you, all...until now I had read too much into the bypass ratio. Now, with the fairly high bypass ratio of the GE90 (9:1, was it?), what would the net thrust ratio be?

4:1 to 5:1 is about right. It will vary with throttle setting, since the core speed doesn't change as much with throttle as the fan speed.

Tom.

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