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videns
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Jet Engine Thrust Percentage

Mon Aug 09, 2004 4:03 pm

How much thrust is generated by what parts of a jet engine?
Is most of the thrust coming from the "visible" blades?
I keep coming across explanations that say "most" thrust is generated by the (visible) engine blades...
What I'd like to know, is how much thrust each engine stage creates in terms of general engine performance.
Does it change depending on altitude or speed?
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LineMechQX
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RE: Jet Engine Thrust Percentage

Mon Aug 09, 2004 4:09 pm

On a modern turbofan engine most of your thrust is being produced by the fan blades you would see on the outside of an installed engine. Most of the air brought in by these blades "bypasses" the engine compression and combustion cycle completely. Therein lies the name high bypass turbofan. Not much different then a turboprop engine, where an even higher percentage of air that passes through the prop blades doesn't enter the engine. Yet a small amount of "turbo" thrust is still produced known basically as ESHP (equivalent shaft horsepower).

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Spitfire
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RE: Jet Engine Thrust Percentage

Mon Aug 09, 2004 5:19 pm

Let's say 80% of the thrust by the fan and 20 by the nozzle to have an idea.
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kalakaua
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RE: Jet Engine Thrust Percentage

Mon Aug 09, 2004 5:56 pm

The distribution of thrust forces can be calculated by considering each componet in turn. The thrust produced by the engine is mainly the product and the velocity increase imparted to it (i.e., Newtons 2nd Law of Motion), however, the pressure difference between the inlet to and the outlet from the particular flow section will have an effect on the overall thrust of the engine and must be included in the calculation.

Thrust = (Area of flow section x Pressure) + [Mass flow*Velocity of Flow/gravitational constant]

*units in English

When applying the above method to calculate the individual thrust loads on the various components it is assumemd that the engine is static.
Gravity explains the motions of the planets, but it cannot explain who set the planets in motion.
 
kalakaua
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RE: Jet Engine Thrust Percentage

Mon Aug 09, 2004 6:59 pm

Gravity explains the motions of the planets, but it cannot explain who set the planets in motion.
 
Yikes!
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RE: Jet Engine Thrust Percentage

Wed Aug 11, 2004 7:09 am

I posted an explanation and numbers for this question about 5 years ago and it can be found in a search, FWIW. The short answer is that in a modern high bypass engine, at takeoff thrust, about 75% of overall thrust comes from the fan; in cruise about 60% of the overall thrust comes from the fan. During reverse thrust on landing, the same 75% of the thrust comes from the cascading fan thrust. Core thrust is a factor but only about 1/2 of the fan's thrust.

Yikes!
 
Okie
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RE: Jet Engine Thrust Percentage

Wed Aug 11, 2004 9:05 am

The CFM56 a common turbine power plant, the 56 refers to 5.6 to 1 ratio of fan in relationship to the core.

I do not know if that refers to thrust ratio in terms of output or mass airflow ratio at the inlet.

1 / 5.6 = .17857 or roughly 18% thrust from the core or airflow going to the core however it is figured.

Okie
 
MITaero
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RE: Jet Engine Thrust Percentage

Wed Aug 11, 2004 10:33 am

You guys are talking about two different things here. Some are discussing component-by-component performance, and some are discussing bypass flow vs. core flow.

What Okie gave was a bypass ratio (mass flow ratio of bypass air to core air). It doesn't matter where it's measured, it's the same at the exit and the inlet due to conservation of mass. The momentum change of the core air is much greater, so expect the thrust ratio to favor the bypass air a bit less than the mass flow ratio does. This is consistent with Yikes's remarks.

In regard to the original post (and considering a modern turbofan), all air goes through the "visible" blades, but some of it passes into the core after being compressed by those visible blades. The air that doesn't (most of it) bypasses the combustion process and passes through a low pressure turbine (which drives the fan) before exiting.

The core air undergoes a stronger compression, then is combusted (where fuel is added), then is routed through a high-pressure turbine before being expelled at a much higher velocity than the bypass air.

What do you mean by "stage"?
 
splitzer
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RE: Jet Engine Thrust Percentage

Thu Aug 12, 2004 2:02 am

Minor correction - bypass air doesn't drive a turbine - wouldn't that be some kind of perpetual motion - with the fan powered by the turbine and the turbine by the fan? Where does the energy come from?  Big grin

It is driven by a low-pressure turbine spun by the core flow.
 
MITaero
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RE: Jet Engine Thrust Percentage

Thu Aug 12, 2004 6:13 am

Oh yeah, whoops. My bad. Thanks for the catch  Smile
 
XFSUgimpLB41X
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RE: Jet Engine Thrust Percentage

Thu Aug 12, 2004 6:45 am

On the CF34's that the CRJ has...its about 80% from the fan, and 20% from the core. I heard that the C-130 has that same ratio on it's prop haha. No wonder we poop out around 22,000.

Something less known, in cascading type thrust reversers, the only air going backwards is the bypass air. The air going through the core does not get deflected at all.
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phollingsworth
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RE: Jet Engine Thrust Percentage

Thu Aug 12, 2004 10:38 pm

Thrust = (Area of flow section x Pressure) + [Mass flow*Velocity of Flow/gravitational constant]

*units in English

When applying the above method to calculate the individual thrust loads on the various components it is assumemd that the engine is static.


Though for most commercial aircraft, which operate with subsonic exhausts (the exhaust would be sonic at most), the pressure terms is eliminated. It is actually (Pressure-exit - Pressure-ambient)*Exit Area; however, for subsonic flow the pressures will equal each other. Therefore, all thrust comes directly from momentum change.

Something less known, in cascading type thrust reversers, the only air going backwards is the bypass air. The air going through the core does not get deflected at all.

Not strictly complete. For separate flow turbo-fans this is the case. However, there have been aircraft with mixed flow turbo-fans that used cascade style reversers, e.g. the 727. These reversed both core and bypass flow. I don't know how the MD-90s thrust reversers work.

[Edited 2004-08-12 15:44:16]
 
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Starlionblue
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RE: Jet Engine Thrust Percentage

Thu Aug 12, 2004 11:53 pm

mixed flow turbo-fans

Huh? Ok, now someone has to explain to explain to me what this is  Big grin
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N328KF
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RE: Jet Engine Thrust Percentage

Fri Aug 13, 2004 12:23 am

The coolest one is the Pratt & Whitney JT11D. It generates 80% of the thrust at the nacelle!

[Edited 2004-08-12 17:24:30]
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phollingsworth
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RE: Jet Engine Thrust Percentage

Fri Aug 13, 2004 12:28 am

mixed flow turbo-fans

Huh? Ok, now someone has to explain to explain to me what this is.


If you are being serious and not making fun of my absurd separation of words (which I will admit has happened here). A mixed-flow turbofan is one where the bypass flow and the core flow are mixed prior to being exhausted through a nozzle. Most Low bypass TFs are mixed flow (JT8D, F404, F100, F101, F110, etc.), also the CFM56s on the A340's, The MD-90 motors, and the RB211s on the 757 are mixed-flow installations. This is compared to the typical high-bypass TF which is separate flow, different nozzles for the fan and the core flow, e.g. GE90, CF34, etc.

Mixed-flow TFs are generally more efficient but have a higher installed weight, greater duct losses, and require are more refined design as fan-duct and turbine exit pressures will have to match.
 
grandtheftaero
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RE: Jet Engine Thrust Percentage

Sat Aug 14, 2004 5:02 am

"The CFM56 a common turbine power plant, the 56 refers to 5.6 to 1 ratio of fan in relationship to the core."

Actually CFM56 is a meaningless acronym. As you might know CFMI, the company that produces the CFM56, is a 50/50 joint venture between GE and SNECMA. CFM56 is a combination of the names of engines that GE and SNECMA make, specifically the CF6 and the M56. True story!

--Shane
 
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N328KF
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RE: Jet Engine Thrust Percentage

Sat Aug 14, 2004 5:16 am

Yes, the PowerJet SaM146 name is pretty much the same thing.
“In the age of information, ignorance is a choice.”
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