MidEx216
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Thrust From Bypass Air

Mon Aug 13, 2007 5:51 am

I was wondering, how much thrust does an average airliner's turbofan engine get from the bypass air that is not combusted?

In much the same way, how much does thrust does an average turboprop get from the combustion cycle, as compared to that coming off the prop? And is just me, or does it seem like they might be able to get more if the exhaust was better placed?
 
SlamClick
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 6:16 am

On high bypass turbine engines a large majority of the thrust comes from the fan discharge and the minority from the "core" air. Doubtless someone better informed than I am will pop up with some valid numbers soon.

As an illustration of just what the ratios are, note that the CFM-56 as installed on, say a B-737-400 only reverses the fan air and not the core or exhaust efflux. As a result when you spool up in reverse thrust the engine core is still providing its forward thrust. Even though reverse thrust is not terribly efficient the forward thrust of the core air is not hard to overcome, giving a net "reverse" thrust vector.

A high-bypass (any modern engine) is, in effect, a ducted turbofan.

Can't speak much to turboprop. I am sure that most produce a modest amount of thrust from the exhaust.

I've flown larger recips (piston) with "augmenter tube" exhaust. It was always asserted that the augmenter tube produced "measurable" thrust but I don't recall ever getting anything more specific than that.
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kaddyuk
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 9:32 am

I believe its as low as 8% on modern engines...
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lehpron
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 10:03 am

Quoting MidEx216 (Thread starter):
I was wondering, how much thrust does an average airliner's turbofan engine get from the bypass air that is not combusted?

Almost all of it, the functional purpose of the core is to run the turbofan. If the bypass ratio was 10, that means ten times that of the flow going through the core goes around the core (bypassing) from the turbofan itself, resulting in 9% core thrust and 91% bypass thrust.

Turboprops have super high bypasses, above the teens. Problem is that bypassed air doesn't have the higher pressure thrust like high bypass jets, resulting is lower cruise speeds. But, high bypass does have better fuel efficiency, which is why turboprops have better fuel economy at lower than jet speeds.

[Edited 2007-08-13 03:11:46]
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tinpusher007
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 12:09 pm

Quoting MidEx216 (Thread starter):
I was wondering, how much thrust does an average airliner's turbofan engine get from the bypass air that is not combusted?

Its pretty commonly accepted that high-bypass turbofans will produce up to 80% of rated thrust off of the fan, leaving about 20% from the core. Of course these number could slightly more or less than 80/20.
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pilotpip
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 3:22 pm

Turboprops don't get much from the exhaust. If I recall, the exhaust from the TPE-331s on the King Air 100 accounts for about 30-40 hp.
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tdscanuck
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 7:16 pm

Quoting MidEx216 (Thread starter):
In much the same way, how much does thrust does an average turboprop get from the combustion cycle, as compared to that coming off the prop? And is just me, or does it seem like they might be able to get more if the exhaust was better placed?

All the power comes from the combustion cycle, but I assume you meant exhaust flow. On a PT-6 I believe you get something like 500 lbs thrust from the exhaust (depending on the model).

Yes, you could get more thrust from some turboprop designs if you changed the exhaust arrangement. However, the thrust from the exhaust is so small that it's probably not worth the additional weight and complexity in the exhaust system. You'd be better off investing that engineering effort in extracting more power to the prop.

Tom.
 
troubleshooter
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RE: Thrust From Bypass Air

Mon Aug 13, 2007 9:19 pm

Quoting MidEx216 (Thread starter):
And is just me, or does it seem like they might be able to get more if the exhaust was better placed?

On a turboprop the turbine nearly extracts all the energy from the gas to drive the prop (and the engine incl. accessories). This results in a very small remaining thrust at the exhaust. I don´t have exact figures at hand but I would say it is less than 10%.
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mrocktor
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 3:01 am

Quoting Lehpron (Reply 3):
If the bypass ratio was 10, that means ten times that of the flow going through the core goes around the core (bypassing) from the turbofan itself, resulting in 9% core thrust and 91% bypass thrust.

Not quite. The core accelerates that mass a lot more than the fan does, so one mass unit of core flow produces more thrust that one mass unit of bypass air.
 
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Starlionblue
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 4:10 am

Quoting SlamClick (Reply 1):
Can't speak much to turboprop. I am sure that most produce a modest amount of thrust from the exhaust.

I have read on this board that some turboprops have the turbine mounted backwards. Is this true?
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2H4
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 4:30 am

Quoting Starlionblue (Reply 9):
I have read on this board that some turboprops have the turbine mounted backwards. Is this true?

The PT-6 is commonly referred to as a "reverse-flow" engine. The cycles are no different than any other turbine engine, it's just that the intake air flows backwards relative to the fuselage. In other words, the air enters the engine at the point closest to the trailing edge of the wing, and exits at the point closest to the leading edge of the wing:




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Jetlagged
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 4:46 am

The Bristol Proteus is another example of a reverse flow turboprop. Even more convoluted gas path than the PT-6.
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Starlionblue
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 4:58 am

Cool. And what's the reason for this reverse flow stuff?
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mrocktor
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 5:40 am

Quoting Starlionblue (Reply 12):
Cool. And what's the reason for this reverse flow stuff?

Not having concentric shafts. Reverse flow means you can have the "cool" portion of the engine in the back and the "hot" portion (HPT and LPT) in front. This means the LPT can drive the prop directly without the drive shaft having to go through the combustion chamber, compressor, etc.



[Edited 2007-08-13 22:42:58]

[Edited 2007-08-13 22:44:58]
 
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Starlionblue
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 10:35 am

Quoting Mrocktor (Reply 13):

Not having concentric shafts. Reverse flow means you can have the "cool" portion of the engine in the back and the "hot" portion (HPT and LPT) in front. This means the LPT can drive the prop directly without the drive shaft having to go through the combustion chamber, compressor, etc.

Makes a lot of sense. Thx!
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flyf15
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 11:04 am

On the CRJ's engine.. the CF34... bypass air accounts for roughly 85% of total thrust.
 
MarkC
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RE: Thrust From Bypass Air

Tue Aug 14, 2007 10:13 pm

Even the old JT9-7R4 running only 5:1 bypass ratio would do 46,000 lbs forward, 36,000 reverse. This would roughly translate to 41K for the fan, and 5K for the core, but its probably a bit less for the core because reverse is not as efficient. The object is to extract the work out of the core, and it does a good job of it.
 
baroque
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 1:11 am

Somewhere you should be able to read of the successful attempts to recover useful thrust out of the radiator systems on the Merlin Spits. Between the exhaust ports and the radiators I think they got somewhere in the 100 to 300 lbs range. IIRC it was enough to add 10 to 20 mph to the top speed - in other words useful. Someone has probably not archived their books as deeply as I have might be able to tell you.

Quoting Mrocktor (Reply 13):
Not having concentric shafts. Reverse flow means you can have the "cool" portion of the engine in the back and the "hot" portion (HPT and LPT) in front. This means the LPT can drive the prop directly without the drive shaft having to go through the combustion chamber, compressor, etc

Thanks, I always wondered the reason for this apparent eccentricity. Now I know!
 
lehpron
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 2:28 pm

Quoting Mrocktor (Reply 8):
Quoting Lehpron (Reply 3):
If the bypass ratio was 10, that means ten times that of the flow going through the core goes around the core (bypassing) from the turbofan itself, resulting in 9% core thrust and 91% bypass thrust.

Not quite. The core accelerates that mass a lot more than the fan does, so one mass unit of core flow produces more thrust that one mass unit of bypass air.

Yes the core flow velocity is faster due to being hotter, but

Quote:
Bypass ratio is a ratio of mass flow rate which goes around the core and mass flow rate going through the core.

Hll Peterson. (1992). Mechanics and Thermodynamics of Propulsion. 2nd ed. Ch 5.5 p184.

mass flow rate is m-dot, which is the area * density * velocity. While core has high velocity, the fan section has a greater area CS = more thrust comes from the bypass flow, period.
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rwessel
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 3:28 pm

Quoting Lehpron (Reply 18):
Yes the core flow velocity is faster due to being hotter, but

Only in an indirect sort of way. While the heating of the core flow certainly increases its volume, that only results in a higher speed exhaust if the cross sectional area is not expanded proportionately. In fact, in many turbojets, the exhaust cross section is *smaller* than the input cross section, and the exhaust velocity is basically proportional to (To/Ti)*(CSo/CSi). Assuming, of course a proper expansion of the flow so that you don't waste energy dumping a pressurized stream out the back end.

The high velocity core flow is not very efficient for subsonic propulsion. The only reason it's not further reduced (by adding an additional turbine stage, for example), is that you've hit the point of diminishing (or even negative) returns as the available energy gets more diffuse (and thus harder to extract), and that less efficient energy extraction system has to compete with the easy to use residual core thrust. Note that a turboshaft engine (optimized for that application), will naturally tend to have more LP turbine stages driving the shaft, since any extra energy in the core flow is pure waste (and in many non-aviation applications the extra weight is a non-issue).

Quoting Lehpron (Reply 18):
mass flow rate is m-dot, which is the area * density * velocity. While core has high velocity, the fan section has a greater area CS = more thrust comes from the bypass flow, period.

If the (mass) bypass ratio is 3:1, and the core flow velocity is more than three times the velocity of the bypass flow, the core flow will generate more thrust.
 
tdscanuck
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 6:13 pm

Quoting Rwessel (Reply 19):
If the (mass) bypass ratio is 3:1, and the core flow velocity is more than three times the velocity of the bypass flow, the core flow will generate more thrust.

That is true. However, in practical turbofan engines, the speed difference between the core and fan flow isn't nearly that large since the vast majority of the thrust really does come from the fan.

Tom.
 
mrocktor
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 9:13 pm

Quoting Lehpron (Reply 18):
mass flow rate is m-dot, which is the area * density * velocity. While core has high velocity, the fan section has a greater area CS = more thrust comes from the bypass flow, period.

More thrust comes from the bypass flow, no one said otherwise. Per unit mass, however, more thrust comes from the turbojet. It is not a large difference, because:

Quoting Rwessel (Reply 19):
The high velocity core flow is not very efficient for subsonic propulsion.

The only thing this means is that you cannot say 90% of thrust comes from the fan if BPR is 9. Yes, 90% of the mass is going through the fan, but since the fan accelerates this mass less than the core, it produces a little less than 90% of the total thrust.
 
DeltaGuy
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 9:29 pm

Quoting Tdscanuck (Reply 6):
However, the thrust from the exhaust is so small that it's probably not worth the additional weight and complexity in the exhaust system.

The most it's good for on smaller turboprops like a Meridian is merely for staining the fuselage from nose to tail.  Smile
A more useful application for the exhaust would be to drive an accessory- seeing as a small turboprop like a PT-6 doesn't have a ton of room for a bleed air-driven accessory.

DeltaGuy
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2enginesonly
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RE: Thrust From Bypass Air

Wed Aug 15, 2007 10:25 pm

The thrust of the fan from the average turbofan engine ( B747/B767/MD11 ) is around 75-80%.......roughly.
On the Fokker 50 the engine delivered 2500hp ( PW125 ) and the exhaust delivered around 300lbs of thrust....not that spectacular  Smile
I presume similar turboprops like the ATR and Dash 8 have similar figures.

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YYZYYT
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RE: Thrust From Bypass Air

Thu Aug 16, 2007 2:59 am

No one has yet mentioned that high bypass engines reverse thrust by redirecting bypass air only. Thus when thrust is used at landing the core air will thrust forward but it is more than offset by reversed by-pass air.
 
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Jetlagged
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RE: Thrust From Bypass Air

Thu Aug 16, 2007 7:59 am

Quoting MarkC (Reply 16):
Even the old JT9-7R4 running only 5:1 bypass ratio would do 46,000 lbs forward, 36,000 reverse. This would roughly translate to 41K for the fan, and 5K for the core, but its probably a bit less for the core because reverse is not as efficient. The object is to extract the work out of the core, and it does a good job of it.

That calculation doesn't look right. You appear to have taken forward thrust and reverse thrust and assumed the difference is the core thrust times two.

Reverse net thrust is much less than forward thrust for a number of reasons. Firstly the compressor discharge pressure limit is reduced in reverse which means max EPR/N1 achievable is less. Secondly the fan airflow is deflected forwards at an angle, not directly forwards. Thirdly, the core airflow is still producing forward thrust, offsetting the reverse effect of the fan.

Also I doubt the -7R4 could achieve 36,000 lb net thrust in reverse. A CF6-50 (roughly comparable) has a net reverse thrust of about 7,000 lb when static, around 14,000 lb at 100 knots, 90% N1. The PW4158 produces 4,000 lb reverse thrust at 0 knots, 12,000 at 100 knots, 90% N1. (Figures from Airbus Takeoff Safety Training Aid for the A300B4 and A300-600, which can be found at http://www.smartcockpit.com/pdf/flightops/safety/0002/ ).

With a bypass ratio of 5:1, you would expect core thrust to be about 16.67% of the total, based on mass flow alone, assuming the exhaust velocity of the core and bypass is the same. Actual core jet velocity will be higher than fan airflow velocity, so the core thrust will be a higher percentage than that.
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lehpron
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RE: Thrust From Bypass Air

Thu Aug 16, 2007 9:56 am

Quoting Mrocktor (Reply 21):
Per unit mass, however, more thrust comes from the turbojet. It is not a large difference,

That is only due to the added mass of fuel per unit mass of intake air (meaning specific thrust).

Quoting Mrocktor (Reply 21):
The only thing this means is that you cannot say 90% of thrust comes from the fan if BPR is 9

I didn't. A bypass of 9 would create a 1-(1/9) = 88% bypass; a bypass of 10 would allow 1-(1/10) = 90% bypass -- ideally.
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tdscanuck
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RE: Thrust From Bypass Air

Thu Aug 16, 2007 3:42 pm

Quoting YYZYYT (Reply 24):
No one has yet mentioned that high bypass engines reverse thrust by redirecting bypass air only.

That's not always true. The PW2040 can do full reverse of both fan and core flows. This capability is used on the C-17. The same engine (commercial version) is used on the 757 but they don't bother reversing the core flow in that application. I'm not aware of any commercial airplane that reverses the core flow on a high-bypass engine.

Tom.
 
mrocktor
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RE: Thrust From Bypass Air

Fri Aug 17, 2007 4:05 am

Quoting Lehpron (Reply 26):
That is only due to the added mass of fuel per unit mass of intake air (meaning specific thrust).

No, it is not only because of that. I've explained it twice already.

Quoting Lehpron (Reply 26):
I didn't. A bypass of 9 would create a 1-(1/9) = 88% bypass; a bypass of 10 would allow 1-(1/10) = 90% bypass -- ideally.

A bypass of 9 means 9 mass units of air through the fan for each mass unit of air though the core. IF both produced the same mass specific thrust as YOU are saying, that would mean the fan would be producing 9/(9+1)=90% of the thrust.

But the core produces more mass specific thrust than the fan.
 
MarkC
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RE: Thrust From Bypass Air

Fri Aug 17, 2007 7:13 am

The 2040 cannot reverse the core. No commercial 2000 can. Its only the F117-PW-100. It is essentially a late build PW2000 with a few differences, the most visually obvious being the core reversing tailcone in place of the normal fairing. Its still bolted to the same exact TEC with the same number of bolts, even though it weighs a heck of a lot more, and transmitts load.

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