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Thrust From Bypass Air  
User currently offlineMidEx216 From United States of America, joined Jun 2005, 651 posts, RR: 4
Posted (7 years 2 months 2 weeks 1 day 22 hours ago) and read 6838 times:

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?


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29 replies: All unread, showing first 25:
 
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 1, posted (7 years 2 months 2 weeks 1 day 22 hours ago) and read 6827 times:

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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User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Reply 2, posted (7 years 2 months 2 weeks 1 day 19 hours ago) and read 6773 times:

I believe its as low as 8% on modern engines...


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User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 3, posted (7 years 2 months 2 weeks 1 day 18 hours ago) and read 6763 times:

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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User currently offlineTinPusher007 From United States of America, joined Feb 2005, 977 posts, RR: 1
Reply 4, posted (7 years 2 months 2 weeks 1 day 16 hours ago) and read 6729 times:

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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User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 5, posted (7 years 2 months 2 weeks 1 day 13 hours ago) and read 6690 times:

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.


DMI
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 6, posted (7 years 2 months 2 weeks 1 day 9 hours ago) and read 6659 times:

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.


User currently offlineTroubleshooter From Germany, joined Feb 2005, 423 posts, RR: 5
Reply 7, posted (7 years 2 months 2 weeks 1 day 7 hours ago) and read 6633 times:

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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User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 49
Reply 8, posted (7 years 2 months 2 weeks 1 day 1 hour ago) and read 6572 times:

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.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 9, posted (7 years 2 months 2 weeks 1 day ago) and read 6543 times:

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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User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 10, posted (7 years 2 months 2 weeks 1 day ago) and read 6539 times:
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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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User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 11, posted (7 years 2 months 2 weeks 23 hours ago) and read 6529 times:

The Bristol Proteus is another example of a reverse flow turboprop. Even more convoluted gas path than the PT-6.


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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 12, posted (7 years 2 months 2 weeks 23 hours ago) and read 6526 times:

Cool. And what's the reason for this reverse flow stuff?


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User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 49
Reply 13, posted (7 years 2 months 2 weeks 22 hours ago) and read 6519 times:

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]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17069 posts, RR: 66
Reply 14, posted (7 years 2 months 2 weeks 18 hours ago) and read 6471 times:

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!



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 15, posted (7 years 2 months 2 weeks 17 hours ago) and read 6461 times:

On the CRJ's engine.. the CF34... bypass air accounts for roughly 85% of total thrust.

User currently offlineMarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 16, posted (7 years 2 months 2 weeks 6 hours ago) and read 6409 times:

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.

User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 17, posted (7 years 2 months 2 weeks 3 hours ago) and read 6374 times:

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!


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 18, posted (7 years 2 months 1 week 6 days 14 hours ago) and read 6296 times:

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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User currently offlineRwessel From United States of America, joined Jan 2007, 2368 posts, RR: 2
Reply 19, posted (7 years 2 months 1 week 6 days 13 hours ago) and read 6287 times:
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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.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (7 years 2 months 1 week 6 days 10 hours ago) and read 6267 times:

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.


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 49
Reply 21, posted (7 years 2 months 1 week 6 days 7 hours ago) and read 6247 times:

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.


User currently offlineDeltaGuy From , joined Dec 1969, posts, RR:
Reply 22, posted (7 years 2 months 1 week 6 days 7 hours ago) and read 6244 times:

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


User currently offline2enginesonly From Netherlands, joined Jun 2005, 91 posts, RR: 0
Reply 23, posted (7 years 2 months 1 week 6 days 6 hours ago) and read 6229 times:

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.

Arjan


User currently offlineYYZYYT From Canada, joined Apr 2005, 971 posts, RR: 0
Reply 24, posted (7 years 2 months 1 week 6 days 1 hour ago) and read 6185 times:

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.

25 Post contains links Jetlagged : 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 t
26 Lehpron : That is only due to the added mass of fuel per unit mass of intake air (meaning specific thrust). I didn't. A bypass of 9 would create a 1-(1/9) = 88
27 Tdscanuck : 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 v
28 Mrocktor : No, it is not only because of that. I've explained it twice already. A bypass of 9 means 9 mass units of air through the fan for each mass unit of ai
29 MarkC : 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
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