luisde8cd
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Turbofans Vs Turbojets

Tue Dec 14, 2004 10:39 am

Can anyone explain the difference between these two kind of engines? are current engines either a turbofan/turbojet or those types were only used to designate 1st generation jets?

Thanks for the input!
 
TWAMD-80
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RE: Turbofans Vs Turbojets

Tue Dec 14, 2004 11:42 am

I'll give this a shot. A pure turbojet engine is one that doesn't have any bypass air. An example would be the engines on an old DC-9. A Turbofan is a turbojet engine that has a fan strapped to the front of it. Some of the air from the fan is ducted around the center turbojet section.

In addition to that you have low-bypass turbofan engines and high-bypass turbofan engines. An example of a low-bypass fan engine is an MD-80's JT8D. There is not a lot of air that bypasses the actual turbojet section. A good visible example of a high-bypass fan engine is the GE fan on the 747-400. If you look in pictures you can see a gap between the jet section and the engine cowling - this is where the bypass air flows through. Hope this helps.


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EMBQA
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RE: Turbofans Vs Turbojets

Tue Dec 14, 2004 11:43 am

I guess the short answer would be with a Turbojet..100% of the air passes through the engine giving you the thrust. With a Turbofan only 20% of the air actually passes through the core engine. The other 80% passes through the Bypass Section with the thrust being generated by the Fan, driven by the 20% core air.
"It's not the size of the dog in the fight, but the size of the fight in the dog"
 
luisde8cd
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RE: Turbofans Vs Turbojets

Tue Dec 14, 2004 1:08 pm

Now I get it. Thanks for the quick replies!
 
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Starlionblue
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RE: Turbofans Vs Turbojets

Tue Dec 14, 2004 1:45 pm

A pure turbojet engine is one that doesn't have any bypass air. An example would be the engines on an old DC-9.

All DC-9s have had Turbofans. For pure turbojets, early marks of the 707 and DC-8 are an example.



I guess the short answer would be with a Turbojet..100% of the air passes through the engine giving you the thrust. With a Turbofan only 20% of the air actually passes through the core engine. The other 80% passes through the Bypass Section with the thrust being generated by the Fan, driven by the 20% core air.

The proportions don't have to be 80/20. Many early turbofans had less than half of the air as bypass air. Only later models have had proportions of 80/20 or so. The important thing, as you say, is that in turbofans there is a bypass air component.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Newark777
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 10:07 am

On the FAA's N-code look-up site (http://162.58.35.241/acdatabase/nnum_inquiry.asp), they list airliners' engine type as turbo-jet, even though the planes are obviously turbo-fan. Do they simply say all jet aircraft are equipped with turbo-jet as a means of saying it is a jet powered aircraft, or is this a mistake by the site?
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QantasA332
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 10:19 am

Good catch, Newark777. Turbojets and turbofans should indeed be differentiated on there, though I guess it's not of that much importance for a rego database...

Cheers,
QantasA332
 
411A
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 12:00 pm

Early turboJET engines such as the Pratt & Whitney JT4A-17 used on the B707-320 had lots of thrust, with very good high altitude performance, but suffered from poor runway performance, as all the air went thru the core.
Early turboFAN engines (JT3D-3B for example) used on the 707-320B had much better runway performance, but above FL370, suffered from rather large altitude thrust loss, as the fan was optimused for lower altitude performance.

Later designs of course improved on this and in addition, turboFAN engines have better specific fuel economy.
 
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HAWK21M
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 7:12 pm

Examples would be:-
JT3Ds of the B707s were Turbojets.
JT8Ds of the B732s were Low Bypass Turbofans
CFM56s of the B737NG were High Bypass Turbofans

Turbjets:- 100% of the Air sucked in thru the inlet is used for combustion
Turbofans:- Part of the Air sucked in thru the inlet is used for combustion the rest is used as bypass air,mainly generates the thrust.
In Low Bypass the ratio of core Air to Bypass air would be 1:1,in High bypass it can be 5:1.Speaking of the Examples above.

What is the Bypass ratio for the GE90s.
regds
MEL
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aloges
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 10:53 pm

"What is the Bypass ratio for the GE90s."

According to a not-so-convincing source, a GE90-115B has a bypass ratio of an impressing 9:1. Still very much possible, but the site didn't look that professional.
Don't cry because it's over, smile because it happened.
 
air2gxs
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RE: Turbofans Vs Turbojets

Wed Dec 15, 2004 11:33 pm

Hawk,

One correction: on a pure turbojet (JT3) 100% of the air is not used for combustion. 100% goes down the core. I'm not sure of the ratio, but a substantial component of core air is called secondary air. This is the air that is used to cool the liner and act as a buffer between the flame front and the liner. And let's not forget bleed demands from the aircraft and the internal cooling needs of the engine.

As an instructor of mine once said: (to paraphrase), you can't breathe the exhaust from a 4 cycle engine, but if you can stand the heat and taste, you can breathe the exhaust of a pure jet.
 
Tarantine
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RE: Turbofans Vs Turbojets

Thu Dec 16, 2004 1:59 am

Also, a fanjet makes much less noise than a turbojet; this was a BIG deal in the early 1960s, probably more so than fuel efficiency.
 
timz
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RE: Turbofans Vs Turbojets

Thu Dec 16, 2004 2:53 am

"JT3Ds of the B707s were Turbojets."

JT3Cs, you mean. And JT4As. JT3Ds and JT8Ds (pre -200 series JT8Ds that is) had bypass ratios around 1:1.
 
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HAWK21M
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RE: Turbofans Vs Turbojets

Fri Dec 17, 2004 5:24 pm

Air2gxs....You are correct.It would be less than 100%.
Timz....What was the Differences between the JT3C & 3D.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
timz
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RE: JT3C

Sat Dec 18, 2004 2:29 am

The original 707-121, -123, -124, -131, -138 and -139 had JT3C turbojets, rated 13000 to 13500 lb thrust with water injection. Pan Am's -121s and AA's -123s were all converted to JT3D turbofans, rated 17000 (?) lb with no need for water injection. All 707s built after... 1962? had turbofans.
 
philhyde
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RE: Turbofans Vs Turbojets

Sat Dec 18, 2004 4:31 am

I stumbled across this site a while back. It's a little funky, but the information is good.

http://www.thaitechnics.com/engine/engine_intro.html
Canon junkie - Aviation Nut

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