sudden
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Jet Engines (without The Fan)

Wed Feb 14, 2007 11:26 pm

As a jet engine is basically 2 engines in 1, my question is if you would be able to rotate, or cruise, with only the jet engine?

I know they work in harmony sort of, but still, would the Jet produce enough thrust to lift. or cruise, a modern airliner?

Can imagine that the fuleburn would be enormous if it was to work, but lets just leave that out for now.

Thanks.

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Sudden
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mrocktor
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RE: Jet Engines (without The Fan)

Wed Feb 14, 2007 11:31 pm

Not sure what you mean about two engines in one. If you are talking about the fact that modern turbofans have two rotating groups with concentric axles, then yes. Early jet engines had only one rotating group exactly as you describe.
 
chksix
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RE: Jet Engines (without The Fan)

Wed Feb 14, 2007 11:34 pm

The old low bypass engines (737-100, DC-9) were "pushing" through the power of the jet thrust only.

I think that at least 80% of the power comes from the fan on the modern engines, so removing the fan would make the engine very weak.

[Edited 2007-02-14 15:35:23]

[Edited 2007-02-14 15:35:47]
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airfoilsguy
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RE: Jet Engines (without The Fan)

Thu Feb 15, 2007 5:44 am

If you take the blades off of a turboprop you would get a similar scenario. Even at max thrust I don't think you would get far.
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KELPkid
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RE: Jet Engines (without The Fan)

Thu Feb 15, 2007 5:50 am

Quoting Airfoilsguy (Reply 3):
If you take the blades off of a turboprop you would get a similar scenario. Even at max thrust I don't think you would get far.

I was talking to a Cessna Caravan salesman at an airshow once. They had brought a Caravan on floats to the show, and I was talking to him about it (of course, I'd never in my wildest dreams be able to afford one  Wink ), and he mentioned that many floatplane pilots have a little difficulty transitioning to the floatplane version of the Caravan because with the throttle at idle, the engine still produces enough thrust to keep the Caravan in motion on the water...not sure if it's the idle thrust that the prop produces or the thrust coming out the PT6's exhaust...
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hangarrat
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RE: Jet Engines (without The Fan)

Thu Feb 15, 2007 6:03 am

Quoting KELPkid (Reply 4):
not sure if it's the idle thrust that the prop produces or the thrust coming out the PT6's exhaust...

I remember reading when the floatplane version of the Caravan was introduced that the tailpipe puts out about 40 pounds of thrust.
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speedracer1407
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RE: Jet Engines (without The Fan)

Thu Feb 15, 2007 1:58 pm

Quoting Chksix (Reply 2):
The old low bypass engines (737-100, DC-9) were "pushing" through the power of the jet thrust only.

The JT8-D on those planes were, as you say, low bypass engines, and had a bypass ratio of something around 1:1 give or take. Any turbo fan, including low bypass ones like the JT8-D, produce some of the thrust via the fan. Only turbojets rely on jet thrust only.
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Gemuser
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RE: Jet Engines (without The Fan)

Wed Feb 21, 2007 2:58 pm

I think what you are describing is a "turbo jet" in which ALL of the air which passed thru the engine entered the combustion camber, was burnt with fuel, then expelled out the exhaust. These were used on all the early airliners such as the Commet, B707-120 series and DC8s (early series). They were total fuel hoggs. They were replaced by the low by pass engines referred to in reply 6, where only some of the air entered the combustion chamber and the rest by passed it and went directly to the jet. Hence turbo jet, fan jet, low by pass ratio and high by pass ratio engines.

As a point of intrest the recently restored QF B707-138 VH-XBA was built with turbo jet engines, which were later replaced with early low bypass fan jet engines. When that was done it became a B707-138B.

Gemuser
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Scooter01
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RE: Jet Engines (without The Fan)

Wed Feb 21, 2007 5:26 pm

Quoting HangarRat (Reply 5):
I remember reading when the floatplane version of the Caravan was introduced that the tailpipe puts out about 40 pounds of thrust.

The RR Dart engines on the F.27 (depending on type) gave additional 350-500 lbst. from the exhaust at take-off power.

As a curiosity I can also mention that on the first two Bell 204 helicopters that came to Norway had the exhaust pointing sideways. I remember how the pilots were bragging about how much faster they had become after they were modified to having the tailpipe pointing straight back.

Scooter
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