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Turbofan Blades

Thu Nov 18, 2004 1:19 am

The blades you can see spinning not so fast while at the airports, before taking off, do they provide any thrust? Even in flight they don't seem to spin fast as a prop.

Or just they breath the air to be compressed by the next stages?

Is it true that a (great?) portion of the breathed air bypasses the turbine in order to help cool it down at the exhaust section?


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RE: Turbofan Blades

Thu Nov 18, 2004 1:36 am

The fan provides most of the thrust of the engine, by far, and it does spin rather fast in flight.

Most of the air on a modern high-bypass-ratio engine goes outside the core (the bypass ratio is the mass ratio of bypass air to core air). It is not for cooling though. It does make the engine quieter, but the reason for bypassing the core is that it brings the engine efficiency up tremendously to accelerate a lot of air a little, as compared to accelerating a little air a lot.

However, a lot of the air which goes through the core (gas generator) does not take part in the combustion process but is used to cool burner liners, turbine etc. Getting this amount of air down is one of the challenges of making jet engines today.

This site contains more information, dunno if it is good but it should point you further if nothing else.

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RE: Turbofan Blades

Thu Nov 18, 2004 8:00 am

It's all about how much air mass you are moving how fast. You can choose to move a little air very fast or a lot of air less fast. The latter is how a turbofans work. So while they may not seem to move so fast, they are rather large.

In addition to Mr T's site, this should answer a lot:
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RE: Turbofan Blades

Fri Nov 19, 2004 9:09 am

Bypass Air provides 80% of the thrust, Core Thrust provides only 20%
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RE: Turbofan Blades

Thu Dec 09, 2004 10:52 am

To answer the question, the engines do provide thrust while on the ground; this is the thrust they use to taxi.
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RE: Turbofan Blades

Thu Dec 09, 2004 2:32 pm

When the engines are not active, such as at a gate, the large front fans will windmill with very little wind. I don't believe there's any sort of brake on the front fans. On very large engines, 747 class and larger, one can hear a rattle as the fans windmill. This is because the large fan blades are intentionally loose until centrifigal force at higher speed plus some heating seats the blades nice and tight.

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RE: Turbofan Blades

Fri Dec 10, 2004 4:05 am

Out of the 100% air coming out of the turbofan engine is the first 15% to atomize the fuel then another 30% air for enhancing combustion and the rest of the 55% air to cool the Turbine stages? Is this theory correct?

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RE: Turbofan Blades

Fri Dec 10, 2004 4:26 am

No, most of the air goes through the bypass duct. The rest will go through combustion and provide air for cooling and engine bleed.

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RE: Turbofan Blades

Fri Dec 10, 2004 3:58 pm

Depending on the Bypass ratio.
The Thrust is mainly generated by the Bypass Air which passes around the Core Engine.
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RE: Turbofan Blades

Fri Dec 10, 2004 4:55 pm

A bit of clarification is in order, I think. By far the largest portion of the air goes through the bypass duct on a modern engine. This is the most efficient means of generating thrust. Of the air that goes through the gas generator, or core engine, only part takes part in the combustion. Significant amounts are used for cooling combustion chamber liners, turbine blades etc. Yet more is bled off to provide the aircraft systems with pressurized air. All this air which does not take part in the combustion process represents an efficiency loss.

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RE: Turbofan Blades

Sat Dec 11, 2004 3:04 am

Common bypass ratios are 4:1 or 5:1...that ratio is bypass air to core air.

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