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Turbofan Blades  
User currently offlineCaptjetblast From Argentina, joined Aug 2001, 281 posts, RR: 0
Posted (10 years 1 month 5 days 23 hours ago) and read 5780 times:

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?


10 replies: All unread, jump to last
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (10 years 1 month 5 days 23 hours ago) and read 5756 times:

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.


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17178 posts, RR: 66
Reply 2, posted (10 years 1 month 5 days 17 hours ago) and read 5671 times:

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: http://travel.howstuffworks.com/turbine.htm

"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 3, posted (10 years 1 month 4 days 15 hours ago) and read 5534 times:

Bypass Air provides 80% of the thrust, Core Thrust provides only 20%

"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineNewark777 From United States of America, joined Dec 2004, 9348 posts, RR: 29
Reply 4, posted (10 years 2 weeks 14 hours ago) and read 5237 times:

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

Why grab a Heine when you can grab a Busch?
User currently offlineMERSPACE From United States of America, joined Mar 2004, 56 posts, RR: 0
Reply 5, posted (10 years 2 weeks 10 hours ago) and read 5201 times:

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.


User currently offlineSanthosh From India, joined Sep 2001, 545 posts, RR: 1
Reply 6, posted (10 years 1 week 6 days 20 hours ago) and read 5112 times:

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?

Happy Landing
User currently offlineStaffan From , joined Dec 1969, posts, RR:
Reply 7, posted (10 years 1 week 6 days 20 hours ago) and read 5103 times:

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


User currently offlineHAWK21M From India, joined Jan 2001, 31712 posts, RR: 56
Reply 8, posted (10 years 1 week 6 days 9 hours ago) and read 5009 times:

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

Think of the brighter side!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 9, posted (10 years 1 week 6 days 8 hours ago) and read 5004 times:

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.


I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineModesto2 From United States of America, joined Jul 2000, 2819 posts, RR: 5
Reply 10, posted (10 years 1 week 5 days 21 hours ago) and read 4966 times:

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

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