Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Turbofans Thrust  
User currently offlineCaptjetblast From Argentina, joined Aug 2001, 281 posts, RR: 0
Posted (10 years 6 months 1 week 4 days 8 hours ago) and read 4662 times:

This is puzzling me.

How is most of the thrust generated in a turbofan (the most common turbine engine in use today)? By the inner fan (like a "ducted turboprop") or by the exhaust gases?

I know the fan gives extra cooling improving performance and extending engine life.



17 replies: All unread, jump to last
 
User currently offlineDufo From Slovenia, joined May 1999, 798 posts, RR: 4
Reply 1, posted (10 years 6 months 1 week 4 days 8 hours ago) and read 4630 times:

High-bypass fan engines produce 60-75% of the thrust by the front fan (technically this is called N1 rotor).


I seriously think I just creamed my pants without any influence from any outside variables.
User currently offlineUTA_FLYingHIGH From Tunisia, joined Oct 2001, 6495 posts, RR: 51
Reply 2, posted (10 years 6 months 1 week 4 days 8 hours ago) and read 4621 times:

From what I was taught during my internship at CFM :
80% of the thrust comes from the fan in itself, the 20% remaining being a bypass product of the exhaust.
The fan's primary use is to suck in air (duh).
Cheers,
UTA



Fly to live, live to fly - Air France/KLM Flying Blue Platinum, BMI Diamond Club Gold, Emirates Skywards
User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 3, posted (10 years 6 months 1 week 4 days 7 hours ago) and read 4608 times:

Thrust is created by the change in momentum of the air entering and exiting the engine. Momentum is the mass flow (lbs/sec) times the velocity (ft/sec). Since mass is conserved, (mass flow entering the engine = mass flow exiting the engine), the exit velocity must be greater than the inlet velocity to create thrust.

A true turbojet takes a small amount of air and accelerates it to a very high velocity to create thrust. A turbofan uses a different approach where a larger amount of air is accelerated only slightly to create the same amount of thrust.

There are two main reasons why we do this. One, since the exit velocity is much lower, the jet exhaust noise levels are much lower for a turbofan engine. Secondly, a lower exit velocity results in a much higher propulsive efficiency. A higher propulsive efficiency leads to a higher overall efficiency which leads to lower SFC.


User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 4, posted (10 years 6 months 1 week 4 days 6 hours ago) and read 4589 times:

Hola Captjetblast. The bypass engine is separate into two parts usually:

-The fan & the compressor

The airflow goes into the fan, which is the large fan you see at the intake of the engine. The flow then separates in two. One part of the flow goes past the engine internals and exits on the rear of the engine. This is the bypass airflow.

The other part of air goes into the compressor. The compressor is made up of several smaller "fans" close together. They lead the air into the combustion chamber. This stage simply compresses the air which results in a high temperature high pressure airflow.

Once the flow gets into the combustion chamber the fuel and air ingite, and it exits at a great velocity. The exhaust flow passes through a first turbine which is the High Pressure Turbine. The high pressure turbine is connected to the same shaft as the compressor, so they turn at proportional speed. Once the exhaust flow leaves the HPT, it enters the Low Pressure Turbine which in turn is connected to the same shaft of the fan.

The turbines take a lot of energy from the exhaust flow, but they spin the compressor and fan, so energy is sort of recycled. An engine is much more complex but this is a simple way of seeing it. In a summary, the LPT spins the fan, and the fan blows out bypass air, which generates a great amount of thrust, just like the "ducted fan" you say. The rest of thrust is produced by the exhaust hot gases blowing from the rear end of the engine.

On high bypass engines the bypass air generates almost all the thrust as asid on previous posts. Low bypass engines are usually noisier and most of the thrust is made by the 'dirty air', the one that goes into the compressor.

---

The hot exhaust air goes out through the protruding cone shaped part, and the fan air goes out through the ring-hole at the end of the engine case. I hope you're not more confused  Smile

View Large View Medium

Photo © William Jenkins



Bypass (fan) air exits around the hot air exhaust.

View Large View Medium
Click here for bigger photo!

Photo © Takashi Takahashi




Regards
-Alfredo


EDIT: I found the DC-10 photo later.

[Edited 2004-01-15 16:20:26]

User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 5, posted (10 years 6 months 1 week 4 days 5 hours ago) and read 4568 times:

I once read a description of a turbofan engine being little more than a turboprop engine with a shrouded, fixed-pitch propeller. It's actually a pretty accurate description. As mentioned above, the thrust from a turbofan engine comes from two different sources - the fan and the tailpipe. At low altitudes, the most of the thrust comes from the fan. At higher altitudes, the fan loses efficiency and a greater percentage of the thrust comes from the tailpipe. Also, you need to remember that (in very simplistic terms) turbine engines are not supercharged, but rather normally aspirated - in other words, they lose power with altitude just like a Cessna 152. The percentage of N1 (on most turbofans) or EPR (many turbojets) required to obtain the engine's full rated thrust will vary significantly depending upon airport elevation and outside air temperature.

Jetguy


User currently offline747Teach From United States of America, joined Nov 2000, 176 posts, RR: 1
Reply 6, posted (10 years 6 months 1 week 4 days 4 hours ago) and read 4521 times:

Captjetblast: The Pratt & Whitney customer training manual (page 2-11 and 2-78) says the JT-9D-7/A/F/J/R4 engine fan (called the first stage rotor by Pratt) produces 78% of total engine take-off thrust at sea level, standard day. Regards,

User currently offlineBruce From United States of America, joined May 1999, 5042 posts, RR: 15
Reply 7, posted (10 years 6 months 1 week 4 days 2 hours ago) and read 4487 times:

Click here to go to GE's engine page and click on "engines 101" to get a quicktime movie of a cutaway engine & how they produce thrust.

http://www.geae.com/education/index.html

this kind of illustrates some of the concept that Bio15 was talking about

bruce



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineCaptjetblast From Argentina, joined Aug 2001, 281 posts, RR: 0
Reply 8, posted (10 years 6 months 1 week 4 days 2 hours ago) and read 4485 times:

Your explanations were incredibly simple and comprehensive!

No more turbine engines puzzling me!

Now I must convince my wife to let me build a turbofan at home (ha ha!).



User currently offlineJetguy From , joined Dec 1969, posts, RR:
Reply 9, posted (10 years 6 months 1 week 4 days 1 hour ago) and read 4471 times:

"Now I must convince my wife to let me build a turbofan at home (ha ha!)."

Actually, there was a segment on (I believe) "Junkyard Wars" over the holidays where they had the teams build jet-powered dragsters from stuff out of a junkyard. One team made a turbojet engine using a turbocharger out of a diesel engine. It actually worked!

Jetguy


User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 10, posted (10 years 6 months 1 week 4 days 1 hour ago) and read 4458 times:

>Since mass is conserved, (mass flow entering the engine = mass flow exiting the engine), the exit velocity must be greater than the inlet velocity to create thrust.

Well.. if you neglect the mass flow of the fuel  Big grin

Just joking, great explanation DarkBlue

(sorry if I confused anyone, the fuel is pretty much negligible.. not like a rocket engine, where the fuel is the only thing being expelled)

And "Junkyard Wars" is cool.


User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 11, posted (10 years 6 months 1 week 3 days 7 hours ago) and read 4354 times:

Yep that's true MITaero, I shouldn't forget my assumptions. Another one not to forget is any bleed off-takes from the engine to the aircraft which pound-for-pound can be of similar magnitude as the fuel flow. Eventually any bleed air must find it's way back to the atmosphere, but not at jet exhaust velocities.

Anybody see "Monster Garage" this past week? They made a jet powered car by putting a Rolls-Royce Viper in a Toyota Celica. Even though they didn't have to build their own turbojet like on Junkyard Wars, it was still pretty cool.


User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Reply 12, posted (10 years 6 months 1 week 1 day 1 hour ago) and read 4225 times:

A turbojet can simply be thought of as a turbofan engine without the big front fan. Here is what puzzles me: Why is it so much louder? Is it because it has to work twice as hard as a turbofan to generate the same level of thrust.


Fly one thing; Fly it well
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 26
Reply 13, posted (10 years 6 months 1 week 1 day ago) and read 4221 times:

Why is it so much louder? Is it because it has to work twice as hard as a turbofan to generate the same level of thrust.

Depending on the size of the engine, not true. Turbofans are more efficient (they have a lower SFC, for reasons mentioned by me and others many times before) -- they don't necessarily produce greater thrust. That is for two engines of approximately the same size.
If other variables change, sure, turbofans can produce more thrust.

qantasA332


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 14, posted (10 years 6 months 1 week 23 hours ago) and read 4216 times:

Thrust,
mainly since there is no "wrapping" of slower fan air around the noisy core exhaust air.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 15, posted (10 years 6 months 1 week 21 hours ago) and read 4193 times:

Much of the noise of an engine is due to the exhaust flow. This sound is created from the friction of air moving at different velocities. In the case of a turbojet, the velocity of the exhaust air is much higher than the freestream air, creating a lot of friction and sound. As Fred stated above, a turbofan surrounds the core exhaust air with slower bypass air. The friction of the bypass air with both the freestream air and the core exhaust air is much lower, creating less sound.

User currently offlineThrust From United States of America, joined Sep 2003, 2688 posts, RR: 10
Reply 16, posted (10 years 6 months 5 days 19 hours ago) and read 4110 times:

Thanks for answering my question, guys. Appreciate it.


Fly one thing; Fly it well
User currently offlineBrons2 From United States of America, joined Sep 2001, 3007 posts, RR: 4
Reply 17, posted (10 years 6 months 4 days 16 hours ago) and read 4052 times:

One team made a turbojet engine using a turbocharger out of a diesel engine. It actually worked!

I tried this myself in my garage. There are a lot of websites about it if you do a search for home-made turbines. Actually, this is the site that inspired me http://www.reality.demon.co.uk/gasturb1.htm

I used a Garrett T56 turbo, a 50 gallon propane tank and a long slender metal vase as a combustor. I drilled a bunch of holes in the vase. I mounted a magnetic pickup on the turbo for N1, calibrated to 30,000 RPM for 100% N1 (measured as manually spun by an industrial electrical engine). Holes were drilled in the vase for air supply, and 2 car sparkplugs were used as igniters. The output of the turbine went into a tailcone of sorts, and I soldiered another propane quick connect on there.....for Afterburning!

I put hundreds of hours into getting this set up, mind you. It was mounted on a metal workbench which was bolted to the garage floor with 1" bolts into contrete.

I could get the thing started by rotating it to 11% N1 with the electrical motor and introducing a small amount of gas. I never could get it above 18% though cause the flame would extinguish. Even at that low setting, it burned through all the propane within 20 minutes of operation. (from what I have read online, those who acheived full throttle only lasted mere seconds to a few minutes. the thirst of the beast is prodigious).

I researched the web for a new combustor design. I think I drilled too many holes in the vase. Or maybe not enough. Regardless, when a large amount of propane was introduced, the flame went out from the velocity of the gas rushing through. (perhaps some sort of precombustion chamber would have been helpful)?

Then, I got laid off from my job and decided my efforts were better used elsewhere.



Firings, if well done, are good for employee morale.
Top Of Page
Forum Index

Reply To This Topic Turbofans Thrust
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
?'s About Turbofans/High-by-pass Ratios/Thrust posted Tue Jan 22 2002 08:12:34 by Lehpron
Thrust Levers Out Of Sync In Picture? posted Sat Nov 18 2006 06:25:35 by Jawed
Slat Retraction During Reverse Thrust? 747-400 posted Sun Nov 5 2006 00:29:24 by Ajaaron
Calculation And Application Of Reverse Thrust posted Wed Oct 25 2006 03:05:10 by HighFlyer9790
Dassault Falcon Thrust Reverser posted Sat Oct 21 2006 16:24:55 by Corsair2
BMW And Turbofans posted Sat Oct 21 2006 00:03:15 by Socal
757 Engine Thrust Comparison. posted Thu Oct 19 2006 05:09:36 by BOE773
Reverse Thrust On Props posted Wed Sep 27 2006 06:40:29 by AirWillie6475
Engine Thrust: Rating Vs. Real Differences posted Sun Sep 24 2006 20:49:42 by A342
Question About Thrust Reverser Types posted Tue Aug 15 2006 03:56:23 by YULspotter

Sponsor Message:
Printer friendly format