AirWillie6475
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Airliner Engine Question

Fri Jun 24, 2005 7:46 am

Why is it that when you look at an engine from the back when the plane is taking off, there is no fire or flames like in the fighter jet engines? With all that combustion in the engines why don't we see any flames or redness at the back, how is it just black?
 
flyf15
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RE: Airliner Engine Question

Fri Jun 24, 2005 8:08 am

The flames you are seeing is the afterburner. To put it simply, afterburners spray raw fuel into the exhaust of the engine, drastically increasing thrust. They are usually only found on fighters but are occasionally on other aircraft requiring extra thrust during certain phases of their flight (B-1, Concorde, ..). If the afterburner is not in use, the engine exhaust will look just like that of an airliner, no flames.
 
320tech
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RE: Airliner Engine Question

Fri Jun 24, 2005 8:18 am

You don't see any flames because they aren't long enough. A jet engine has four sections, generally speaking. There's the intake, the compressor section, the combustion section, and the turbine and exhaust. The combustion section is where the flames are. The turbine and exhaust section is quite long, and of course the turbine has several rows of blades (rotors and stators) that will block the view into the rear of the combustion section.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
KFLLCFII
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RE: Airliner Engine Question

Fri Jun 24, 2005 8:30 am

Because conventional airliners don't have afterburners. The Concorde, on the other hand, does have afterburners which inject fuel directly into the hot exhaust gas after the normal combustion process, and is visible since it occurs in the exhaust nozzle:


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Photo © Ian Nightingale



Here is a great shot showing the afterburner itself:


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Photo © Den Pascoe



Just in front of the last turbine blades, there is a metal ring that lies about halfway between the center dome and the outside wall. This is from where the fuel is injected into the exhaust.

If you look closely in this shot, you can see that ring in action spraying fuel into the hot gas:


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Photo © Voortmans Dirk



And that's it!

For an expanded explanation, see:
http://en.wikipedia.org/wiki/Afterburner



PS: You actually don't even need an afterburner to shoot flames from your engine. All you have to do is become an MD-11 with a serious problem:


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Click here for bigger photo!

Photo © M.Oertle

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efohdee
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RE: Airliner Engine Question

Fri Jun 24, 2005 1:08 pm

In a regular non afterburning turbine engine the expanding gases lose temperature when passing through the stages of turbine rotors and nozzles. By the time the gas exits through the exhaust is is too cooled to be incadesant, but it is still very hot for us humans!!
 
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HAWK21M
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RE: Airliner Engine Question

Fri Jun 24, 2005 2:20 pm

Quoting KFLLCFII (Reply 3):
PS: You actually don't even need an afterburner to shoot flames from your engine. All you have to do is become an MD-11 with a serious problem:

Any Idea on the MD-11 Snag.What was the reason.Presuming the Aircraft returned back normally.Any Link/Details.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
KFLLCFII
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RE: Airliner Engine Question

Fri Jun 24, 2005 3:18 pm

Quoting HAWK21M (Reply 5):
Any Idea on the MD-11 Snag.What was the reason.Presuming the Aircraft returned back normally.Any Link/Details.

Oh I have no idea. I just remembered the pic, and though it was somewhat relevant to this thread. I seriously doubt anything became of it, as I don't remember hearing of any VASP MD-11 accidents.. Sorry I don't have anymore details.
"About the only way to look at it, just a pity you are not POTUS KFLLCFII, seems as if we would all be better off."
 
HAL
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RE: Airliner Engine Question

Fri Jun 24, 2005 5:26 pm

There is at least one modern fan engine where you can see a red glow, but only at night. When I taxi behind a 737 with CFM engines there is a dull red glow when you look into the turbine section. It isn't flames, but the glow of heat from the combustion section. It gets a little brighter as they power up to move after being stopped, but it is dim enough that you'd never see it during the daytime.

Other engines however are completely dark, even at night. I've looked at the IAE engines on A320's, RB211 on 757's, and PW 4060's on 767's, and there is no detectable glow there.

HAL
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northseatiger
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RE: Airliner Engine Question

Fri Jun 24, 2005 5:43 pm

Also if you carry out a wet start, you can get some pretty mean flames, I know a guy who was doing some mx and didn't dry up all the fuel down the side of the A/C, he then had a major wet start, huge flame and then the side of the A/C caught fire momentarilly !!
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727EMflyer
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RE: Airliner Engine Question

Fri Jun 24, 2005 10:11 pm

Yet to be mentioned, afterburners are fuel wasters! Unless you need the power to go supersonic you would be a fool to use an afterburner.

At the ripe old age of 11 I flew a UA 732, sitting behind the engine, and was highly dissapointed that I didn't see any flame. The thrust reverser deploying made up for it though!
 
efohdee
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RE: Airliner Engine Question

Sat Jun 25, 2005 5:13 am

I have seen the hot red glow in the CFM-56!! Also when taxiing the 737, if you look right up the exhaust so can see the last stage/ exit guide vanes glwing red hot!! Maybe someone can snap a good photo for the database.
 
bohica
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RE: Airliner Engine Question

Sat Jun 25, 2005 11:10 am

Ride on a BAE-146 at night. Get a window seat toward the back of the plane and you can see up the back of the inboard engine on your side. It will turn a reddish orange color on takeoff and will be white during cruise. It's really neat to see that. Depending where you're sitting, you might also be able to see the same thing on the outboard engine.
 
Matt72033
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RE: Airliner Engine Question

Sat Jun 25, 2005 6:53 pm

is this the glow of the combustion chamber? or just an optical illusion?

http://www.boeing.com/companyoffices/gallery/images/c17/c1705.htm

[Edited 2005-06-25 11:55:12]
 
jeb94
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RE: Airliner Engine Question

Sat Jun 25, 2005 7:13 pm

That is an illusion. The glow you're seeing with the globemaster is in the bypass duct. The engine core is the dark area in the center. These engines aren't running as the airforce certainly wouldn't have equipment and vehicles the close to the inlets and crew men just standing around in the blast area of the exhaust.
 
Silver1SWA
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RE: Airliner Engine Question

Sat Jun 25, 2005 7:22 pm

Quoting Efohdee (Reply 11):
I have seen the hot red glow in the CFM-56!! Also when taxiing the 737, if you look right up the exhaust so can see the last stage/ exit guide vanes glwing red hot!! Maybe someone can snap a good photo for the database.

I have never seen this before. Maybe the ramp is too bright? This is interesting. I will be paying closer attention.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
 
jeb94
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RE: Airliner Engine Question

Sun Jun 26, 2005 5:03 am

They can do that. If for some reason the flame goes out momentarily and then restarts after the fuel mist builds up you will get a huge flame shoot out of it. That is how tail pipe fires start one the ground during start operations. I've been witness to a ten foot flame from the APU exhaust of a DC9 (too much fuel in the combustion liner) and 40 to 50 foot flames out of JT8Ds, both the little ones and the big 200 series. That photo is not fake and of course meant an engine change.
 
ArmitageShanks
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RE: Airliner Engine Question

Sun Jun 26, 2005 5:27 am

I've seen a 747 engine spew fire like that after a failure on take off, too.
 
air2gxs
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RE: Airliner Engine Question

Sun Jun 26, 2005 5:30 am

Yes, I've witnessed engines torching before. I've seen tailpipe fires and am well aware of the mechanism which causes them to occur.

Jeb,

I see your're an AMT. Please explain to me how a FADEC controlled CF6 installed on an MD11, on takeoff, can produce that well defined flame.

Compressor stall? No.
Flame-out and relight? Doubt it. Engines at TO power rarely flame-out and if they do, it is usually a fuel starvation issue. No fuel for a re-light.
Trashed turbine? Lots of junk out the back end, but rarely fire. The engine instantly ceases to turn, which brings the fuel pressure down to boost pump pressure. Boost pump pressure produces a dribble at the nozzles, if that.

I just don't buy it. Call me stubborn.
 
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HAWK21M
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RE: Airliner Engine Question

Sun Jun 26, 2005 6:32 pm

Quoting Air2gxs (Reply 17):
Please explain to me how a FADEC controlled CF6 installed on an MD11, on takeoff, can produce that well defined flame

Exactly the Question.
On T/O what could have caused this Event.Turbine distress,Combustion Chamber Shift.
What event transpired after the Flames emerged.presumingly the Engine was shutdown.Pilot would notice Abnormal EGT rise and/or Vibration.
Anyone having details on the Incident.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
MrChips
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RE: Airliner Engine Question

Mon Jun 27, 2005 6:18 am

Quoting HAWK21M (Reply 5):
Any Idea on the MD-11 Snag

Referring to our discussion in an earlier thread, Mel, that would be an incident, not a snag, as that engine would probably have to be shut down.
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HAWK21M
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RE: Airliner Engine Question

Mon Jun 27, 2005 4:03 pm

Quoting MrChips (Reply 19):
Quoting HAWK21M (Reply 5):
Any Idea on the MD-11 Snag

Referring to our discussion in an earlier thread, Mel, that would be an incident, not a snag, as that engine would probably have to be shut down.

Agreed.See my Post just above yours,Note the last line  Smile

So what happened on that day.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
broke
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RE: Airliner Engine Question

Mon Jun 27, 2005 10:24 pm

Turbine engines are designed to contain the flame in the combustion section of the engine. If the flame were to reach the turbine, it would quickly damaged the turbine vanes.
If you could see the combustion and turbine cases of any engine operating at high power, you would see that whole section of the engine glowing cherry red due to the temperatures these areas experience. You don't see it, because the by-pass ducts cover those sections of the engine. Looking at an engine from the rear and at a distance, you would see the red glow at high engine powers. You had better have some distance between you and the engine if you try this or you will get blasted by high velocity high temperature air.
 
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Jetlagged
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RE: Airliner Engine Question

Mon Jun 27, 2005 10:48 pm

The notes from the photographer refer to 3 or 4 very loud bangs, which sounds like it could be a compressor stall. (The centre engine is probably vulnerable to disturbed airflow from the fuselage during rotation.) Compressor stalls can cause external flames if violent enough. Airflow through the engine is reduced, in severe stalls the airflow can reverse. Fuel continues to be scheduled for takeoff power. At the very least you will get a high EGT, but you could get a flame such as the one pictured, especially if the engine momentarily flamed-out.
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aloges
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RE: Airliner Engine Question

Mon Jun 27, 2005 11:17 pm

For the record, check this pic:

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Photo © Piotr Marek

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lightsaber
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RE: Airliner Engine Question

Tue Jun 28, 2005 6:44 am

Quoting AirWillie6475 (Thread starter):
Why is it that when you look at an engine from the back when the plane is taking off, there is no fire or flames like in the fighter jet engines? With all that combustion in the engines why don't we see any flames or redness at the back, how is it just black?



Quoting Broke (Reply 21):
Turbine engines are designed to contain the flame in the combustion section of the engine. If the flame were to reach the turbine, it would quickly damaged the turbine vanes.

If I may expand:

As someone who has over 5 years of combustion education and almost 5 years of gas turbine combustor design experience, I feel very qualified to answer this question in more technical detail.

The combustion must be done by the exit of the combustor for fuel efficiency reasons. As soon as the combustion gas enters the turbine the flame is "quenched" by several mechanisms.
1. Turbine cooling air is 25% to 33% the combustor air. This chills the flame below its arrhenius activation temperature. In other words, conversion of CO to CO2 stops.
2. The turbine extracts energy out of the gas, this also ools the gas below its arrhenius temperature.
3. There are strict emission ICAO emission limits on CO, NOx, unburned hydrocarbons and smoke. In no way can reaction (you could say "flames") occur after the combustor and have all of the goals met. Yes, augmentors pollute like a dog. The concorde had an emission and noise waiver. Military aircraft are not regulated by ICAO. If you recall, when the JT8D-17's where pulled from Europe, the FAA threatened to pull the concord's waiver. There will never be another emissions waiver given for a class of airframes.

Recall that a gas turbine works by "squeeze bang blow." The more heat you add at the peak pressure, the more efficient the engine is. An augmentor (proper name for an afterburner) typically adds 200% more fuel for 50% more thrust. (Or 300% fuel for 150% thrust) For the power of the engine comes entirely by the volumetric expansion of the gas and a volumetric expansion of a low pressure gas doesn't allow one to extract much of the available energy.

As to flames shooting out of an engine, nothing beats the pw4168 (ok, pw4172...) Talon II combustor demo engine. It would hock out of the engine a 25 foot diameter fireball during startup to a typo in the startup software having an extra few zeros and an offset time constant...  Smile Now that's a fire! Management and the customer was not amused that the combustion engineers considered it a harmless bonus "proof of light" sensor. It didn't hurt anything... not that much fuel was lost... So what if the fireball was spit out of the *front* of the engine!  flamed 

Lightsaber
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DeltaGuy
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RE: Airliner Engine Question

Tue Jun 28, 2005 7:16 am

Not that it's a full-blown turbofan engine, but at night, you can see a glow coming from the CRJ's APU, as well as from a Gulfstream's APU...you'd never see it during the day, but at night, it looks like the last few embers in a fire, still glowing orange.

DeltaGuy
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Gilligan
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RE: Airliner Engine Question

Wed Jun 29, 2005 7:51 am

You don't want flames shooting out of the passenger jet engine, it scares the hell out of the passengers on that side of the plane. I've pushed 737 a/c before that had a wet start, lots of smoke and then WUMPHHH a big long shot of flame out the back end. Not a minute later the pilot came over the headset requesting that we tow him back up to the gate so somebody could get off. The second time the pilot came over and asked in an indignant tone what the bright flash was and I replied, "I don't know, maybe 5 gallons of jet fuel igniting all at once, it was bright enough for 10 but experience tells me more like 5". Didn't hear another word from the guy about it.
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