747400sp
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Why A Jet Stop Smoking When They Go Into A/B

Thu Apr 26, 2007 12:38 pm

I notes that when a fighter or bomber take off on military power, they will have a dark smoke tail. But when they go into after burner, there exhaust clear up. Why? I would think the extra power they get from to afterburners would make more smoke.
 
Fly2HMO
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RE: Why A Jet Stop Smoking When They Go Into A/B

Thu Apr 26, 2007 1:20 pm

Quoting 747400sp (Thread starter):
But when they go into after burner, there exhaust clear up. Why? I would think the extra power they get from to afterburners would make more smoke.

Jet engines with the A/B on still make plenty of smoke, but what I've noticed is that the color of the smoke changes, more to an very light brown/ash-grey color instead of the dark black soot you would see otherwise, it's just that the grey blends more easily towards the sky. Next time your at an airshow or at an AFB or wherever, pay pretty close attention specially when the aircraft is flying level away from you, if the plane stays level long enough with tha A/B on, the smoke trail will seem quite obvious.

I guess you could also say that with the A/B on you get a more complete combustion, been a while since my turbines class so don't quote me on that.  Smile
 
avioniker
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RE: Why A Jet Stop Smoking When They Go Into A/B

Thu Apr 26, 2007 7:27 pm

Quoting FLY2HMO (Reply 1):
I guess you could also say that with the A/B on you get a more complete combustion, been a while since my turbines class so don't quote me on that.

Actually I'd say you stated it quite right.
The J-79 in the F-4E was modified to the -17 level and classified as a "smokeless" engine after the engine hot section was modified and the EGT was raised about 35 degrees in 1984-86.
 Smile
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
sovietjet
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RE: Why A Jet Stop Smoking When They Go Into A/B

Tue May 01, 2007 11:17 pm

Same on Mig-29, all the blakc smoke clears up because the black smoke is usually unburned fuel and with the A/B on the combustion is more complete. Of course more fuel is sprayed into the A/B but lets just pretend that doesn't happen  Smile
 
Jerald01
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RE: Why A Jet Stop Smoking When They Go Into A/B

Wed May 02, 2007 12:57 pm

I'm not an expert on jet engines but my own thinking is this:

The fuel being burned in the engine's combustion chamber is being carefully mixed with a given ratio of air, one which provides the maximum amount of thrust per a given amount of fuel burned. Some of the calculation behind that maximum amount of thrust concerns purposely including incompletely-burned fuel material - it's mass is greater than the mass of the heated air it would have produced had it been completely burned, hence it provides more thrust. Essentially this would be an over-rich mixture.

When fuel is injected into the afterburner of a so-equipped engine, that fuel is more completely burned because it has a greater amount of oxygen molecules available to it than that which is burned in the combustion chamber. Afterburners are pretty much just a fuel ring squirting raw fuel into an already-burning mass of air. That ring is in such a location that the burning fuel can obtain oxygen not from just the engine inlet, but also from areas surrounding the engine itself. Nacelle / fuselage design is such that this oxygen feeds into the afterburner area to give a more lean mixture than is obtained in the combustion chamber itself, thereby burning the fuel more completely.

That, in itself, leaves less unburned matter to show up as a smoke trail.
"There may be old pilots, and there may be bold pilots, but there are darn few green cows"
 
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mighluss
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RE: Why A Jet Stop Smoking When They Go Into A/B

Wed May 02, 2007 10:49 pm

Quoting FLY2HMO (Reply 1):
very light brown/ash-grey color

I noticed that too!


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More than smoke, those Olympus engines are throwing pure heat!!
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