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Why A Jet Stop Smoking When They Go Into A/B  
User currently offline747400sp From United States of America, joined Aug 2003, 3584 posts, RR: 2
Posted (7 years 3 months 4 weeks 1 day 15 hours ago) and read 3233 times:

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.

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User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 3 months 4 weeks 1 day 15 hours ago) and read 3221 times:

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


User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 2, posted (7 years 3 months 4 weeks 1 day 9 hours ago) and read 3167 times:

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
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2596 posts, RR: 17
Reply 3, posted (7 years 3 months 3 weeks 3 days 5 hours ago) and read 2862 times:
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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

User currently offlineJerald01 From United States of America, joined Oct 2006, 161 posts, RR: 2
Reply 4, posted (7 years 3 months 3 weeks 2 days 15 hours ago) and read 2745 times:

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"
User currently offlineMighluss From Spain, joined Oct 2001, 948 posts, RR: 8
Reply 5, posted (7 years 3 months 3 weeks 2 days 5 hours ago) and read 2684 times:

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!!



Miquel.
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