What determines contrails?

[superberkut]

I remember an air show I went to some time ago, where a squadron was doing their act in the sky. However, while all the planes were moving at the same pace and in the same maneuvers (or so it seems), only one plane drew sharp contrails. Is it the use of the afterburners? Or is it engine heat or exhaust? Or something else?

(I hope this is not a stupid question; I am genuinely unknowledgeable)

 

[Duke]

If it was a show, probably that one plane was making smoke by dumping oil into the exhaust.

Contrails generally don't happen at lower altitudes because the air near the ground is too warm. Contrails are formed at high altitude where the air is A) colder, B) lower pressure, and C) thinner. A sharply-defined "contrail" within a few thousand feet of grade is almost certainly from a show smoke system.

Contrails are formed when hot humid air from the exhaust hits the cold ambient air. It cools almost instantly, forcing the water to condense out and then freeze into tiny ice crystals. This is why they are often so bright white on a clear blue sky. They also can 'seed' clouds when additional airborne moisture freezes around these crystals.

However, down near the ground (and particularly on a hot humid day) you will often see aerobatic and high-performance aircraft leaving little "smoke" trails from the wingtips as they put on their demonstrations. This is actually water vapor being crushed out of the humid air by the sudden, dramatic increase in air pressure at the tips as the plane performs some brutal maneuver.

 

[Keef]

Contrails are condensation trails. Usually they happen in cold, moist air high in the sky. Usually those trails are made when the moisture from the combustion in the jet engine goes poof when it hits the freezing cold air. The shape of a wing naturally makes zones of low air pressure, and that drops the temperature, sometimes making moisture in the air condense.

/paraphrasetomakemesoundsmart

 

[Greycap]

That sounds like the wingtip vapour trails to me, and to my understanding those are caused by the weight of the aircraft under heavy G loading pressurising the air under the wings, which in turn leads to the vapour in the air condensating and escaping as a small but dense cloud where the air pressure is the lowest, usually at the wingtips. And this is what you see as those trails.

I'm not half sure I wrote that correctly... and it seems Duke stealthily edited his post when I was typing this.

 

[superberkut]

That sounds like the wingtip vapour trails to me, and to my understanding those are caused by the weight of the aircraft under heavy G loading pressurising the air under the wings, which in turn leads to the vapour in the air condensating and escaping as a small but dense cloud where the air pressure is the lowest, usually at the wingtips. And this is what you see as those trails.

I'm not half sure I wrote that correctly...

No, I think you wrote that correctly, Greycap.

At what altitude would this air pressurization occur? Or does it even matter?

EDIT: Never mind, I read the post thoroughly... And Duke's as well.

By the way, thanks for all the responses, people!

EDIT AGAIN: Duke: But would the result of dumping oil into the exhaust create a black contrail?

 

[Duke]

EDIT AGAIN: Duke: But would the result of dumping oil into the exhaust create a black contrail?

No, unburned fuel is black - that's why jet exhaust from a hard-running turbine (say, and airliner taking off, or a B-52) shows up as black smoke.

Oil burns white (though it can be dyed to make red and blue smoke as well). The same is true with cars - a car running rich will leave a black exhaust cloud, whereas a car that needs piston rings will leave a white exhaust cloud.

 

[wfooshee]

And they won't use the smoke trail in an airshow on afterburner, if anybody's interested.

To Superberkut, The trails behind planes in a show is oil dumped into the exhaust. The "fog" you see in high-g maneuvers is the extreme reduction in air pressure causing the water vapor to fall out of solution, to condense.

These are smoke trails from oil:




These are pressure-drop condensation. The 3rd and 5th pictures are flat passes, but stinking fast, just under sonic.



Ooh, found one with both! The location of the oil nozzle is very plain in this shot.


A true contrail happens at high altitude, in thin, cold air. These are what you see when an airliner passes high overhead, and there's a group of people that actually believe the contrail is a chemical spray sent up by the government! Google "chemtrail" and be amused. Contrails are not limited to jets, but they are more common with modern jets because of the altitude needed for them to occur. Not many piston planes get into the 25-30 thousand foot range. And to emphasize, they are NOT chemical in nature, simply water vapor condensed to ice by the sharp temperature and pressure drop on exiting the engine. Water is a MAJOR component of engine exhaust.