2 contrails from 1 engine?

skephu

Senior Member.
I just encountered this picture that tries to argue that two trails come from each engine of the plane in the photo, and one is an ordinary contrail while the other must be a chemtrail.

The original photo is from airliners.net.



Indeed, it looks like there are two distinct parts of the trails behind each engine. One is thinner and whiter, while the other one is thicker and also is of a darker color.
What do you guys think the two parts are? I thought the thinner one may be an aerodynamic contrail while the other one an exhaust contrail.
If this was already discussed, please just point me to the corresponding thread.
 
Last edited by a moderator:

Mick West

Administrator
Staff member
I think it's just a combination of the lighting and the forced perspective. Because the contrail is viewed from a shallow angle, it magnifies the effects.

However, theorising, it might be something to do with the pressure differences or difference in mixing between the top and bottom of the contrail. This might make the contrail a bit more dense on top, which is only apparent from this acute angle. Interesting possibility.

Both parts of the trail are from the exhaust contrail though. There is an aerodynamic trail, but it's just the thin iridescent sheet between the main trails.
 

Mick West

Administrator
Staff member
Here I've enhanced the image to greatly magnify the contrast:


There does seem to be a distinct separation between the "pink" and "blue" parts of the trail (note these are not the real colors). I've seen this photo many times before, but never really noticed that.

The difference is explained simply be different densities within the trail.The question is why are there different densities, and why does there seem to be such a sharp demarcation.
 

pseacraft

Active Member
I don't think its a density issue as the exhaust gases should be rather balanced. To my layman's eye it looks more like the effects of the aerodynamics of the wing are actually pulling the contrail "apart" and the wingtip vortex's are also having a affect.
 

Mick West

Administrator
Staff member
I don't think its a density issue as the exhaust gases should be rather balanced. To my layman's eye it looks more like the effects of the aerodynamics of the wing are actually pulling the contrail "apart" and the wingtip vortex's are also having a affect.

I was thinking that the vortex sheet could be having an effect on the contrail in the same way a wake vortex creates the "hybrid" contrails - i.e. an area of lower pressure. Or simply, as you say, pulling the contrail apart.

Here's an animation of the vortex sheet. Could it be splitting the exhaust contrail into upper and lower sections? Possibly with different pressures or mixing speeds, and hence different visual densities?


(That's a simplified plane above, not the tail).
 

skephu

Senior Member.
The weird thing is that the separation occurs right at the engine. If you look at the part of the trail closest to the engine, the separation is clearly seen. It's almost as if there were two engines, one below the other.

That's why I suspected that the two trails might actually have different sources. On the whole photo, a rainbow-colored band can be seen behind the plane, a sign of an aerodynamic contrail. That's why I thought that one trail could be the exhaust contrail, while the other may be an aerodynamic contrail created by the engine pod. Otherwise, if it's a pressure and density effect, I don't see how a sharp boundary between the two trails could be created.

Do we have photos of planes where we know that an aerodynamic and an exhaust contrail are simultaenously present? We could compare those with this photo.
 

Mick West

Administrator
Staff member
The weird thing is that the separation occurs right at the engine. If you look at the part of the trail closest to the engine, the separation is clearly seen. It's almost as if there were two engines, one below the other.

That's why I suspected that the two trails might actually have different sources. On the whole photo, a rainbow-colored band can be seen behind the plane, a sign of an aerodynamic contrail. That's why I thought that one trail could be the exhaust contrail, while the other may be an aerodynamic contrail created by the engine pod. Otherwise, if it's a pressure and density effect, I don't see how a sharp boundary between the two trails could be created.

Do we have photos of planes where we know that an aerodynamic and an exhaust contrail are simultaenously present? We could compare those with this photo.

I've never seen an aerodynamic contrail come from an engine (they are usually either wingtips or the end of the flap), nor have I seen one at cruise altitude (except where coming from the whole wing). Also there's a gap between the engine at the start of the trail. Aerodynamic contrails do not have a gap.

 

Pete Tar

Senior Member.
Could the wake created by the air going past the front of the the engine or the engine strut be influencing it? (ie, the air is being split to go around the engine structures)
 

Mick West

Administrator
Staff member
It's a 747, and they have a fairly distinctive contrail pattern, with this quite pronounce twist of the outside contrails, and then the inner contrails often spread out flatter. Probaby worth looking at some other examples of 747 contrails from different angles:

http://www.airliners.net/photo/Air-India/Boeing-747-437/0560567/L/


http://www.airliners.net/photo/Air-China/Boeing-747-4J6M/0922038/L/


https://www.flickr.com/photos/aerosavvy/10982504554


http://www.airliners.net/photo/Pakistan-International-Airlines/Boeing-747-367/0332946/
 
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pseacraft

Active Member
I think what you are seeing as the "boundary" is caused in part by the angular preservative and the lighting showing the outer edges of the hot exhaust plume meeting the cold atmospheric air first where the center of the plume maintains its temperature longer giving the impression of separation. You can not see the same effect on the left side of the image again due to the angle of perspective but if you could it would look the same as the right side. There is nothing really weird here at all, it just looks crazily imposing in an ugly yet beautiful manner.
 

TWCobra

Senior Member.
I filmed a 747 contrail about a month ago when we flew under one. The 747 creates very complex trails. Anyway, I'd be interested in the views of what happens here. It seems to me that the contrails created by the inboard engines last longer, perhaps due to some acceleration of the vortex created by the wing.

 

deirdre

Senior Member.
I filmed a 747 contrail about a month ago when we flew under one. The 747 creates very complex trails. Anyway, I'd be interested in the views of what happens here. It seems to me that the contrails created by the inboard engines last longer, perhaps due to some acceleration of the vortex created by the wing.

I'm getting a "private" message. cant see video
 

skephu

Senior Member.
Now in this picture, we again see that each engine appears to emit two separate contrails. So can we agree that this is caused by the center of the exhaust plume being hotter, as pseacraft suggested?
But even here, there is a thin line separating the two "sub-contrails". May this be caused by the structure of engine somehow, e.g. the center of the engine not emitting exhaust gases because the axle is there?
 

TWCobra

Senior Member.
May this be caused by the structure of engine somehow, e.g. the center of the engine not emitting exhaust gases because the axle is there?

It is a rotating spindle. I suspect the split is caused by the large sideways (outboard) flow of air under the wing, in effect splitting the contrail from the rear of the engine. Just speculation though. Any one have a wind tunnel handy?
 

skephu

Senior Member.
On this site there are two photos showing similar split contrails. They call it "bifurcated contrails":




But the site seems to be some chemtrail awareness site and they don't offer any explanation.
I'm still not sure what causes these split contrails.
 

Mick West

Administrator
Staff member
Clearly there's going to be some kind of structure to the exhaust as it comes out the engine.


The middle of the exhaust is hotter, and the exhaust mixing happens from the outside. So in general the middle will take longer to become visible.

Then the exhaust goes along the underneath of the wing, and then interacts with the vortex sheet coming off the top of the wing.
 

skephu

Senior Member.
Possible. But let's look at this detail image:

If we use the presence of condensation (white color) as a temperature proxy, and look at the right side of the picture, it looks like there are two separate contrails, each having a hotter center: we see 4 whiter lines, with blue regions in between. Like two tubes next to each other, each with a hotter center: cold-hot-cold, cold-hot-cold.
Also, the thin blue strip in the middle does not diseappear, it persists along the contrail as long as we can see it in the photo.
I might also interpret it as a single thick-walled tube, with an empty center. And the inner wall of the tube cools down just like the outer wall. But the center remains empty.
 

Mick West

Administrator
Staff member
Yes, thinking of it as concentric cylinders explains the initial structuring

Interesting though that in this image the structure of the two contrails seems to be reversed between left and right engines.

This appears to be an illusion though, with the upper contrail being in shadow.
 

WeedWhacker

Senior Member
Interesting though that in this image...

Here is the same airplane (the visible registration number caused me to look it up), not making contrails yet (but evident that it does have markings, contrary to the many, many "chem"trail enthusiasts' claims):

 

Jason

Senior Member
Here I've enhanced the image to greatly magnify the contrast:


There does seem to be a distinct separation between the "pink" and "blue" parts of the trail (note these are not the real colors). I've seen this photo many times before, but never really noticed that.

The difference is explained simply be different densities within the trail.The question is why are there different densities, and why does there seem to be such a sharp demarcation.
OT: A bit off topic but not really, do contrails actually look this pronounced from above. I can't believe how thick and heavy the contrails appear from this angle, is this typical?
 

Jason

Senior Member
Clearly there's going to be some kind of structure to the exhaust as it comes out the engine.


The middle of the exhaust is hotter, and the exhaust mixing happens from the outside. So in general the middle will take longer to become visible.

Then the exhaust goes along the underneath of the wing, and then interacts with the vortex sheet coming off the top of the wing.
Is the air flow coming out of #9 and #2 moving at a different velocity from the compressed air in #8?
 

Mick West

Administrator
Staff member
OT: A bit off topic but not really, do contrails actually look this pronounced from above. I can't believe how thick and heavy the contrails appear from this angle, is this typical?
It varies - but the angle greatly magnifies the density.
 

Mick West

Administrator
Staff member
Is the air flow coming out of #9 and #2 moving at a different velocity from the compressed air in #8?

Wikipedia says:
#9 and #2 are the bypass flow. 8 is the "much higher velocity engine exhaust", where the extra heat and water is.
 

WeedWhacker

Senior Member
Is the air flow coming out of #9 and #2 moving at a different velocity from the compressed air in #8?

Yes, #2 (in Mick's animated GIF) represents the air entering the N1 Fan. #9 shows how this air is forced aft by the action of the Fan (which is essentially a multi-bladed shrouded propeller), and through the design, the constriction causes a venturi effect. That "bypass" air (about 80% to 85% of the engine's thrust is derived from it) accelerates, and helps impart thrust.

The central and very hot core exhaust gasses provide some minimal thrust (the remainder, from the 80%-85% from N1 Fan).

Since the engine is circular, the hot central portion appears to be 'bifurcated" (hence the illusion of "2" contrails from "1" engine), but this is thinking in 2 dimensions. We see a "clear" columnar-shaped area centrally, that in photos appears as a "split".
 

Jason

Senior Member
Wikipedia says:
#9 and #2 are the bypass flow. 8 is the "much higher velocity engine exhaust", where the extra heat and water is.
So this simple fact should easily explain why at times it appears as if 2 contrails are coming out of one engine. I often wonder what the affect would be if we put a huge fan in the upper atmosphere that could push air at the same the velocites you see in the "bypass" air exhaust portion of the engine. Without the mixing and heating created by condensing the air flow. I wonder if contrails would be created as a result. But honestly, this reminds me of the "vortex" we discussed a few days ago in the smoke ring above England. These contrails seem to be made in a similar fashion but on a much grander scale

That "bypass" air (about 80% to 85% of the engine's thrust is derived from it) accelerates, and helps impart thrust.
This is very interesting to be honest with you, and not what a laymen would expect. I never would've thought that the "bypass" air in #9 and #2 would be responsible for most of the plane's thrust, I would've thought that it would be the other way around to be honest with you...
 

Mick West

Administrator
Staff member
This is very interesting to be honest with you, and not what a laymen would expect. I never would've thought that the "bypass" air in #9 and #2 would be responsible for most of the plane's thrust, I would've thought that it would be the other way around to be honest with you...

Compare the above (a turbofan) with the below, a turboprop. That's a jet engine where the jet propulsion produces no real thrust, and the jet turbine drives a propellor:


The turbofan is kind of like taking that, attaching the prop directly to the shaft, adding some blades, pointing the exhaust straight backwards, and enclosing it all.
 
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WeedWhacker

Senior Member
I should add that as fuel prices were rising (during the 1980s), and along with development next-generation of the High Bypass TurboFan designs (to improve power and fuel consumption efficiency), there was a brief foray into a sort of hybrid Turbo-prop, multi-blade unshrouded fan design that could be adapted for use in large commercial passenger airplanes

The engineers called a "PropFan":





Can only guess as to how THOSE contrails would have appeared. Obviously though, the research wasn't promising enough to indicate their use as alternative propulsion methods.
 

MikeC

Closed Account
Yes engines are much more efficient (generate more power per unit of fuel) if they move a large mass of air slowly, rather than a small mass fast.

But moving a large mass slowly requires a larger diameter engine - hence the size of high-bypass-ratio jet engines and propellers - and it eventually gets too large to be practicable - prop tips start moving supersonically for example, which is a massive waste of fuel.
 

MikeC

Closed Account
I notice that the "thin" bit of the bifurcated contrails always seems to be in the middle as you look at it - is it just a case of the circular contrail has less apparent thickness there as opposed to hte edges - like looking through the walls of a tube??

tube.jpg
 

Ross Marsden

Senior Member.
Yes, thinking of it as concentric cylinders explains the initial structuring

Interesting though that in this image the structure of the two contrails seems to be reversed between left and right engines.

This appears to be an illusion though, with the upper contrail being in shadow.

The upper (right side) contrail is mostly in shadow. There is a thin sliver in the bottom of the trail just in sunlight between the trailing edge of the wing and the horizontal stabilizer. Immediately after that it is in complete shadow from the tail section, then fully in sunlight after that.
 

Jason

Senior Member
I should add that as fuel prices were rising (during the 1980s), and along with development next-generation of the High Bypass TurboFan designs (to improve power and fuel consumption efficiency), there was a brief foray into a sort of hybrid Turbo-prop, multi-blade unshrouded fan design that could be adapted for use in large commercial passenger airplanes

The engineers called a "PropFan":





Can only guess as to how THOSE contrails would have appeared. Obviously though, the research wasn't promising enough to indicate their use as alternative propulsion methods.
OT: So how exactly did this engine work. I'm assuming the "bypass" air flow would be the airflow on the outside which would be forced through the propellers. So does this engine have the same set up as above with "bypass" air flow and forced condensed air flow or is it purely forced condensed air
 

WeedWhacker

Senior Member
So how exactly did this engine work.

Same principle as any rear propeller arrangement. (....actually, think of boats' propellers, as an analogy). There are also some piston-engine "pusher" propeller aircraft examples, they're easy to find online.



I'm assuming the "bypass" air flow would be the airflow on the outside which would be forced through the propellers. So does this engine have the same set up as above with "bypass" air flow and forced condensed air flow or is it purely forced condensed air

In those "PropFan" arrangements, there would be a very hot central column of core exhaust, not that much different from the High Bypass Turbofan design. The differences might have been seen in the way that the cooler air, in the surrounding and larger column (propelled by the REAR fan blades) would have resulted in interesting airflow pattern mixing, out the aft ends.

Much as seen in the WW2 examples of piston engines, with tractor propellers...still, the large cooler column of air had the effect of tempering the much hotter central exhaust air. Contrails would form, regardless.
 

MikeC

Closed Account
Yes the centre of the "prop-fan" is a jet engine - apart from scale and speed the only difference between it and a "normal" turboprop is that the exhaust is exiting through the hub. There are a few turbo-prop pushers around such as the Piaggio Avianti P.180 - but you can see the exhaust there exits to either side of the prop hub -

 
I notice that the "thin" bit of the bifurcated contrails always seems to be in the middle as you look at it - is it just a case of the circular contrail has less apparent thickness there as opposed to hte edges - like looking through the walls of a tube??

tube.jpg

The air in the center/middle is the super-hot combusted fuel-air mixture that just came out of the core of the engine after going through the turbines that drive the fan and compressor sections.
This hot air is enveloped inside the cold ambient air pushed by the fan.
The hot air from the core needs to cool to ambient temperature before the ice crystals become visible. This is not the case with the ambient bypass air.

The bypass air probably doesn't create contrails since it's not having any moisture added to it from combustion, but you do have two different flows of air with different temperature, pressure and moisture content which are mixing together.

(I ended up in this 4 year old thread in the course of google searching something related to contrails as I am finishing up the 2 hour "chemtrail" debunking video I have been working on gradually for 3 months... It will be given a misleading title to get believers to watch it so they see the truth/science of persistent contrails/relative humidity).
 

Mick West

Administrator
Staff member
The air in the center/middle is the super-hot combusted fuel-air mixture that just came out of the core of the engine after going through the turbines that drive the fan and compressor sections.
This hot air is enveloped inside the cold ambient air pushed by the fan.
The hot air from the core needs to cool to ambient temperature before the ice crystals become visible. This is not the case with the ambient bypass air.

The bypass air probably doesn't create contrails since it's not having any moisture added to it from combustion, but you do have two different flows of air with different temperature, pressure and moisture content which are mixing together.
I don't think that's going to relate to the tube effect though. You still essentially only have the ice supersaturated air (ambient and bypass) and the water supersaturated gases (exhaust). The tube effect comes from the briefly un-condensed water in the center.
 
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