Jet Pilots Fear "Chemtrail" Attacks

Jazzy

Closed Account
can increase the frequency of persistent contrails within the tropopause
Certainly. The engines are not only larger, they burn more fuel. It is BURNT FUEL (water) that they're adding. More water added at any time is bound to beat more conditions into making trails, so the amount that appears will increase. Or maybe the trails will lengthen. maybe both.

Also the frequency of trails depends on the frequency of flights, which is still, AFAIK, increasing exponentially (although I bet that exponent has recently dwindled some).

Mick, engines that were originally 5000 lb thrust are now 80,000 lb thrust. They aren't 1600% efficient, they are burning more fuel (as well as being significantly more efficient). Whether the air comes out really hot or hotter than that makes no difference once things are down at -40. It is the water added per unit volume air, and the ambient temperature and humidity, that create trails.

I didn't mean to infer that the trails widened proportionately according to their weight. But still they will be denser than trails that start with less material. Trails can fall 22,000 feet and if they start big, and no dry layers intervene to prevent their growth, then they're bound to end up bigger still, certainly if they are shearing as they fall four and a half miles downwards. And I guess be a hundred feet wider... :)
 

George B

Extinct but not forgotten Staff Member
Certainly. The engines are not only larger, they burn more fuel. It is BURNT FUEL (water) that they're adding. More water added at any time is bound to beat more conditions into making trails, so the amount that appears will increase. Or maybe the trails will lengthen. maybe both.

Also the frequency of trails depends on the frequency of flights, which is still, AFAIK, increasing exponentially (although I bet that exponent has recently dwindled some).

Mick, engines that were originally 5000 lb thrust are now 80,000 lb thrust. They aren't 1600% efficient, they are burning more fuel (as well as being significantly more efficient). Whether the air comes out really hot or hotter than that makes no difference once things are down at -40. It is the water added per unit volume air, and the ambient temperature and humidity, that create trails.

I didn't mean to infer that the trails widened proportionately according to their weight. But still they will be denser than trails that start with less material. Trails can fall 22,000 feet and if they start big, and no dry layers intervene to prevent their growth, then they're bound to end up bigger still, certainly if they are shearing as they fall four and a half miles downwards. And I guess be a hundred feet wider... :)

The issue is not that larger engines or that more efficient engines can lead to more persistent contrails or contrail induced cirrus nor the fact there are simply more air craft flying in optimal air (that has been acknowledged) . . . it is has global warming increased the frequency and number of persistent contrails . . ??
 

Jazzy

Closed Account
The issue is not that larger engines or that more efficient engines can lead to more persistent contrails or contrail induced cirrus nor the fact there simply more air craft flying in optimal air that has been acknowledged . . . it is has global warming increased frequency and number of persistent contrails . .
You were answered some time back. In that GW exists and the consequences of it will be to increase (by an arguable amount) atmospheric water vapor, and that the 17% given as the saturated proportion of the atmosphere will increase, and be a figure greater than before, and that the greater saturated area will intercept more passenger flights, and thus produce more trail, though NOT necessarily a significantly greater number of them.

Can you still wiggle? :)
 

George B

Extinct but not forgotten Staff Member
You were answered some time back. In that GW exists and the consequences of it will be to increase (by an arguable amount) atmospheric water vapor, and that the 17% given as the saturated proportion of the atmosphere will increase, and be a figure greater than before, and that the greater saturated area will intercept more passenger flights, and thus produce more trail, though NOT necessarily a significantly greater number of them.

Can you still wiggle? :)
And that my friend is what I asked Dr Minnis . . . . and I assumed the answer would be what you just indicated . . . .but that is not what he indicated . . . he indicated rather that the increase in temperature of (I assume) the high troposphere (whatever altitude and latitude one is talking about) via Global Warming would most likely reduce the frequency and number of flights encountering optimal air . . . and thus possibly fewer contrails and contrail induced cirrus clouds . . . so he thinks the temperature distribution of the troposphere is more important than the RH changes via GW . . .
 

Mick West

Administrator
Staff member
Mick, engines that were originally 5000 lb thrust are now 80,000 lb thrust. They aren't 1600% efficient, they are burning more fuel (as well as being significantly more efficient). Whether the air comes out really hot or hotter than that makes no difference once things are down at -40. It is the water added per unit volume air, and the ambient temperature and humidity, that create trails.

The temperature DOES make a significant difference, as it affects the slope of the mixing curve. Lower exhaust temperatures means a wider range of condition where contrails will form, so more contrails. In this classic photo:

The newer plane is leaving contrails primarily because of the lower exhaust temperature (due to higher efficiency), because it's a higher bypass engine.

Look at these charts. With a change in temperature the position of point B moves, hence the mixing line is less likely to go into the "cloud" region that is needed for contrails to form:


So all other things being equal, a hotter exhaust can cause a contrail not to form.
 
Last edited:

MikeC

Closed Account
and of course there is the fairly well known study on propulsive efficiency and contrail formation -

707 airbus.JPG

On the left an A340 with high bypass ratio engines, on the right a 707 with low bypass ratio ones.

the significance of this in regard to temperature is that high BPR engines have a much larger mass of "cool air" around the hot core that supports combustion compared to low BPR engines.
 

Billzilla

Senior Member.
and of course there is the fairly well known study on propulsive efficiency and contrail formation -

707 airbus.JPG

On the left an A340 with high bypass ratio engines, on the right a 707 with low bypass ratio ones.

the significance of this in regard to temperature is that high BPR engines have a much larger mass of "cool air" around the hot core that supports combustion compared to low BPR engines.

It's a very good demonstration of what I meant by modern engines having the core run hotter and so more likely to make contrails.
 

MikeC

Closed Account
Can you explain that to me?

While modern engines are able to run hotter due better materials, AFAIK efficiency is gained by extracting as much of that heat energy as possible. So the highest efficiency is gained by having as much of a difference between the combustion temperature and the exhaust temperature - so a cooler exhaust is better since it indicates more energy has been extracted to power the fan - which is where most of the thrust is generated.
 

Jazzy

Closed Account
Yes. I understand. The slope of the line from pos B. Latent heats account for the second curve. The ice, once formed, needs energy from somewhere in order to sublime. Good explanation, but a tough one to put across. :)
 

Jazzy

Closed Account
Can you explain that to me? While modern engines are able to run hotter due better materials, AFAIK efficiency is gained by extracting as much of that heat energy as possible. So the highest efficiency is gained by having as much of a difference between the combustion temperature and the exhaust temperature - so a cooler exhaust is better since it indicates more energy has been extracted to power the fan - which is where most of hte thrust is generated.
Bilzilla's right. Maximum combustion efficiency is utterly dependent on pre-combustion pressure. That will inevitably result in the hottest core possible.

It's amazing to me the improvement in axial flow compressors (and the exhaust turbines) which has occurred over the last 45 years. Much of it is due to the confidence that one could force-cool materials WITH AIR well beyond their apparent capabilities, that was learned by the research carried out while I was a lowly apprentice. Much thought and computation has been put to cooling passage dynamics since my slide-rule days.

Now the trick is to use as much of the coolant energy as one can to warm the fanned air while it is confined to a degree*, so that its further expansion results in overall thrust. I'm always surprised that present configuration achieves this. It would seem that cooling air would have to move forward, against the flow, but evidently the airstream itself provides the confinement. But that's bye-the-bye...

* Ideally all that energy extracted in order to keep core and blades cool ought to be made to reappear, evenly-distributed, immediately after the fan. Air could be dispensed with, and the core steam-cooled from the rear, the steam to be discharged and distribute itself as a fan trail, contributing quite dramatically to the thrust...

...but more dramatically to the trail, of course. George would love it. :)
 
Top