Contrail formation temperature for piston engines?

[Everett]

A bit of an unusual topic, but here goes. Since this place is certainly the home of the contrail experts, I'll ask here. I fly a lot of big, piston engined propeller-driven 1950's airliners in a flight simulator. No jet or turboprop engines - just four huge radials with 18 or even 28 cylinders, and up to 71 liters and 3400 horsepower. Each. They also fly decently high, from 22,000 feet in one plane to 30,000 feet in another.

Now the question. All of the ones I fly have the contrail formation temperature (in flight simulator) set at -20 C. Is that correct for piston engines, or should it be -40 C, like jets?

 

[Mick West]

The -40C temp is for freezing contrails. Water needs a nucleus to form ice on at temperatures above about -40C (it varies with pressure). At -40C it freezes "homogeneously".

I can't think of a reason why -20C would be a significant temperature. The physics is basically the same. It's possible that due to the inefficient nature of older engines they are pumping out a lot more water per foot, and so you'd get denser liquid water clouds forming - but that would happen at any cold temp (like your breath on a cold day, or a steam engine).

So I'm not 100% sure, but I think that -20C is a mistake.

 

[George B]

This Blog is a historical review of Contrails from WWI forward and even cites Contrail Science. Has several references to propeller driven aircraft.

http://swallowingthecamel.blogspot.com/2012/06/chemtrails-iii-contrails-and-clouds.html?m=1

Contrails most commonly form behind jets flying at high altitude (over 26,000 feet), but they can occur at lower altitudes under certain conditions. And other aircraft can produce them as well. In fact, the first known contrail sighting occurred during WWI, over 20 years before the first jet was in the air.
In 1918, Captain Ward Wells of the U.S. Army Medical Corps, who was serving in France during the Meuse-Argonne campaign, wrote of seeing "several strange and startling clouds" in the air. When he and his fellow ground observers looked at these "long, looping, graceful ribbons of white" more closely, they found "some distance ahead of each cloud point the tiny speck of a chasse [sic] plane." (2)
These contrails developed on a cloudless day, giving lie to claims that contrails don't appear when the sky is clear and blue. (Above Top Secret forum member "cutbothways": "So, what it boils down to, is that on a clear day, it's very, very unlikely that a contrail would form, let alone persist.")
Captain Wells's brother, Everett Wells, was so fascinated by the phenomena that he reported it to Scientific Americanmagazine. (“Clouds formed by Airplanes“, June 7, 1919)

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The first known sighting of a persistent contrail was made in 1921, 18 years prior to the first jet flight. A La Pere plane flown by Lt. J. A. Macready left what was described as "long feathery white streamers" at an altitude of 26,000-27,000 feet. This was a discontinous contrail, with a gap between the first and second "streamers". The contrail lingered for about 20 minutes before spreading and merging with existing cirrus clouds. (2)

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Wakes of war: contrails and the rise of air power, 1918-1945 Part I--early sightings and preliminary explanations, 1918-1938.

http://www.thefreelibrary.com/Wakes of war: contrails and the rise of air power, 1918-1945 Part...-a0164870066

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[George B]

This is a rather good discussion about the evolution of aircraft capabilities and the emergence of contrails.

While this operational ceiling is still below the band between 25,000 and 40,000 feet where atmospheric conditions are most often conducive to contrail formation, (4) planes flying at 20,000 feet and even lower can generate contrails under the proper conditions of temperature and humidity. Therefore, in the later stages of the Great War, contrail-generating flights would have become increasingly common as the operational ceilings of first-line aircraft increased. Given the number of planes flying over the Western Front and the number of men on the ground with a vital interest in watching the skies for hostile aircraft, it was virtually inevitable that substantial numbers of people would eventually notice that at least some high-flying planes were producing long thin clouds as they crossed the skies.

http://www.thefreelibrary.com/Wakes of war: contrails and the rise of air power, 1918-1945 Part...-a0164870066

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[Everett]

Those engines might actually provide a nucleus - they burned 135 octane aviation gasoline, which had an octane rating that high due to tetraethyl lead, and lots of it. I don't 'think' that gets burned in the engine, so it goes right out the exhaust. That, and possibly burnt engine oil (a 3800 mile flight consumes about 100 gallons), but since jets consume oil too, I doubt that's a factor. At high power settings (which doesn't apply in this case), they're also cooled by unburned fuel going out the exhaust - but I lean the mixture anyway at cruise, and power is much lower, so that's not it.

Most likely, they wanted more common contrails, since -40 C doesn't come up all that much at 18,000-24,000 feet. The other plane I fly is usually 25,000-30,000, with most of the flight toward the low end.

 

[WeedWhacker]

Now the question. All of the ones I fly have the contrail formation temperature (in flight simulator) set at -20 C. Is that correct for piston engines, or should it be -40 C, like jets?

Basically, the OAT [Outside Air Temperature] depends on altitude. For jets, because of the higher speeds, we use TAT (Total Air Temperature) due to what's called "Ram Rise" from friction, due to airspeed. Lighter, slower and lower airplane pilots are familiar with OAT (which I mentioned first), but for jets we use the equivalent called SAT (Static Air Temperature). SAT is derived (computed) by the Air Data Computers to approximate OAT, and it simply uses airspeed and measured TAT to do the calculations.

Long-winded explanation...short version: -20°C (SAT or OAT) is typical for pressure altitudes of around ~20,000 feet....this will vary by latitude and specific atmospheric conditions in any airmass, of course.

-40°C is seen usually up in the range of ~30,000 feet.

 

[Mick West]

Those engines might actually provide a nucleus - they burned 135 octane aviation gasoline, which had an octane rating that high due to tetraethyl lead, and lots of it. I don't 'think' that gets burned in the engine, so it goes right out the exhaust. That, and possibly burnt engine oil (a 3800 mile flight consumes about 100 gallons), but since jets consume oil too, I doubt that's a factor. At high power settings (which doesn't apply in this case), they're also cooled by unburned fuel going out the exhaust - but I lean the mixture anyway at cruise, and power is much lower, so that's not it.

Most likely, they wanted more common contrails, since -40 C doesn't come up all that much at 18,000-24,000 feet. The other plane I fly is usually 25,000-30,000, with most of the flight toward the low end.

All engine exhaust provides lots of water nuclei, this helps the initial water droplets condense. However ice is a different story - ice nuclei has a specific structure, and while the nuclei in exhaust work well for liquid, they don't work well for freezing.

 

[Mick West]

Basically, the OAT [Outside Air Temperature] depends on altitude. For jets, because of the higher speeds, we use TAT (Total Air Temperature) due to what's called "Ram Rise" from friction, due to airspeed. Lighter, slower and lower airplane pilots are familiar with OAT (which I mentioned first), but for jets we use the equivalent called SAT (Static Air Temperature). SAT is derived (computed) by the Air Data Computers to approximate OAT, and it simply uses airspeed and measured TAT to do the calculations.

Long-winded explanation...short version: -20°C (SAT or OAT) is typical for pressure altitudes of around ~20,000 feet....this will vary by latitude and specific atmospheric conditions in any airmass, of course.

-40°C is seen usually up in the range of ~30,000 feet.

I don't think that relates to the question about contrail formation though - you are just describing how temp is measured, and what temp it typically is at different altitudes.

 

[WeedWhacker]

I was thinking that -20°C and ~20,000 feet might not be conducive to contrails.

But again, in winter, in proper conditions, and even the 20,000 to 25,000 feet range, a big piston engine can make contrails. As we see from the WW2 examples.

 

[deirdre]

kinda off topic but if the exhaust lingers in -20 on an 'updraft?' (like what keeps hail up), and slowly cools it would eventually look like a contrail, no? the plane would be long gone of course.

 

[Jason]

Saw this on "outrageous acts of science" the other day with my son and it's quite amazing to say the least. The guy makes a cloud form at ground level inside an enclose room under the right conditions. I thought this could help explain how contrails form.

 

[Mick West]

kinda off topic but if the exhaust lingers in -20 on an 'updraft?' (like what keeps hail up), and slowly cools it would eventually look like a contrail, no? the plane would be long gone of course.

No, if the exhaust did not form a liquid water cloud on exiting the engine, then cooling the air is not going to change anything. The very minor increase in local humidity of the fully mixed air is not going to be significantly different to the surrounding air in an updraft.

 

[WeedWhacker]

Saw this on "outrageous acts of science" the other day with my son and it's quite amazing to say the least. The guy makes a cloud form at ground level inside an enclose room under the right conditions. I thought this could help explain how contrails form.

Neat, and shows how condensation, with appropriate nucleii present will occur. For contrails, then the water droplets freeze, and either persist, or sublimate, depending on conditions.

In the VAB (Vehicle Assembly Building) at the Kennedy Space Center (KSC) at Cape Canaveral, Florida, it rains...indoors:

The interior volume of the building is so vast that it has its own weather, including "rain clouds form[ing] below the ceiling on very humid days", which the moisture reduction systems are designed to minimize.

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http://en.wikipedia.org/wiki/Vehicle_Assembly_Building#Construction

Naturally, that much moisture is problem, so there are attempts to de-humidify as much as possible, but still it's Florida!

Then, we have the result of combining Liquid Oxygen (LOX) and Liquid Hydrogen (LH)....to form H2​O:


And, despite the title...it doesn't so much as make "rain clouds", as make low-level stratus....AKA fog. Of course, if conditions were suitable, then what's in the condensation could eventually combine and form larger droplets, which could then fall as rain. But, only in a very limited radius of the test facility itself.

 

[deirdre]

I mean the conditions are all right for contrails EXCEPT for the outside temperature. it only takes -40 because the exhaust is hot right? so if its -20 it may take 5 minutes for the exhaust to cool enough to freeze at -20. am I making sense?

 

[Jason]

I mean the conditions are all right for contrails EXCEPT for the outside temperature. it only takes -40 because the exhaust is hot right? so if its -20 it may take 5 minutes for the exhaust to cool enough to freeze at -20. am I making sense?

It would freeze instantly once it exited the exhaust, here's a simple youtube demonstration of how boiling water freezes instantly when it mixes with sub freezing temperatures (-41 degrees)

https://www.youtube.com/watch?feature=player_detailpage&v=jKMNSvpB9dY

 

[Mick West]

I mean the conditions are all right for contrails EXCEPT for the outside temperature. it only takes -40 because the exhaust is hot right? so if its -20 it may take 5 minutes for the exhaust to cool enough to freeze at -20. am I making sense?

The exhaust condenses and then freezes in a fraction of a second. So if you don't see it condense, you won't see it freeze.

 

[Ross Marsden]

Back to the fundamentals...
There is water vapor in the exhaust gas (hot), and there is usually some water vapor in the environmental air (cold).
As the exhaust mixes with more and more environment air, the temperature decreases, the water vapor content usually increases, and the Relative Humidity changes.

The changes in Relative Humidity (with respect to water) of the exhaust-environment mixture require our close attention.

Initially RH is low because the mixture is mostly exhaust with a little bit of environment air.
As more cold environment air mixes in, temperature lowers and the RH gradually rises.
Late in the process, the RH is lowering again because the mixture is then mostly environment air with very little water vapor content (compared to the exhaust), and even more is being added.

Somewhere in the middle of the process the RH reached a maximum value.
If it reached 100%, then the water vapor would have condensed on the abundant naturally occurring cloud condensation nuclei. Contrail!
If the RH of the mixture didn't reach 100%, no condensation, and the exhaust-environment mixture would remain invisible.

The question is this: Is the exhaust of piston aero engines cool and moist enough that water condensation can be reached in an environment as warm as -20°C? No, I don't think those engines were that efficient.

In Calculations of Aircraft Contrail Formation Critical Temperatures there are tables for various engine types (contrail factors). I think the original Appleman contrail factor of 0.0336 g kg−1 was for piston engines. And here is the table.


So at 24,000 feet (400 hPa) , which may be typical cruise altitude for a piston engine aircraft, the environment temperature needs to be below -39°C (moist air) to -48°C air (dry air) for contrails to form.

The combustion products do contain CCN, and these do contribute to the condensation. More importantly there are ice nuclei in the exhaust products, like soot, SO2, nitrates, etc. It's on these that the condensate freezes.