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4 Answers

Wingtip vortex cloud trail

Asked by: 3386 views Aerodynamics

What causes and why do you observe long linear cloud trails when a wing is loaded.

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4 Answers

  1. Chris Carlson on May 13, 2013

    Seeing that this has gone a few days without answer, I’m going to try my best, but I am open to any corrections.

    Contrails, which you refer to as long linear clouds behind an aircraft, are a combination of water vapor and exhaust. As the aircraft passes through the air, it creates a forest off of each wing (wingtip vortices, to be explained in many other threads). Inside the vortices is low pressure, which is conducive of creating clouds. We now have water vapor and exhaust, which encourages nucleation of water vapor, in a low pressure system. That is a great recipe for a cloud. At the altitude that most contrails are seen, it is below freezing, and it is possible that most contrails are actually ice. The humidity of the local area of the contrail is the variable that determines the length of time that the contrail will stay as a visible cloud.

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  2. Steve Butler on May 13, 2013

    May I give it a try (not an engineer or CFI so take it with a grain of salt). It is my understanding that there are two completely different types of contrails. One is the result of condensation of water vapor from cooling exhaust gases and this type is prevalent in the 25,000 to 30,000 ft. altitude range. The other is the result of wingtip vortices which is very often seen in warm moist air at lower altitudes. Wing tip vortices are not a major factors at high altitude during straight and level flight. When they do become a major factor is during high angle of attack flight such as landing, and high g turns and pull ups. The kitchen refrigerator works on the principle that when gases are compressed, cooled somewhat, and then the pressure is rapidly lowered, you get significant cooling of the gases. At a high angle of attack, the air is compressed under the wing and as it spills out over the wingtips creating the vortices, the pressure is rapidly reduced as in a tornado. The pressure reduction lowers the temperature of the air below the dew point. If there is sufficient moisture in the air, it will create a cloud which is the contrail. You will very often see this type of contrail when a heavy jet makes its initial pull up during take off and also sometimes at air shows during high speed, high g turns and pull ups. Does this make sense to anyone?

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  3. Brian on May 16, 2013

    I suspect you’re referring to trails seen when a fighter jet performs a high g maneuver. The answer is a combination of adiabatic cooling and dew point.

    When dew point is reached moisture becomes visible. We learn this at a young age the first time we exhale in a cool environment, watching as our breath becomes visible. Warm moist air leaves our mouth only to be rapidly cooled by the surrounding air until reaching it’s dew point.

    What about this adiabatic cooling though, what is that? The term adiabatic means temperature change will result from changes in pressure. Wing tip vortices, even in a level flight condition, exhibit an area of low pressure. In fact, if we were to stick a temperature probe in the vortex and compare that temperature with the one by our wind shield we would find the temperature within the vortex to be lower. The stronger the vortex the lower the pressure and the lower the temperature.

    With the above we are equipped to answer your question. In a high g turn wing tip vortices become strong enough that the pressure change results in adiabatic cooling sufficient to cool the surrounding air to it’s dew point. The result are those cool white lines of moisture seen on a heavily loaded wing.

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  4. Brian on May 16, 2013

    I almost forgot, Chris, great explanation. I do question the correlation you make to contrails being exhaust though. I might just be misreading this though.

    Either way, contrails stands for condensation trails. In the case of exhaust, mostly comprised of carbon dioxide and water vapor, the water vapor is the trigger. Much like when we breath into a cool air the local relative humidity is increased and the air reaches saturation point. The contrails formed from exhaust at a high altitude is better synonymous with our breathing into cold air.

    Tip vortices, on the other hand, produce contrails by lowering the temperature of the air to it’s dew point. Rather than adding moisture to the air, as we see with the jet exhaust system.

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