Clarification about Aircraft Over-Wing Fog Formation (Adiabatic)

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Discussion Overview

The discussion revolves around the phenomenon of fog or cloud formation on the upper wings of aircraft during takeoff and landing, particularly under humid conditions. Participants explore the underlying physical principles, including adiabatic expansion, pressure differences, and the effects of wing design and operation on this occurrence.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the fog formation as resulting from lower pressure on the upper wing leading to adiabatic expansion and a temperature drop, which causes condensation.
  • Another participant notes that the dew point varies with atmospheric conditions, explaining why fog does not always form on the wings.
  • Several participants discuss the relationship between wing velocity, angle of attack, and the magnitude of low pressure affecting fog formation.
  • There is a mention of the effect of spoilers on lift and how their deployment correlates with the disappearance of fog, although the timing of spoiler deployment is debated.
  • One participant emphasizes the importance of the landing gear compression as an indicator of lift shedding, rather than solely relying on spoiler deployment.
  • Another participant highlights the variability of fog appearance during a constant bank due to changes in air temperature and humidity.

Areas of Agreement / Disagreement

Participants express both agreement and disagreement on various aspects of the phenomenon. While there is consensus on the role of pressure and temperature in fog formation, the specifics of how spoilers and landing gear affect this process remain contested.

Contextual Notes

Participants acknowledge that the discussion involves complex interactions between aerodynamic principles and environmental conditions, which may not be fully resolved in their exchanges.

Who May Find This Useful

Aerospace engineers, aviation enthusiasts, and students studying fluid dynamics or aerodynamics may find this discussion relevant and insightful.

RobbyQ
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You may have noticed when airliners land or take off on humid days a fog or cloud develops on the upper wing. My understanding is that the upper wing has a lower pressure (Bernoulli's) than the atmospheric pressure (QNH) at the airfield. When the air molecules hit the upper wing there is a rapid adiabatic expansion due to the lower pressure and work is done, resulting in a drop in temperature within 2.5C of the dew point. Condensation onto dust particles occurs and the fogging is observed. I also liken this to that small cloud you get when you crack open a can of fizzy drink. Would this be correct?
Here is a link of the fogging effect:
 
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That is basically correct, but dew point changes according to atmospheric conditions, reason for which the foggy cloud on top of wings not always happens.
Also, the magnitude of the low pressure also varies according to wing velocity and angle of attack.
 
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Lnewqban said:
That is basically correct, but dew point changes according to atmospheric conditions, reason for which the foggy cloud on top of wings not always happens.
Also, the magnitude of the low pressure also varies according to wing velocity and angle of attack.
Good feedback. Thanks.
Also, did you notice on the touch down in the video where the spoilers were deployed, lift dump occurs and the fogging disappears as the upper wing is back to the airfield pressure.
 
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Perhaps the best indicator is that at about 0:53 in the video. As the jet touches down and load shifts from the wings to the landing gear, that wing fog disappears proportionately to the landing gear compression - clearly indicating that it is the direct product of the lift.
 
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.Scott said:
Perhaps the best indicator is that at about 0:53 in the video. As the jet touches down and load shifts from the wings to the landing gear, that wing fog disappears proportionately to the landing gear compression - clearly indicating that it is the direct product of the lift.
I think that's lift dump as spoilers deployed and upper wing pressure is back to standard. Also killing the lift, as you implied, means all the weight is back on the wheels and improves braking efficiency.
 
RobbyQ said:
I think that's lift dump as spoilers deployed and upper wing pressure is back to standard. Also killing the lift, as you implied, means all the weight is back on the wheels and improves braking efficiency.
At 0:52, the wing fog is gone and the spoilers are not deployed. The first sign of the spoilers is over a second later at 0:53. Because spoilers increase the stall speed, it is normal practice to land before fully deploying the spoilers as brakes.
I think the best indication of the shedding of the lift is the compression of the landing gear suspension system.

I have piloted aircraft with air brakes (not spoilers). I love them. They give super precise control of your descent angle.
 
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.Scott said:
At 0:52, the wing fog is gone and the spoilers are not deployed. The first sign of the spoilers is over a second later at 0:53. Because spoilers increase the stall speed, it is normal practice to land before fully deploying the spoilers.
I think the best indication of the shedding of the lift is the compression of the landing gear suspension system.
The spoilers are armed and deploy when the rea wheels touch the runway. I don't think we can argue about what happens within 1 second here. Also it's just coming out of the flare so angle of attack is decreasing. Spoilers are 2 fold: 1. During the descent procedure they act as air brakes (if needed) 2. On touch down they kill the lift and get that 1G back onto the wheels for efficient braking.
 
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RobbyQ said:
Good feedback. Thanks.
For the airplane flying a constant bank (timelapse 1:41 through 2:07), note that the above-wing pressure (and AOA) should remain more or less constant but fogging appears and disappears according to the air temperature and humidity in different zones.
RobbyQ said:
Also, did you notice on the touch down in the video where the spoilers were deployed, lift dump occurs and the fogging disappears as the upper wing is back to the airfield pressure.
Indeed.
You can also see how the bending (smiling) along the wingspan disappears.
The weight is transferred from the hanging wings onto the rolling gear.
 
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Lnewqban said:
For the airplane flying a constant bank (timelapse 1:41 through 2:07), note that the above-wing pressure (and AOA) should remain more or less constant but fogging appears and disappears according to the air temperature and humidity in different zones.
Indeed. Very noticeable. There's a lot going on here. Time for some Navier-Stokes :smile:
 
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