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Inside a cloud, the plane flies in a medium (air + water droplets) with a higher density than 'normal' air. If airspeed and AoA are the same, lift should be higher. It is really so...?

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  • $\begingroup$ And the moisture should add weight. $\endgroup$ – user7241 Dec 13 '17 at 14:38
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    $\begingroup$ Are you certain that air parcels in clouds have a higher density than air parcels outside clouds? The opposite is often the case. $\endgroup$ – J Walters Dec 14 '17 at 21:45
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    $\begingroup$ Also, and this might be critical to providing an accurate question, what do you mean by "all other things equal"? A cloud parcel of air cannot be equal to a non-cloud parcel of air. The factors that determine cloud formation (temperature, saturation, pressure, air mass stability, etc) necessarily render something not equal between the two parcels. What the difference is might determine the answer. $\endgroup$ – J Walters Dec 14 '17 at 21:46
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If we consider droplets "floating" in the air, they should hit the wings at the angle of attack and provide some lift. As they are more dense than the air, the lift should be more that just hitting the amount of air that could substitute the droplets.

There are also other effects like humid air is less dense and if the droplets are large enough to fall they should hit horizontal surfaces, pushing down. The overall effect depends on all these factors and may be complex to define.

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No, higher humidity air is less dense than lower humidity air. This is because water molecules are lighter than the nitrogen and oxygen they would replace in an airmass.

https://science.howstuffworks.com/nature/climate-weather/atmospheric/is-humid-air-heavier-than-dry-air.htm

any water vapor that gets added replaces either nitrogen or oxygen in our free-moving air. Nitrogen and oxygen make up the majority of our atmosphere, and they're displaced -- or evaporated -- when water takes their place in the air. And water vapor molecules are lighter than both nitrogen and oxygen. In other words, humid air is going to have less heavy nitrogen and oxygen -- and lighter hydrogen and oxygen -- in its place. Remember that they have the same number of molecules, but the air with water vapor is simply less dense

Humidity, therefore, is also an important factor in density altitude calculations.

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    $\begingroup$ This is a good thought, but by its very nature, at least some portion of the water present in a cloud is necessarily in a non-vapor form: either ice crystals or liquid water droplets. I'm not sure the water vapor density facts behind this answer apply to this question. $\endgroup$ – J Walters Dec 14 '17 at 21:49
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No, or at least not lift due to the airfoil (see below). The difference between the air inside of a cloud and that directly adjoining it will be relatively small. Also, the air inside a cloud is less dense in most cases than the surrounding air because it is warmer. The following diagram illustrates the effect:

enter image description here

The air outside the cloud is cooler and more dense. The air inside of a cloud, therefore rises, and outside of it, the air sinks. Any glider pilot experiences when passing beneath a cloud: the glider rises as it travels under the cloud, but as soon as it exits the area under the cloud, the glider sinks.

An aircraft will experience lift inside of a cloud, but this is due to rising thermal air currents, not lift granted by the wing's airfoil.

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    $\begingroup$ I can follow you on the warm and rising air below the cloud. But isn't droplet formation caused by a drop in temperature? Warm air can hold more moisture than cooler air. $\endgroup$ – user7241 Dec 13 '17 at 16:06
  • $\begingroup$ Yes, the air in the cloud is colder than the air BELOW it, but the air alongside of the cloud is colder than the air inside the cloud. $\endgroup$ – Tyler Durden Dec 13 '17 at 16:10
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    $\begingroup$ The cloud forms because the air is rising. The air is rising because it is less dense than surrounding air. As the air rises pressure decreases causing droplet formation. The droplet formation in turn causes the drop in temperature. Aerodynamic lift doesn't change. The AoA might go up briefly as you enter the cloud, but will quickly return to the trimmed state. Flying at straight and level AoA through rising air causes positive climb rate. Same way the same airspeed with a tailwind yields a higher ground speed $\endgroup$ – TomMcW Dec 13 '17 at 19:01
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    $\begingroup$ Many clouds do not include an instability lifting force. Certain types of clouds do. Many cloud types include both updrafts and downdrafts. Other cloud types are stable with little or no associated up or down drafts. An aircraft my or may not experience some form of increase or decrease of lift inside a cloud. $\endgroup$ – J Walters Dec 14 '17 at 21:57
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    $\begingroup$ @jjack Stability refers to an airmass's tendency to resist vertical displacement. Conversely, instability refers to an airmass's tendency toward vertical motion. Different types of clouds are related to stable or unstable airmasses with accompanying temperature and density dynamics. $\endgroup$ – J Walters Dec 15 '17 at 13:12

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