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I am posing this question from the perspective of a novice. I read an article, from Scientific American, titled "No One Can Explain Why Planes Stay in the Air". The article explains how, while we understand how to create flight (with airplanes, for example), we still don't understand why there is lower pressure above the wing, allowing for the air above the wing to flow more quickly, thus generating lift. If you care about this topic, read the article I am referencing, it lays out the two competing theories for explaining why air pressure is lower above an airplane wing in flight. It also shows how both of the theories are incomplete.

I found it fascinating that we still don't understand this, so I decided to think about it a little. I am not a physicist, but I would say I have a basic understand of physics (having at least studied it during high school). The first aspect of what is going on when an airplane is flying that I decided to think about is (to me) the most obvious force involved in flight: gravity. I then conjured up the question, "how does gravity act on air?" If you type that into a google search, it will bring up this text box at the top:

As gravity hugs the blanket of air to the Earth's surface, what physicists call a density gradient is set up in the air. The air near the ground is pulled on by gravity and compressed by the air higher in the sky. This causes the air near the ground to be denser and at a greater pressure than air at higher elevations.

Google cites this as coming from https://www.uu.edu/dept/physics/scienceguys/2001Oct.cfm - I am assuming it is reputable if google is choosing to present it as a "quick answer" (if you will) for the search "does gravity act on air".

In this explanation, I think what it is saying is, the air that is closer to the ground is more closely packed together. It mentions the "density gradient," and I interpret that concept to mean that, as you get closer to the edge of our atmosphere (starting at ground level), the air will become less and less dense as you go higher and higher. So if that is how air behaves under gravity, and an airplane is flying through the air, this means that, as it is being propelled sideways (as it's wings "cut through the air"), by nature, it is going to split the density gradient of the air (which I think can also be directly linked to air pressure - more dense air, air closer to the ground, is higher pressure, and less dense air, air further from the ground, is lower pressure). In saying this, then I think I can say that, as the wing cuts through the air, there is no reason why it wouldn't cut through the air in a way where it creates a situation around the wing, where there is lower pressure above the wing, and higher pressure below the wing (even if it is only a small difference - much force is being applied sideways in this situation by the airplane), thus allowing for lift. There is no reason for the air that is higher up (the inherently less dense air) to pass below the wing, because it is already above the more dense air (the air that is already closer to the ground) as described by the density gradient. It seems to me, that this notion supports the idea that there would be lower pressure above an airplane wing (allowing for the air to move more quickly above the wing), and higher pressure below it. Everything I have said kind of seems pretty obvious, and seems to support the idea that lift will be inherent in any situation where something is slicing horizontally through the air at a rate of speed where it is able to negate any other forces that may act in an effort to make it move non-horizontally. I'm guessing I am missing something, and it isn't this (seemingly) simple - either way, with what I think I have discovered through my brief research, I basically want to know why what I said is wrong, if it is (I guess it would be pretty cool if I just figured out this problem haha).

Thanks!

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    $\begingroup$ This has been asked many times, and it is simply not true that aerodynamic lift is unexplained. Please see physics.stackexchange.com/questions/13030/… $\endgroup$
    – D. Halsey
    Jan 31, 2020 at 22:57
  • $\begingroup$ Well than it seems my next question is why did scientific american put out an article in support of the notion that we still don't understand why lower pressure is above a wing while in flight? It seems the answer in the post you have referenced provides a comprehensive answer, but at the end he still says, "This is the absolute best explanation I have. If you assume that the fluid is incompressible it works great...The bold text is the best answer I have and I think it's a good one." So is his answer complete? $\endgroup$
    – ewizard
    Jan 31, 2020 at 23:09
  • $\begingroup$ 🦗 🦗 🦗 🦗 🦗 🦗 🦗 🦗 🦗 🦗 $\endgroup$
    – ewizard
    Feb 1, 2020 at 14:31
  • $\begingroup$ It's been known for 100 years how airplanes fly, but that doesn't stop questions like this from being asked. If you want people to go read something, YOU go read something: av8n.com/how The only thing tricky is something called the "Kutta Condition". It's the reason for the sharp trailing edge of the wing. It's why you can blow a candle out, but you can't suck it out. $\endgroup$ Feb 1, 2020 at 23:38
  • $\begingroup$ I will check out you link - my first move was to ask a question here, (google isnt the only resource on the internet) if you don't feel like answering or partaking, than don't. Furthermore, you go on to say the Kutta Condition is "tricky" it seems like if something "tricky" is going on, while people clearly understand flight, it seems my question still makes sense to ask - it is possible to understand how to fly, while the concept of why lift exists remains a "black box" (as scientific american says it is still). I still haven't heard anyone say anything that conclusively answers my question. $\endgroup$
    – ewizard
    Feb 2, 2020 at 16:18

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"I'm guessing I am missing something"

Yes you are. Two things in particular:

Firstly, hot air rises, so there is a density gradient in opposition to gravity. This will tend to provide regions where there is effectively no density gradient, and therefore no lift, under your idea outlined above.

Secondly, a modern airliner has a wing depth, from it's top surface to it's undersurface, of say 100 cm on average. Earth's atmospheric density gradient is far, far less than this, so these aircraft manage to fly in air that has no density gradient, in real terms.

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  • $\begingroup$ I have some questions though. 1) i understand hot air rises, but that doesn't necessarily speak to pressure, what i found on google says that the air is pulled harder by gravity at low altitudes, and there is less pull at high altitudes - ergo, high altitudes are associated with less pressure, so im not sure if what you are saying about air temperature is correct (according to what I found about gravity and air on google). also, if there are areas with no density gradient, im assuming it wouldnt matter if those areas are not large, how big can the areas without density gradient be? $\endgroup$
    – ewizard
    Feb 1, 2020 at 1:44
  • $\begingroup$ what do you mean by "earths atmospheric density gradient is far, far less than [100 cm]" that doesn't seem to make sense to me either, the density gradient stretches through the atmosphere, so at whatever altitude the plane is at, the wing is passing through the air with a 100cm height (as you say) and so what is above that is at a lower pressure, than what is below the wing, that is what i am wondering about. $\endgroup$
    – ewizard
    Feb 1, 2020 at 1:50
  • $\begingroup$ Okay, I'll bite :). ...as far as pressure goes, think of my answer the next time you get turbulence in flight caused by hot air rising from mountain areas in the middle of summer, I guarantee you that's air pressure that's making you pray :)....in Earth's atmosphere, pressure and temperature effects are inseperable......as far as areas of no (or at least balanced) density goes, Google cloud formation then look up and estimate on any given day how much rising air there must be. $\endgroup$
    – StudyStudy
    Feb 1, 2020 at 2:08
  • $\begingroup$ There is always a density gradient, my point is that over a vertical distance of 100 cm, it is a neglible pressure difference...it might affect a fly, but not a 550 ton Boeing $\endgroup$
    – StudyStudy
    Feb 1, 2020 at 2:12
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I will first summarise the question as I understand it, so that if I'm wrong you can quickly correct me without having to read through a lot. Here goes:

(1) There are competing theories about lift on a plane, and an SA article claims they are both incomplete.

(2) You have a third, simpler interpretation, and you want to know if that is complete.

Now, your interpretation is solely based on natural density differences in the atmosphere, and the upward force generated on these for the same reason. Consider two counters to this:

(a) A plane can also land, or lower it's altitude in the same atmosphere just as it can raise it's altitude. Going down does not require more fuel/effort than going up, which you would expect if density gradients were keeping it upfloat. Note that I'm assuming no change in speed here.

(b) It is possible for a plane to fly very close to the ground, where the density difference is pretty low (atmospheric density doesn't actually vary linearly with height).

There is a reason for airfoil design, and for flexible ailerons, namely that they can change the direction of airflow above and below the wing. These airflow directions actually generate the pressure differences and generate lift, in a manner basically similar to what you propose. Just that it's coming from airfoil design and not natural density differences.

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  • $\begingroup$ I ha e some more questions. In this situation we are talking about lift only, not the plane’s ability to go up or down any substantial amount. That being said if you can find somewhere to show me where it says going down costs exactly the same amount of energy going up that would help. As i see it there is a problem with uour ligic here. When a plane goes up it is fighting gravity and when a plane goes down it does not, so that would take precedent in the situation where we are measuring effort of up or down directions. So i wonder if that is a problem with (a) $\endgroup$
    – ewizard
    Feb 1, 2020 at 14:18
  • $\begingroup$ In respnse to (b). I guess i am not saying that what i think would actually affect whether or not a plane can fly close to the ground. Even if there is only a very small difference in density of air close to the ground, there still is one, all i think i am saying is that, the very fact that there is any difference in air pressure at all is the source of lift. While airplanes weigh a lot. They also can propel themselves forward very quickly. $\endgroup$
    – ewizard
    Feb 1, 2020 at 14:24
  • $\begingroup$ ...If they are able to negate the downward force of gravity by going sideways, then what i am saying is it seems like the difference in air pressure (no matter how small) is what allows for lift, given that the airplane has flaps (and its own power source) so that it is able to direct itself. I basically wonder. If the density gradient were reversed (not possible, i know) that would mean that higher pressure is above lower pressure at all times, i wonder if that is a situation that wouldnt allow for lift $\endgroup$
    – ewizard
    Feb 1, 2020 at 14:25
  • $\begingroup$ i guess to relate it back to airfoil design, how you talk about at the end of the above answer, are there any airfoil designs that would work in air with a reverse density gradient (and gravity would still exist too obviously)? Or is there one that would work, it would just be a different design than the one we have now? $\endgroup$
    – ewizard
    Feb 1, 2020 at 14:37
  • $\begingroup$ @ewizard - I see your point. See, gravity is there anyway; when we talk about lift we are implicitly considering lift sufficient to overcome gravity. If the lift generated by density differences is less than gravity, no point considering it as a force responsible for flying, right? About fuel, you will find a fairly close example in the excellent link given by Mike as a comment, section 1.2.3. It takes more fuel to go up than to coast at the same height, which you wouldn't expect if density differences were the major factor involved. $\endgroup$ Feb 2, 2020 at 7:24
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While you blame pressure gradient for the lift, why do you need the plane to "slice" through the gradient? There is a pressure gradient across the wing as it is waiting to be boarded. Here's another thought in the same line of reasoning : use a wing that has an even thicker profile, thereby creating more lift due to greater pressure gradient exposed and once the plane is airborne, shrink the profile to reduce the enormous drag. The thing is that pressure gradient produces negligible gradient, which in fact would be changed merely by the temperature gradient effected by the engines, etc. Coming to the article in Scientific American, unfortunately, popular science book writers, in an attempt to attract interest from a wide audience, pretty often make bold statements. Just like "no one can explain why planes stay in the air", exclamation mark. Actually it is very well explained, as the article later admits. "But by themselves, equations are not explanations" it goes on to add, which is just a semantic game. Equations are just representations of explanations. If you can't explain it, you can't generate equations just by scribbling on a piece of paper. Then the article accepts this but does not give up and claims "but the theorem does not constitute a complete explanation of lift". Actually it does, but the author is just not aware of it. I personally would suggest reading more serious stuff than Scientific American.

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