There is an analogy between this and infeasibility of "Explainable AI".
For the Lift Theory we have Navier-Stokes equations that give you the lift if you solve them numerically or, maybe, figure an analytical solution. But most people won't accept it as an "explanation" - they want a simpler description in more coarse-grained concepts and relationships. And, it seems, that there is no such satisfactory reduction for the Lift Theory - there are just too many interrelated things happening at the same time.
Same thing goes for some decision-making AI. One might like to have an "explanation" for why AlphaZero decided to make a particular move. And sometimes there could be a general high-level pattern rule, that one could extract from the model. But, it seems, that in most cases no simple rule can be expected.
I upvoted this, because I think the fundamental thesis is right: most of the time, for physical phenomena, there is a wrong-but-close approximate explanation that is within the understanding of someone with high-school science, or at least within intuitive grasp thereof. This is the basis of popular science.
The errors with the approximation reduce as you apply better models, and those models are the domain of practitioners.
The question of why airfoils generate lift seems to be resistant to the popular science technique. The first useful approximations all require calculus and are the result of whole-system effects at a macro-level. So all the explanations you can get to from high-school science are contradicted by real-world experience.
"There is no gravity in outer space. Gravity ends at the top of Earth's atmosphere. That's why astronauts in the ISS are weightless."
Simple and concise. Easily understood by little kids. Textbooks in the 1950s actually taught this "fact." What could be the problem?
It's not "simplified," instead IT'S WRONG...
...and it creates a massive physics-misconception, where if a student actually believed it, later they'd have enormous trouble in physics classes. (Also, if they later became a teacher, then they'd give all their students the same destructive misconception. If later they became a textbook author for K6 grades, millions of students would become infected! Heh, education is a viral effect: a meme-spreading process, and "mental diseases" like the above are common.)
I like this explanation that likens the search for principles to compression:
The natural human urge to look for simple explanations is a compression problem. If you can find a simple mechanism for operating in a complex universe, you can teach it to your children more easily.
Training AIs has a similar motivation — a simpler function is cheaper to store and compute.
Perhaps we can call the simplest explanations we've found so far "Principles". We want to believe they are universally true. For example, conservation of energy, or humans should have equal rights. They seem to fit our current reality, and they are vast simplifications over doing things ad-hoc, but are they more than lucky simplifications of a complex universe?
On the other hand, simple intuitive explanations may not lead to simple mathematical analyses. Things which are obvious and simple to grasp, may involve near-insoluable systems of equations! This seems to be the case with lifting force.
We can easily explain lifting-force if first we get rid of the wingtips. Just explain 2D airfoils in a 2D "flatland world." Infinitely-wide wings are almost trivial to analyze.
There is a simple "explanation" of wing lift, though: the wing pushes air down, it applies downward force on air masses. This is a trivial consequence of Newton's third rule and the facts that a) there's gravity, b) airplanes don't fall from the sky.
If you want to know what exactly is happening to the air around the wing you need fluid dynamics of course.
I don't know if we'll get any such "conservation of X" principle from the AI world, but that would be pretty cool.
I'd argue that that is not really an "explanation", because you cannot meaningfully generalize it. Yes, there is a lift because there is upward force on the wing, because there is a downward force on the air... But, how do you use it in another context? That feels more like a sequence of tautologies, than an explanation.
But if air is pushed down, then it must continue moving down (much like with a hovering rocket, or a helicopter.)
But this effect does not appear in typical airfoil flow-diagrams. Instead, the air approaches horizontally, and leaves horizontally. It's a symmetrical pattern, as guaranteed by "circulation theory." In these diagrams, the wing doesn't fling any air downwards. (Heh, the same thing happens with helicopters that fly inside a giant vertical pipe, where the blade-tips slide along the inside surface of the pipe.)
When I was studying for my PPL, they taught us using the equal transit theory, though the instructor couldn't help himself from telling us that it was a simplification. This video from Cambridge University illustrates what happens to the airflow over an aerofoil: https://www.youtube.com/watch?v=UqBmdZ-BNig
Euler equations explain the lift, but it can be still hard to grok.
Newton’s Laws of Motion gives the clearest correct explanation: If the wing stays in the air, there must be a force that is equal but opposite to the gravity.
If you divide the air around the wing into equal mass chunks of air (not equal volume) and visualize the velocity and acceleration of each chunk, you get very informative picture of where and when of the lift. If the wing stays in the air, air around the wing must be accelerating downwards.
In 2D airfoil diagrams, first the parcels approach horizontally. Then they are mysteriously accelerated upwards. Then the airfoil accelerates them downwards. Then behind the airfoil, they're mysteriously accelerated upwards again, so they depart horizontally, for zero net acceleration applied to each parcel.
As a whole, air around the wing ISN'T accelerating downwards.
As long as we ignore the "mysterious" part, or try to sweep it under the rug, our explanations won't satisfy.
A complete explanation must include the ground surface ...and then we discover that we're actually explaining a kind of "venturi effect," where the ground is a part of the system, and those mysterious accelerations before and behind the airfoil are caused by interaction with the ground.
This is a "big AHA!" situation: a 2D airfoil can only explain ground-effect flight, where the Newtonian force-pair is located between ground and the airfoil, so no net downward acceleration of air exists. (In other words, if we erase the ground from our diagram, we've just violated Newton's 3rd law.)
A 2D airfoil diagram without the ground surface ...that's a sort of "inertial drive," where the upward force on the airfoil has no corresponding down-force (it does not give a net downward acceleration to the air as a whole.) Easy to fix. Just add the ground surface back in. And then confront the fact that this is a venturi, and not an explanation of flight.
Expanding on that, the downwash theory is the easiest to understand in my opinion. If you think about the flow of water over the back of a spoon then imagine increasing the amount of the water flow and the size of the spoon until you could eventually feel the force pulling your hand opposite the direction the water is deflected.
"Longer path" is an incorrect explanation for why the air above the airfoil moves faster. Yet the air above the airfoil really does move faster, as that webpage correctly describes. (That webpage doesn't say why the air moves faster! Thus they avoid the "equal transit fallacy.")
- "Airplane wings are shaped to make air move faster over the top of the wing."
True. Giving the airfoil some camber will change the upper/lower velocities, making it produce lift, even at zero attack. No mention of "equal transit-time."
You remember? Action requires opposite and equal reaction?
Plane is (mostly) not a rocket and so to be able to fly it has to act on something and the only something up there is air. If the plane is being lifted it means a bunch of air needs to be pushed/dragged down.
Helicopter is a better way to visualize it.
Now, in a plane, once that air is pushed/dragged down it immediately starts doing funky things like vortices and so on, but at least initially the air is being pushed/dragged down.
I write "pushed/dragged down" because the air is deflected on both sides of the wing. On the low pressure side of the wing (hopefully for most people this is the upper side) it is being deflected by creating low pressure in a laminar flow. If that laminar low pressure flow state is lost and instead we get turbulences, those turbulences are really bad at inducing the air to go down. We say that "it has stalled".
EDIT:
The only way for something to stay up is to push off something. When you stay on the ground you push off the ground but when you are up there you have to push something else even to just stay at an altitude.
Wings, turbines, rocket engines, propellers, barn doors, these are just different ways to accomplish the same task -- push air (and very infrequently some other material) downwards.
A plane is different from a rocket in that the material is mostly air (and very tiny amount of fuel) and main mechanism of pushing it are wings and air breathing engines/turbines.
Some planes can have so much power that their main pushing mechanism is engine exhaust and additional air sped up by the turbine.
But most planes have engines that do not produce anywhere near what is needed to stay up this way. These planes use the engines to push (mostly) forward (there is slight downwards vector due to engine exhaust being in line with the plane body but AoA positive). These planes rely on air stream being deflected downwards as seen by the plane to be economical.
First of all, rockets are no exception. Just throwing things backwards will make the object fall to the ground, no matter how hard it pushes.
It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it.
Vorticity has to come into the explanation, and explains both the points further up.
Imilcin is precisely right. Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing.
Rockets are easiest understood by concervation of momentum. In the case of a rocket, this is for the combined momentum of the rocket and the spent fuel/exhaust. The rocket "trows" the exhaust backward with a momentum exactly opposite to the momentum gained by the rocket.
A wing or the propellar. They accelerate the aircraft by "throwing" air in some direction, with a momentum per unit of timemass equal to the acceleration they cause. (In order to maintain their altitude, this acceleration must counteract the "gravitational acceleration", g of 9.81m/s^2)
This all follows from Newton's laws, where "Force" is the time derivative of momentum F=(dp/dt)=ma => a=(dp/dt)/m. With conservation of momentum for the aircraft + air, one gets:
> Rockets are an exception, in that they work perfectly well in a vacuum, where a propellar would do nothing.
A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity.
> Airodynamic theory, including Bernoulli and Navier-Stokes equations only serve to explain _how_ the wings accelerate the air down.
But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation?
> A sideways rocket would... fall to the ground. Just like anything that doesn't have a way to oppose gravity.
Well, a helicopter pointing it's rotor horizontally also falls down. Why do you assume that the rocket should point horizontally? (Technically, of course, rockets _can_ and _do_ point horizontally, after going fast enough to maintain orbit.)
> But those are the very point of this discussion. Does the force, enough to hold up the entire aircraft, come from essentially pinging tiny tennis balls off the wing, or does it perhaps come from a complex interaction of the air with the wing, involving ideas of boundary layer separation?
There are multiple factors at play. Deflection of air downwards is a side effect of those factors that creates lift.
From your previous post:
> It is indeed the air that pushes the wing up, but you have to ask yourself why the wing would need to be shaped in a certain way if it's merely deflection. If it was that, you could just have flat boards at an angle and be done with it.
Creating lift can be done perfectly well by a flat board. The wings are shaped as they are to minimize the drag/lift ratio at the speeds and angles of attack the wings will operate under. And for that all the complexity of the airodynamics and air/wing interactions come into play.
You can have 'planes with wings that are "flat boards at an angle."
You're right that it's not "merely deflection", but the reason you give here is not conclusive.
PS: I've tried replying to you by email so we can coordinate our efforts, but I don't seem to have been able to deduce your email from your profile. If you want to contact me, my email is in my profile.
According to the NASA site, the theory is not wrong but incomplete as it ignores the dynamics on the upper wing. This means, you probably could build a plane with this theory but would be surprised by the amount of lift you get.
Yes, it is. It cannot be any other way; momentum is always conserved. You need to get that 9.81m/s^2 acceleration from somewhere!
To answer your two questions:
1) A simple wing is basically a teardrop shaped fairing around a flat plate - not unlike a rudder. Like a rudder, it can redirect the flow in either direction, depending on the angle it meets the air. (In fact, most aircraft struggle to fly upside down - if they're not aerobatic they tend to have wings that are like fairings around curved plates, to more efficiently redirect air in a single direction.)
2) The aircraft in front of you has just imparted a ton of downward momentum to the air. Compared to flight through clean still air, you're on an elevator that's going down.
Regarding your 2nd answer, taking off quickly after another aircraft has departed is difficult because of wake turbulence. The most hazardous part of wake turbulence is the spiraling wingtip vortices which can be observed in this neat video: https://www.youtube.com/watch?v=dfY5ZQDzC5s
One notable wake turbulence incident was in 2006 at Boeing Field in Seattle where a Cessna 172 and Boeing 747 were both on final approach for parallel runways. The Cessna 172 got flipped almost upside down and recovered 150 feet over the terrain: https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?E...
A wing generates lift in relation to the relative airflow over it, so a plane with some velocity "straight upwards" generates lift exactly equivalent to the lift generated when flying "horizontally" at the same velocity.
You can have aircraft with engines powerful enough that they can overcome gravity with thrust alone so can fly upward indefinitely but their wings will still be generating lift, even if they are travelling vertically and it's not useful for overcoming gravity.
Through surfaces on the horizontal stabilizer, the pilot controls the angle of attack of the wing, and hence the "lift" force perpendicular to it. If the pilot doesn't want any lift, there's no lift. The "vomit comet" for instance achieves weightlessness by flying a totally ballistic trajectory; the wings produce no lift for up to 30 seconds at a time.
Yes AoA and it's relation to the wing generating lift is just a technical term for "a wing generates lift in relation to the relative airflow over it".
That's a simple, introductory explanation for kids and that sentence is contradicted twice on that page as well. For example:
"Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction."
And:
"Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion."
So I'm not wrong, even according to the page you cite.
Lift is defined to be perpendicular to relative wind (and drag parallel to it). What you cite is the simplification that works eg in unaccelerated straight and level flight.
I'm wondering whether I'm missing something, because whenever this topic comes up I think "the wing accelerates air downwards and the inertia of the air lifts the wing". What's incomplete or complicated about that?
I like searching for "downwash clouds" to see the effect :-)
It's not complicated, instead it's HERETICAL. It directly violates Bernoulli/Euler, because with downward accelerated air, the parcels continue moving down, long after the wing has passed by ...meaning that energy has been injected into the atmosphere.
Yet Bernoulli equation is entirely based on having zero energy injected. Bernoulli only works if we eliminate all net downward acceleration (so, the parcels MUST be left unmoving after the wing has passed by. As an example of this, look at any 2D airfoil diagram. The air approaches horizontally, and leaves horizontally.)
In my opinion, that issue seems to be the source of the entire controversy.
---
In real wings, Bernoulli equation cannot be employed, because parcels remain moving downwards, which means that lift is an example of propulsion. Bernoulli does not describe propulsion. Real wings are air-pumps, and they fling air downwards, leaving behind a wake of descending air. The wing is adding net energy to air parcels. Yet in fluid dynamics for beginners, all the low-level explanations reject this notion, and instead eliminate the air-pump (eliminate the energy-injection, eliminate the vortex-creation, eliminate the downward "exhaust-plume" created by all short wings. That way Bernoulli equation can apply.)
How do they do this? Just get rid of the wingtips! Make the wingspan be infinite.
In other words, we only analyze the flow around airfoils in a 2D world. In a wind-tunnel, a two-dimensional airfoil must have sliding contact with the walls of the tunnel, and the airfoil doesn't pump any net air downwards. It acts as a small slice of an infinitely-wide wing. It creates a pressure distribution which presses against the floor and ceiling of the wind tunnel ...even if the wind tunnel is very very tall.
To make Bernoulli equation applicable, we must only analyze airfoils having infinite wingspan.
But unfortunately, a 2D airfoil only depicts "ground-effect flight," because an infinitely-wide wing can never fly high enough to escape 100% ground-interaction. (It would have to fly higher than infinite altitude. A wing is in 100% ground-effect whenever the altitude is << wingspan, and if wingspan is infinite, then we're only explaining venturi-effect flight close to the ground surface, where no air is flung downwards on average, and the Newton's-3rd force-pair is between the wing and the ground surface.)
In other words, our explanations of lift have the ground surface built into the explanation (which then lets us employ Bernoulli/Euler equations, since zero energy is injected into the air.) The wing pushes indirectly against the ground, and the ground pushes indirectly against the wing. No need for net acceleration of the air. Yet real wings only push upon air, and the ground-surface plays no role.
But then we don't bother to mention any of this to our students. And, our 2D flow diagrams completely violate Newton's 3rd, because they don't even depict the ground!
Instead, real wings only create propulsion forces, and they fly much like helicopters: flying far from the ground, while leaving behind a momentum-carrying "exhaust plume" of descending air. Indeed, if first we explain a hovering helicopter, then later shove it rapidly sideways, we end up with a complete explanation of lifting force in fixed-wing aircraft. But then we're forbidden from applying Bernoulli, since Bernoulli cannot tolerate propulsion, or air pumps, or descending plumes left behind by airfoils.
I'm reminded of the car-keys joke. Someone is searching for their car keys at night, only searching under a bright streetlight, because the light is brighter there. But they dropped their keys far away, in the dark.
We've done the same with airfoil explanations: altering them until they no longer can explain flight ...but they do allow us to use much simpler math (employing a Bernoulli-based description.)
Unfortunately, if we want an intuitive and satisfying explanation of lifting force, first we must decide to remove Bernoulli entirely. Then sit ourselves down and figure out an alternative approach. This might even be easy! But it remains totally impossible while, right at the start, we insist on using Bernoulli concepts, where helicopters and propulsion and downwash-plumes are utterly forbidden.
It was started by a famous paper from Ludwig Prandtl, in ?1920?. The math is easy, if vortex-shedding is removed, and our wing has infinite span. Nobody realizes that an infinite wing is permanently trapped in ground-effect mode, and only works by instant ground-forces, just like a venturi. Just like those snow-speeders from Star Wars.
Other Prandtl papers analyze short, non-infinite wings. But they avoid all the insoluable equations by having the wing fly at infinite velocity! This way, no air moves downwards on average. Prandtl forgets that if the tip-vortices don't move downwards, then also the vortex-shedding goes to zero, and the tip-vortices vanish! By flying at infinite velocity, then pretending that tip-vortices are still created, Prandtl is "searching for car-keys under bright streetlights, where the job is easier" when the actual explanation of lifting force is still hidden elsewhere, out in the darkness.
Heh, another of Prandtl's papers rigorously described the equal-transit-time theory, giving us diagrams, and including it as the explanation for lifting-force. Yet nobody could contradict the Great Prandtl, since his papers were huge walls of interlocking equations, which were all correct. Only his initial assumptions were wrong, and it took about seven decades before physics teachers started seeing the problem.
There's a longstanding interest in the acadame in finding "clean" closed-form analytical explanations for all sorts of real-world problems, which mostly speaks to the historical lack of computing power to do a complete simulation leading to acceptance of bad approximations. Economics is also full of these kinds of equations, and many of the ones taught in undergrad are barely beyond a working hypothesis.
Actually things like that is what got me interested in physics. My physics teacher asked "so does lightning strike from the sky, downwards". Everybody in class agreed. We all "know" this. We all "seen" it in cartoons. We were learning about electricity (electrons) at this time. "But if electricity is electrons moving, ground obviously has more electrons that the sky, lightning must go from ground to the sky". I was so fascinated about actually "learning" something new. Remember running home to tell my father how clever I was now.
I remember showing that article to a colleague of mine who became Very. Angry. Indeed.
They said that all the articles and explanations that are being quoted then picked apart and shown to be inadequate are, were, and always have been simplifications intended to give the non-technical person some insight into what's going on, but have never been intended to "explain flight".
My colleague went on to say that starting with each of the simplified and incorrect descriptions, one can gradually refine each, then successively combine into a bigger picture, until one ends up with a description that "feels right" and agrees with all the numerical simulations.
But they've also stopped trying to be helpful pretty much exactly because of articles like this, that take half-truths as if they are intended to be complete explanations, then debunks them.
So don't read this article from Sci.Am. and believe it uncritically.
He's wrong. All those explanations ARE intended to explain flight! They're given to professional pilots without qualifiers, presented as if they were complete explanations. (If they weren't, then the controversy would instantly evaporate.)
This situation is quite different than, say electrons of atoms. First we learn that atoms are like little solar systems ...but then also hear that this is oversimplified (wrong,) and that we'll have to learn QM and probability-clouds in order to have a correct explanation of atoms' electrons.
That's NOT done with lifting-force explanations. Pilots aren't being taught that the explanations are wrong, oversimplified, and only the professional fluids experts can attain the "bigger picture" which removes all the mistaken concepts.
Did your angry colleague supply a simple correct explanation, to replace the wrong ones in the article? "Feels right" is a cop-out. Pilots don't want hand-wavy fuzz, or some experts who insist "it's just too complicated for beginners."
How about this instead...
First we explain a hovering helicopter. It's an air pump. It pulls air inwards from all directions, then blasts it downwards. Next, shove the helicopter rapidly sideways. That's how wings actually work: they pull air in from all directions, then fling it downwards. (But this is hard to notice, since the downwash-plume of an airplane is all stretched out, and it even seems purely horizontal. It's not.)
Heh, wings are the same as VTOL jet aircraft! But the turbine-blades in the VTOL engine have broken loose and flown off alone. Yet each little blade is still acting as an air-pump, and pulling air inwards from all directions, and flinging it downwards, producing a downwash, a momentum-carrying "exhaust plume."
This feels like a motte/bailey tactic. I remember clearly being taught the equal-transit-time theory as a kid, and it was taught as actual fact, not a simplification. I was uneasy with it but couldn't vocalize why (it didn't occur to me to ask why planes can fly upside down).
If it is meant as a simplification, many HS teachers have not gotten the memo.
Ocean waves work like this: water molecules are piled up in humps, and all the molecules slide across the ocean, while the water underneath the humps does not move. If you poured some dye into the waves, the water-humps would carry it along.
Sound works like this: vibrating objects put energy into air molecules, then the molecules zoom all the way to your ears at the speed of sound. They put the sound vibrations into your eardrum. Then they zoom all the way back to the vibrating object. Then repeat.
Uh, that's all wrong. Not oversimplified. Just wrong. (Airfoil explanations are similarly wrong. Not just oversimplified.)
I can vocalize why I am uneasy with it: why does the separated airflow need to meet back at the same point? How does the top-air know it needs to hurry to keep up with the bottom-air?
I mean, there might be a reason for that, but I would rather someone explained that instead of waving their hands. :-)
The best explanations I've heard is that these theores are not mutually exclusive. They can all be true at the same time and merely describe the physics from a different angle.
That's true for some explanations — for example the various proofs of the Pythagorean Theorem — but you still have to watch out for explanations that are flat-out wrong, and can even be harmful because they obscure the real explanation. I think equal-transit theory seems to be one of those, haha.
Maybe the damage is limited because very few of us will be designing airfoils. That may also be why it has been so hard to correct! The consequences for getting it wrong are minimal.
Something much more dangerous would be an untruth that directly harms people. The debate around those tends to be much more rigorous, although I'm surprised how long some of those can endure when people get attached to them!
Many generations of professional pilots have been taught the "equal transit theory." It's often as an answer their licensing exams.
So, most pilots have little idea how wings actually work. (But since the rise of the internet, things have been sloooowly changing. At least the pilots have started questioning and arguing!)
Almost all object that are airborne, stay so by pushing air in the opposite direction that they want to go. This is Newton's third law. Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards.
> Aeroplane wings take the air coming towards them (caused by the existing velocity of the aeroplane) and direct it down and backwards; this creates a resultant force that is forward and upwards.
Well wings typically don't provide thrust but drag, i.e., a resultant force that is backwards and upwards. So according to Newton they would be pushing air down and forwards.
The engines overcome drag and give a net forward force.
That's a very bold statement, so I'm compelled to ask:
In the light of evidence that conflicts with your statement, why should we believe you?
In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong.
[0] Even faster than the (incorrect) equal transit time idea would imply.
> In the light of evidence that conflicts with your statement, why should we believe you?
Can you clarify what you mean? Which evidence?
>In particular, we know that fluid flows faster over the upper surface[0], so we know from Bernoulli that the pressure over the upper surface must be lower. That appears not to be included in, or covered by your statement, which suggests that your claim that "It is really that simple" is wrong.
These two perspective are in fact congruent. I decided to explain lift by reasoning about the circulation generated by the wings (https://en.wikipedia.org/wiki/Circulation_(physics)). In this picture, the motion of the air in the wake of the wings has a net rotation (clockwise when the wind is coming from left to right) that generates an upwards resultant force on the wing. This is analogous to the pressure argument. I feel this perspective provides a clearer understanding. With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is.
As I stress, it really is a simple and well understood problem. Complexity with fluid dynamics arises from problems like turbulence, multi-phase flow and computational modelling.
> With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is.
That's true. However ...
> ... the circulation generated by the wings ...
You haven't explained why there is "circulation generated by the wings", you have just stated that there is.
Note, if you take the full flow over the wings and subtract the vector field that is the undisturbed average flow then you are left with a circulation. The two are effectively equivalent, so "explaining" by saying that there is "a circulation" is basically saying that the air flows faster over the top and slower underneath.
It's a non-explanation.
I do have an explanation as to why the air flows faster over the top, but this discussion is too small to contain it.
PS: I agree that all the modelling and equations work very well, and from that point of view its "well understood". However, the discussion here shows that for many people, even those who are technically capable, it is not "well understood".
I think we have turned this into a point scoring contest rather than clarifying the issue.
To summarise:
Any statement to the effect of "aircraft lift generation is complex" is probably wrong. It is wrong because a high-level picture given by Newton's third law (momentum conservation) is correct and adequately explains lift generation. You do not need to go into the details of velocity or pressure distribution on the wing. You can step back and look the overall airflow and reason about why lift was generated.
> You haven't explained why there is "circulation generated by the wings", you have just stated that there is.
I only mention circulation, to state that both the arguments are similar, just from a different perspective. In my initial post, I reference only Newton's third law (the conservation of momentum). Naturally the source of the circulation is the same as the source of the velocity/pressure distribution. As stated earlier, you do not need these details to explain lift. You do need them to explain phenomenon like stall or drag.
I suspect we are really not far from each other, but we might have different ideas of what is "intuitive", or "obvious", or "complete", or "well understood".
I've spoken with several aerodynamicists over many years, and they all say that the "conservation of momentum" idea, and the "downwash" explanation are only part of the story, and when you run the numbers the lift you get is not completely explained by those alone.
But at this point I need to go back to work, and I'll come back when I can to see how the overall discussion is going. It does feel unlikely that I'll have anything useful to add.
To clarify the physics, the "conservation of momentum" idea explains all of the lift, not just some of it.
If you draw a bounding box around the wing and measure the momentum at either end, the lost momentum will be because of the wing, some of that will be lift generation (there is also drag + other turbulent losses).
If it's a 3D wing with finite span, then you're well into vortex-shedding and momentum-carrying plumes of gas.
The usual way to avoid this is to make the wingspan be infinite, with no wingtips. But this is dishonest, because it transforms the problem into a "venturi effect," where the airfoil is producing an instant-force against the ground. Then, the Newtonian force-pair exists between wing and ground. (Yet real, non-infinite wings don't need any ground surface to react against. Their force-pair is between the wing and the vortices being launched downwards.)
To simplify: first explain a hovering helicopter. Wings work the same, acting as air-pumps, pulling in air from all directions, then creating a momentum-carrying plume launched downwards. (Helicopters and wings, both are examples of fluid propulsion, where Bernoulli doesn't apply.)
His statement is right, but it requires that tip-vortices and downwash-plumes exist. The air behind the wing is carrying momentum, and is left with downward motion. (In typical 2D airfoil diagrams, this cannot happen.)
Also, his statement is only true if we completely eliminate Bernoulli (since any wing which flings air downwards, is also performing net work, and injecting energy into air-parcels. If the net energy of parcels is changed, then Bernoulli concepts cannot be employed.)
"Simple" is the problem. It's simple to describe wings verbally, but the math is not simple at all. Helicopters are the same: they generate a column of downwash, but this involves performing net work, and involves vortex-shedding, so simple Bernoulli concepts are utterly forbidden.
Simplified: wings are an example of propulsion, and fluid propulsion always involves vortex-shedding, which falls under the heading of Turbulence. It's easy to verbally describe simplified elementary turbulence. It's just a vortex which moves around, and involves work performed on the air as a whole. But jeeze the math involved!
The statement only referred to the net motion of the vehicle and the deflected air. The fact that during that process the air here of there had higher or lower pressure is incidental, and not contradictory to this explanation. The air might have other things changed about it by the process, such as turbulent motion, but again those are incidental and are not contradictory to the general statement.
Bernoulli goes both ways and in this case I think the other way makes more sense. Force of air on the wing is net upwards, therefore the pressure at the bottom of the wing is higher than at the top, therefore, by Bernoulli's principle, the airspeed at the top is higher than at the bottom.
Since we are trying to give higher order explanations for lower level phenomena you can probably dissect this a thousand different ways and conclude "it's not that simple", but I don't believe this explanation is wrong at the granularity at which it functions, and it really is quite simple.
(edit) Furthermore the detail of the air speed difference is not even relevant for the explanation mansoor_ gives, so I would hesitate to call it "conflicting evidence". Of course you can demand of a high level theory that it explains a fact which is not necessary for the high level theory to function, and then say that it's not as simple, but I don't think that's fair.
In that case the 2nd theory must be correct but they say its incorrect for some reason. Which doesnt make sense to me at all.
They say it's incorrect because it doesnt account for the airflow on the top. Well the flow on the top doesnt strike the wing (except its forward facing part, the wing is pitched 'up' slightly) so it doesnt push it down. But the bottom flow does push it up (it strikes the whole surface of the wing).
Really cant see whats wrong with this simple explanation.
It's wrong because the force on the lower airfoil surface is far smaller than the total lifting force.
The flow above the airfoil "sticks" to the airfoil. Since the upper flow is being forced downwards by the curved upper surface, it forces the airfoil to move upwards.
What happens if the upper flow doesn't curve downwards? Simple. That's called "stall," and your airplane drops out of the sky. "Stall" is when the upper flow becomes detached, and goes straight back, rather than following the curved upper surface. (The force on the bottom of the airfoil is far too small to keep the plane in the air. Most of the lift is created by the upper surface! That's why "stall" is such a huge deal.)
OK well we can add that to the simple explanation. If the top airflow is diverted down when it wants to go straight, it would still want to go straight therefore pulling the top surface up, right?.
The numbers still don't add up, you still need to account for the pressure difference (lower on top, higher underneath) due to Bernoulli. The "downwash theory" doesn't give enough lift on it's own.
I think you have a misunderstanding here. You are right that downwash accounts for a tiny proportion of the overall lift, in fact it only become significant close to the wing tips.
Downwash (in the context of lift generating wings) refers to the downwards motions of air generated by tip vortices (look-up "induced drag" for more details). This is not the same as the net downwards motion of the air in the wake, relative to the free-stream air. Take for example, an infinite wing (or a 2D wing). Here, there is no downwash but we still generate lift and have a net downwards motion of the air in the wake. You can see this with online fluid simulators for airfoil sections, many of them are 2D.
> The second page of the article seems to address this and calls it an incorrect lift theory.
If it does, it's wrong. Edit: Now that I have read it, I would have to disagree with you. The article only references the Bernoulli pressure arguments, it does not mention Newton's third law or airflow circulation.
> The real details of how an object generates lift are very complex and do not lend themselves to simplification.
There is no scientific contention about flight. Nobody is currently publishing papers on this topic ("journal of fluid mechanics" if you are interested), it was well understood in the previous century. Newton's third law explains at a high level how object fly. Yes you can dig into the details of fluid motion, turbulent flow, boundary layers etc, but this does not detract from the high-level picture.
It's not just the underside that is responsible for the circulation of the airflow. In fact the majority of the circulation is generated by the top surface.
(Please note ... we are not in disagreement, but I wanted to address this)
Asking how 'planes fly upside-down and putting one's theory to the test in this thought experiment is useful, but it doesn't refute the "downwash" theory. If you have a wing at an appropriate angle of attack, even if upside-down, then provided the flow remains attached, it (the flow) can still be deflected downwards.
If the wing is reasonably asymmetrical then the angle of attack needs to be large, which makes it more likely that the flow will become detached, so we deduce that a 'plane designed to fly upside-down needs to have symmetrical, or near symmetrical, aerofoil profiles ... which they do.
So the downwash theory is not refuted by the "upside-down" question.
(Please note: All that notwithstanding, the unadorned downwash theory is not the complete explanation of how lift is generated)
Yes very true. It is just one among many things that needs to be explained by a theory though. Things like why do stalls happen, why are there deltas on the ends of commercial airliners, and so on.
Point is it's one of the more complicated everyday phenomena to explain.
They don't do so at the same angle of attack I'd wager. Meaning to fly upside down they have to control pitch in a way that turns the wing to the exact needed angle. Same way they have to control pitch when flying normally to achieve lift.
This pic of real jets doing that side by side confirms what I'm saying (nose is pitched up on the normal-flying plane and 'down' but still up relative to the ground on the inverted one): https://imgur.com/loVcrgO
Great question, but the physics is still the same. The aircraft still generates lift by direction air in a downwards direction. For some aircrafts (based on the wing geometry), you can achieve this upside down, within a certain range of angle-of-attack of the wing. Obviously, the aerodynamic efficiency is not the same. Airfoil sections are anti-symmetric across this direction because these designs achieve higher L/D (lift to drag) ratios.
For the Lift Theory we have Navier-Stokes equations that give you the lift if you solve them numerically or, maybe, figure an analytical solution. But most people won't accept it as an "explanation" - they want a simpler description in more coarse-grained concepts and relationships. And, it seems, that there is no such satisfactory reduction for the Lift Theory - there are just too many interrelated things happening at the same time.
Same thing goes for some decision-making AI. One might like to have an "explanation" for why AlphaZero decided to make a particular move. And sometimes there could be a general high-level pattern rule, that one could extract from the model. But, it seems, that in most cases no simple rule can be expected.