I wish every presentation on how planes fly started with an actual flat plane. A wing that has a flat crossection. I think the shape of the airfoil of the wing is absolutely distracting and prevents people from understanding what is really happening.
Every person who ever stuck a flat object outside the window of a moving car knows that you do not need a fancy shape to have lift.
And so many people are stuck thinking that the shape of the airfoil is responsible for the plane to be able to fly, supposedly because the air needs to run a longer way around the foil above the wing than below the wing. And this somehow causes pressure difference due to Bernoulli law and this is what keeps the plane up. Which is almost total BS because planes can obviously fly inverted.
Now I admit I only skimmed the article, and although the animations are beautiful, I am missing what really is key to understanding of what is happening.
I am looking for a bigger, far away view of the wing and showing what happens to the air BEHIND the wing.
Because how the plane really works is as it flies forward, it diverts large masses of air downwards. It pushes off of air.
Part of the air is diverted by the lower portion of the wing, but the much larger portion of lift is generated by larger masses of air above and behind the wing. Those can be thought as being sucked down behind the wing (if you look at it from the point of view of a stationary air mass, not from the point of view of the wing).
And the main role of the airfoil is to keep that mass of air behind the wing stuck to the airfoil at wide range of angles and speeds as possible, because a flat sheet is very poor at doing this.
Yea, I agree and try to explain it this way to friends. Airfoils help, but it's ultimately just the wing pushing air down and why planes can fly upside down.
FWIW, aerospace engineering degree, used xFoil, did tons of fluid sims, etc.
If the diagram shows lift but doesn't show the air being directed downward after leaving the tailing edge of the wing, I basically stop reading. That's the whole thing.
Thank. You. That's exactly what is missing and that's exactly what I have mentioned in my... highly criticised comment. It just shows how pervasive the misconception is.
If you take a step back there is a simple way to think about this. In order for the object to stay up there, there needs to be equal and opposite force from some other body. What is that other body? It is the mass of air that is being directed in the opposite direction of the lift force acting on the plane.
I think the mistake this site is making is trying to model out pressure changes that would create an airfoil-like shape of airflow. That's not how wings work. The pressure at the wing's leading surface is infinite because there's a metal skin there. You don't need a low pressure zone sucking air up 1m above it to explain that.
This view also implies that most of the lift is happening on the very front edge of the wing which I doubt is accurate otherwise we would have very skinny wings.
Unfortunately it partly bitrotted due to using java applets for interactive demos, but I think most of it is still reachable - I'll try to find it later when I'm at the desk.
Personally I learnt from a 1980 book that was still part of mandatory reading for glider pilot course in Poland in 2005.
There is no lift on a sphere or cylinder without rotation dude. The whole point of parent post is that the "proper" discussion does not inlay a good intuitive understanding of lift, which in my opinion, should start with "push air down to go up".
But there's quite different flow and drag around it, which was used as opening for for adding rotation (which would add viscosity effects including lift from rotation) and other changed shapes in better way than starting with flat plane.
To me the most intuitive and practical mental image is imagining two large bubbles of lower pressure above the wing that hold the wing up by suction (you can see those literally as condensation under certain conditions). As you increase the angle of attack the bubbles get larger and stronger, until the angle is so large that they “break off” and the wing stalls.
I once found an explanation that finally made it clear to me why the shape of the airfoil can create lift. Yes, the air above the wing needs to travel a longer distance with the typical section used in wings, which means that it goes faster than the air below the wing. It also leaves the wing moving downwards - and when this downward-moving, faster flux of air meets the slower one from below, the result is that a mass of air is pushed downwards - exactly as needed to lift the plane, as you correctly said.
As the article says, you can have lift by just changing the inclination of a symmetrical airfoil, but an asymmetrical one can generate lift even without inclination (and with lower drag). The article also explains that acrobatic airplanes have symmetrical wing sections exactly because they need to be able to fly just as easily inverted.
> Yes, the air above the wing needs to travel a longer distance with the typical section used in wings, which means that it goes faster than the air below the wing.
Both of these sub-clauses are true, but the "which means" connecting them aren't. There's no law of physics saying a fluid that has a longer path ahead of it speeds up in anticipation.
Isn't there an even more basic explanation: If incoming air hits a flat surface at an angle, and is deflected downwards, then by the law of action and reaction, the surface itself moves upward.
As a child, I quickly outgrew the airfoil explanation when I realized this.
That's exactly what is happening. But it is also not enough for the airplane to fly.
In a normally flying airplane, the wing compresses and pushes an amount of air under its wing. But there is actually even greater amount of air sucked down by the region of underpressure created above the wing and by the laminar flow directing it downward. Here, the drawing at the top of the page makes it clear: http://www.amasci.com/wing/airfoil.html
When you have a stall condition, what happens is that the air below the wing is still being compressed and directed downwards, but the air above the wing becomes turbulent and "unsticks" from the surface of the wing. Rather than being nicely directed downward, it just dissipates a lot of energy in turbulent motion that is not directed in any particular direction.
This turbulent air not only ceases to provide lift, it also prevents the air from below the wing to be directed downwards efficiently.
The main job of an airfoil isn't to create a pressure difference, it is to create conditions for the air to be laminar at as wide range of speeds and angles of attack as possible to make the plane nicely behaving and possible to takeoff and land. It is super critical for landing as you need to have higher the angle of attack the slower you fly and all planes essentially are driven as close to stall as possible during landing. Similar happens at high altitudes and high speeds, but for a bit different reason (read up on "coffin corner" if you are interested in that sort of thing).
Great explanation. In addition, flaps make the wing able to provide lift at slower airspeed at the cost of efficiency. Perfect for takeoff and landing.
Yes and no. The thing you describe happens, but it's not enough to explain the amount of lift generated by a wing, because a surprising amount of air hits also the top of the wing! The difference in pressure between top and bottom wing surface is just a few percent.
The reason wings produce significant lift anyway is that they deflect air far beyond their surface. Air several metres away from the wing is also deflected downward, even though it doesn't actually hit the wing itself.
So yes, Newton's third law is involved, but in a "spooky action at a distance" form, where the wing somehow manages to deflect a bunch of air it doesn't even touch!
My dad who worked on wind tunnels just flat said you can either integrate the pressure over the surface of the wing or the momentum change as the air passes over to derive the amount of lift.
Both give exactly the same results and are convertible mathematically.
For wind tunnel work it was easier to measure pressures.
I'm with you I don't think the standard hand wavy explanation gives you the ability to attack the problem mathematically. So it's basically wrong.
My understanding is that "which means" only makes sense with the assumption that what is being studied is the laminar flow of an incompressible fluid (which was described as a fair assumption for air and a wing at subsonic speed). But thinking more about it, it's probably right that this isn't about the fact that the air above needs to travel a longer distance, which would also be true for a concave wing section, but the fact that the layers immediately above the wing need to travel the same X distance through a thinner Y section - as in a tube which becomes thinner. Which forces the fluid to go at a higher speed, and have a lower pressure.
Unfortunately, your explanation is entirely wrong... and you're attacking a "lies to children" simplification with your mention of "needs to run longer way around" bit.
Well in defense of the GP, the "planes can observably fly upside down" point (and its close cousin the "flat wing cross sections can fly too" point) is a good one, this pokes holes in the usual two-dimensional "the air goes faster on top" themed explanation that omits any discussion of vortex shedding/third-dimensional effects.
Oh, to be quite honest, I loved trolling my high school teachers with "your explanation fails, here is a real world airfoil, please explain it" and I would draw a symmetrical airfoil or - for extra trolling - a trapezoid one. (At that point I had already flown solo)
But the same I found myself unable to pass by someone pushing "flat plane at an angle".
Fortunately the exam questions that involved lift in high school were simple enough they didn't trigger "you're wrong and the textbook is wrong" response XD
Fortunately my exams were open ended not "fill in the circle in answer sheet" so worst case I'd have written a more complete answer and fought it out.
Worrying about having to fight against "answer key" is part of why only one person (and only on a lark) took computer science on Matura exam in my class - which was CS-math-physics focused one
You mean it is more akin to those grills you position to control your AC pushing the air in a certain direction. But with just one surface you get the AoA too high problem. Hell I am gonna stick my hand out next time in a car (being safe about it!) and see the stall angle of my hand.
Every person who ever stuck a flat object outside the window of a moving car knows that you do not need a fancy shape to have lift.
And so many people are stuck thinking that the shape of the airfoil is responsible for the plane to be able to fly, supposedly because the air needs to run a longer way around the foil above the wing than below the wing. And this somehow causes pressure difference due to Bernoulli law and this is what keeps the plane up. Which is almost total BS because planes can obviously fly inverted.
Now I admit I only skimmed the article, and although the animations are beautiful, I am missing what really is key to understanding of what is happening.
I am looking for a bigger, far away view of the wing and showing what happens to the air BEHIND the wing.
Because how the plane really works is as it flies forward, it diverts large masses of air downwards. It pushes off of air.
Part of the air is diverted by the lower portion of the wing, but the much larger portion of lift is generated by larger masses of air above and behind the wing. Those can be thought as being sucked down behind the wing (if you look at it from the point of view of a stationary air mass, not from the point of view of the wing).
And the main role of the airfoil is to keep that mass of air behind the wing stuck to the airfoil at wide range of angles and speeds as possible, because a flat sheet is very poor at doing this.