Comment by onetimeuse92304
2 years ago
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.
And it's an "even more wrong" explanation than the "lies for children" diagram used in school physics class.
For reference, actual "proper" discussion of lift in textbooks on aerodynamics have tendency to start with a sphere/cylinder.
Do you have any recommended reading on this topic? I'd like to brush up.
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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".
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Why is it wrong?
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.
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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".
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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.
> planes can obviously fly inverted
Many (most?) planes cannot sustain inverted flight.
IIRC that is due to other issues, not aerodynamics.
For instance, the engine no longer receiving oil at negative 1 g, or fuel, as the system is designed for gravity flow.
Stunt planes and airplanes capable of long inverted flight need special oiling and fuel systems to keep the engine from starving from either.
Fascinating! Commercial airliners have also optimized their engines enough they don't have the power budget to make up for the difference in lift.