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Comment by rcxdude | Hacker News Reader

A lot of the difficulty of explaining lift of airfoils is that generally explanations try to follow a neat chain of cause an effect. But with the wing there isn't really a clear one. All these statements:

- There is an upwards force on the wing

- The pressure above the wing is lower than the pressure below it

- The air around the wing follows a curved path downwards

- The air above the wing travels faster than the air below it (NB: not in equal time!)

- The air behind the wing has a downward momentum

are related to all the others, but not straightforwardly: they all imply each other to some extent, both caused by and causing some of the others. So basically all explanations try to follow some path through the tangled web, but by doing so they always cause some oversimplification. The only top level chain is: shape of wing and angle of attack -> ????? (tangled mess of fluid dynamics few people fully understand) -> lift!

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Exactly, this is why understanding fluid dynamics is so difficult. You can't look at some physical laws and assume that the right hand side "causes" the left hand side. They all represent relations and it so happens that the fluid configuration that fulfill all the relations (and that the world adopts) is the one that causes lift. Just trying to talk about cause and effect is a misunderstanding.

My favorite example of this was in the air-data computer I was working on for a fighter trainer. I was just on the software side rather than the aerodynamics, but it was notable that the corrections to angle-of-attack and angle-of-sideslip measured by the multifunction probes (which are way up at the nose of the plane) included terms related to the position of the flaps (which are way back at the trailing edges of the wings).

That's awesome!

I'm not surprised about the angle-of-attack needing correction. The angle-of-attack is defined as the angle between the average chord (an imaginary line running from the leading edge of the wing to the trailing edge of the wing) and the relative wind. Since changing the flap position changes the position of the trailing edge, the angle-of-attack will also change.

Honest question: if wing's cross section has a big curve in front, how do determine which point exactly constitutes the "leading edge"?

Usually you draw a line from the trailing edge (which is sharp so unambiguous) to the point on the leading edge that makes the line the longest.

The definition that makes the most sense, though, is to disregard the geometry of the wing and define zero angle of attack as the zero-lift angle, because then lift is proportional to AoA.

> explanations try to follow a neat chain of cause an effect. But [...] there isn't really a clear one. All these [...] are related to all the others, but not straightforwardly

There seems a pattern of misattributing pervasive failures of science education content design, to physical system complexity and student deficiency. A favorite of mine was a PhD thesis "We taught grade G young students common incoherent nonsense about atoms. Surprisingly, that's didn't work out well. We draw the obvious conclusion: students in G are developmentally incapable of understanding atoms." Which might even be valid... for a "regurgitate incoherence" definition of "understand atoms".

Here, I wonder if an atomistic explanation might work better? Could one craft a nicely accessible, coherent, transferably powerful, molecular superball mosh pit story of wings? The confusion and disagreements here sound a bit like "It's net molecular motion! No, surface impacts! No, differential surface impacts!". An abstraction/model fail, rather than underlying irreducible system complexity.

It's easiest to understand as a black box, or a "control volume". Consider the air coming in the front (horizontal) and the air going out the back (velocity is deflected downwards). Momentum change, needs a force to keep things in balance. Simple! Fluid mechanics is all about that kind of thinking.

OK, but that's just saying "the aerodynamic force exists, because we can observe the plane goes up and the air goes down", isn't it?

Classic phenomenological analysis. "This is not reality, but as a model it's good enough for a first pass design analysis"

It's control volumes all the way down. Look up "Finite Volume Method" in CFD.

This is exactly one of the common pathways through that middle section, which is nice and simple but doesn't really explain anything (why is the air deflected downards?).

Because the plane has speed and trading that speed for lift

If I'm not wrong, it seems dead simple when you put it like this:

- Imagine the jet moves the wing forwards some small distance in some small amount of time.

- Due to the shape of the wing, there is now a temporary vacuum above the wing as air particles have yet to rush in and occupy the space where the wing used to be.

- There is now an unbalanced pressure around the wing sufficient to overcome gravity and give lift.

No Bernoulli, no math, just visualizing a bunch of particles getting pushed around.

If you think about air this way it also becomes obvious why a helium balloon moves in the direction of acceleration inside a car. Car moves forward, air in the rear of the cabin is now squished while the air in front is stretched out as it hasn't caught up to the car yet, pressure gradient sends the balloon forwards.

As a purely conceptual illustration of the fact that the air must be deflected downwards by the wing, sure. It doesn't really work, though. For example, there's no reason for the air to move faster over the top of the wing in your scenario, and without that you'd underestimate the amount of lift a wing actually generates.

> there's no reason for the air to move faster over the top of the wing in your scenario

I suppose the compressed air at the top of the wing will find its way into a vacuum and travel a bit faster than the air at the bottom that's encountering normal pressure, but I'm honestly out of my depth at this point. Not sure if the air coming in on the left cancels that out either, I'd have to run the equations. https://webwhiteboard.com/board/KhTCsoDvhyGTy0uJtmPpQNhvldF1...

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