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Comment by camperman | Hacker News Reader

Air does not behave like a bunch of bullets hitting a wing. It's a fluid with velocity and pressure everywhere, not simply in the vicinity of aircraft surfaces. A wing is a pump that sets up circulation in the airflow, upward of the leading edge and downward off the trailing edge. The downward momentum of the air flowing off the trailing edge produces an equal and opposite reaction by Newton's third law which is the lift.

This is clearly demonstrated by this incident. Many tons of air deflected downwards by the A380's wings had enough force to flip the unfortunate plane below several times.

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> Air does not behave like a bunch of bullets hitting a wing

Yes it does. And those bullet impacts are the very definition of pressure.

The aerodynamics snobbery that you're trying to invoke is that those bullets also hit each other, so you can't make ideal gas assumptions and must model the whole system as a giant system of differential equations (c.f Bernoulli) if you want numbers out.

That doesn't change the fact that air pressure at the surface of the body is what causes the force on the body. That busybodies feel the need to argue against this bleedingly obvious point says some very bad things about the way the aviation community has tried to teach aerodynamics.

Nonsense. See here:

https://www.av8n.com/how/htm/airfoils.html#sec-fluid

Yeah, that site is indeed one of the worst offenders. Try a fluid dynamics textbook. I genuinely don't think you understand what I'm saying. Air isn't magic fairy dust that pushes on airfoils from a distance. Pressure is impact.

If you hit a slice shot in tennis, the ball will travel considerably further because the backspin produces circulation which generates lift. If air was a bunch of bullets, the effect wouldn't happen at all. If you raise a spoiler on a Grob's wing, its lift-producing capabilities drop considerably because the spoiler interferes with the circulation. If air was a bunch of bullets, a spoiler wouldn't do anything.

> Air isn't magic fairy dust that pushes on airfoils from a distance.

No. Like I said, it's a fluid that has pressure and velocity everywhere. At low enough Reynolds numbers, like for model gliders, you can even get away with treating it as a non-compressible fluid with decent results.

> Pressure is impact.

And contact between two objects is really about Pauli exclusion surfaces at the atomic level. So what?

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