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Comment by acchow | Hacker News Reader

tl;dr The wings are angled so that air is forced slightly downwards when it collides into the wing. This collision is sustained by engines which force the plane into the air in front of it.

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Just one more bit: "Air has viscosity". Without naming viscosity, the explication doesn't hold. If the air didn't have any viscosity, like it is natural to imagine, planes could not fly.

This is incorrect. A wing can impart momentum to air even in the absence of viscosity. However, the optimal wing shape is different. For an example of a wing that moves air "down" without viscosity, look at turbomolecular vacuum pumps:

http://www.physics.rutgers.edu/ugrad/387/388s06/UHV_LEED/UHV...

Not really - if a wing moves through fluid without viscosity it still has to move the particles down.

The impulse required to move particles of a fluid with zero viscosity would be zero, so the equal and opposite force that we call lift would also be zero. Consequently you couldn't fly if air had no viscosity.

If the particles have mass then the impulse wouldn't be zero.

Is there such a thing as non-viscous fluid?

Yes, one example is superfluid helium.

https://en.wikipedia.org/wiki/Superfluid_helium-4

Viscosity is weird.

https://en.wikipedia.org/wiki/Non-Newtonian_fluido

tl;dr The wings pump air down from above, causing lift not through pressure effects but rather from the volume of displacement.

(I got a little bit lost in the discussion of upwash. It seemed to me that the idea was upwash was less efficient at moving air than downwash, and therefore more air was moved down.)

Pressure effects still take place. Pressure is just force divided by area. If a net force (lift) exists, then a pressure differential must exist per newton's third law. A flat plate at a positive alpha has both a pressure and speed differential across the upper/lower surfaces, see http://www.windsofkansas.com/AZ2Fig5.jpg

That's not correct, then the upper surface of the wing would not have effect on lift, when in fact it contributes over half of it.

All the low pressure above a wing does is reduce a downward force it does not provide lift.

Drop a piece of paper and it drifts slowly. Drop a book and it falls fast. In your model the paper is supported by the air below and pulled up by the void behind it.

However, if you put a piece of paper behind a book it sticks to it and is not pulled up. Thus, all lift comes from the bottom. Edit: You can do the same thing with a piece of paper on top of a wing.

PS: The same is true if you pick up a piece of paper with a vacuum. Even if it seems like the suction is providing lift all the force is from outside air.

> All the low pressure above a wing does is reduce a downward force it does not provide lift.

NASA explains why the "skipping stone" theory of lift is incorrect well [1]. In summary, the upper surface does generate lift.

[1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html

I did say the top of the wing reduced a downward force, unlike the skipping stone theory. But, that's not lift.

Do you think individual air molecules are going to magically hold the top of the wing and pull?

Now, you can use an inaccurate model of what's going on. And mathematically it looks like the top of a wing provides lift. But, physically that's not what is happening.

PS: This is only an issue, because we think an aircraft as zero forces acting on it when it's sitting still on a runway. Instead of being under ~2100 pounds of force per square foot on all sides from air.

I think you've changed definitions on the fly. Both the higher pressure on the bottom and the lower pressure on the top contribute to the lift.

If you use a suction cup to lift a piece of horizontal glass, well, I think most reasonable people would say it contributes a force. Maybe you'd say no it doesn't, you don't actually lift it, you just provide a lower pressure at the top allowing the bottom air pressure to push it up... Well duh, of course.

I don't think science should care what 'reasonable people' believe, because reasonable people are often wrong. The whole point is to build accurate models of the world and perpetuating a myth is harmful in the long term.

PS: Can you point to where you think I changed definitions? I may have been less than clear.

Your definition of lift is very different from the commonly accepted one.

If an aircraft that weighs 1000 kg flies level, then the lift has to be 9800 newtons.

That means the lift is the sum of the effect from both the bottom overpressure and the top underpressure.

You can check it from any dictionary.

https://en.wikipedia.org/wiki/Lift_%28force%29

In level flight every actual force vector above the wing is lowering the magnatude of lift.

If you owe 2000$ and have 2200$ your net worth is positive 200$. But, people don't talk about debt as adding to your net worth even if it's part of the equation debt is reducing the result not increasing it.

PS: I agree you don't need to model the 2.1 tons per square foot of force in most situations. But, just because you ignore something does not mean it goes away.

If that was it, then the wings would have been symmetrical or flat. The angle of attack probably accounts for less than 10% of the lift.

Aerobatic wings are symmetrical. Lift is proportional to CL, which is related to angle of attack:

https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift...

Most non-aerobatic wings are asymmetrical because it produces lift at zero angle of attack, and also because it results in a lower stall speed.

https://en.wikipedia.org/wiki/Camber_(aerodynamics)

Source? The whole article basically argues against this view. What is your evidence to say AOA only contributes 10%?

This is common misconception probably coming from level flight situation. Commercial aircraft wings are made for adequate lift in takeoff and minimal drag in level flight. Which encourages building wings that generate all necessary lift in level flight while being relatively "flush".

> As one can see, the popular explanation, which fixates on the shape of the wing, may satisfy many but it does not give one the tools to really understand flight.

Well, apparently that is it! That also explains why planes don't slam into the ground when flying upside down.

Consider how flaps are shaped. When extended they increase the angle of attack; not the ratio of air travel distance.

Generally, when flaps and slats are extended, they increase the camber of the airfoil, increasing the Coefficient of Lift for the same AoA. Also, the air gaps introduced at the junctions between slats/flaps and the main wing re-energize the boundary layer on the top surface of the wing to retard flow separation on the upper surface (becomes a concern because of the increased camber).

They actually do both, depending on their deployment.

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