[Rapzid]

> If one were to build a large scale on the ground, as an airplane flies over it, the scale would register the weight of the airplane

No, it wouldn't.

I think the article does a pretty good job building a more complete understanding than the simplistic "deflection" mental model.

[turtledragonfly]

I think what they were saying is that from a pure "Newton's 3rd law" standpoint, if the plane has an upwards force, then the air has a corresponding downward force, which must go somewhere. Yes, it is spread out and complicated and turbulent, etc, but ultimately must balance out.

If we could somehow "draw a box around" the entire plane+air system, then the plane's upward lift will create a corresponding downward force on the box, one way or another.

So, in the broad sense that you push the earth away from you when you jump, the plane also pushes the earth away from it when it flies (mediated by a bunch of fluid dynamics).

Or, classic example: if a (sealed) truck full of birds is jostled so that they start flying, does the truck weigh less? [1]

[1] https://www.youtube.com/watch?v=lVeP6oqH-Qo&t=35s

[Rapzid]

It's wrong though. A large, hypothetical scale under the plane would not register the weight of the plane as it flies over. And not just because diffusion but that being one of many reasons.

[turtledragonfly]

I'm curious to know your reasoning more.

Certainly if we flew the plane very low over the ground, the air pretty directly pushes down on it, and the hypothetical scale would register something. Just look at the grass when a helicopter hovers over it.

As the aircraft flies further up, we'd need a bigger scale to capture the full area affected, and if it's moving there would be increasing lag between the location of the plane and the (large) area where the downward force hits the ground.

Or do you disagree with that? At what point does the scale stop working?

Obviously there would be practical limitations — that force is so spread out that it would be hard to measure. But let's not have practice get in the way of theory (:

[Rapzid]

The "stone skip" or "deflection" theory of lift is not accurate; https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...

Planes fly through gas, not solid particulate. Gas has intrinsic kinetic energy when energized. Diffusion plays a huge role in all this of course.

The airflow is split at the leading edge. The area of positive pressure is not entirely below or focused under the wing. The top and bottom of the airfoil are both involved in turning the air flow.

The pressure under the airfoil increases a bit, but the pressure above decreases by as much to much more depending(2-3x or more). This hypothetical scale is under the aircraft but much of the lift occurs by decreasing forces on the top surface.

Scales measure weight/mass. Barometers measure changes in atmospheric pressure. So it's not even the tool for the job even if the stone skip theory of lift was accurate.

[turtledragonfly]

I think I see what you're saying.

Perhaps my mention of Newton's third law gave you the impression that I was advocating for that "stone skip" theory ­— I assure you I wasn't! Especially as presented on that page, it is obviously junk (:

But surely you agree, broadly, that if birds are flying inside a sealed box, the box still weighs the same amount as if they were standing, right? (modulo some fluctuations)

All of the pressure differentials and whatnot have the net effect that an upward force on the wing results in a downward force elsewhere. The purpose of the scale is to measure that force — like measuring the weight of the box with birds in it.

In the hovering helicopter example, wouldn't you agree that a (large) scale directly under the helicopter will measure a weight corresponding to the helicopter's lift force? Like if I blow directly onto a kitchen scale — it will measure some grams.

Edit: I feel we are kinda re-hashing the Bernoulli/Newton discussion also addressed at the nasa site: https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... — 'So both "Bernoulli" and "Newton" are correct ...'

For reference, the correct Newtonian explanation (flow turning) is also covered: https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...

[somat]

The simple newtonian deflection model is correct however, As you engineer your deflector to have the least possible drag the airfoil shape naturally falls out.

Actually that is a bit of a lie, the airfoil shape only falls out due to a third implied force that needs to be accounted for. the wing needs to be strong enough to hold itself up. if you had infinitely strong materials the deflector shape that would fall out would be like a slightly bent piece of paper.

A clarification note on fluids: you are deflecting fluids, and everything this implies. just because I say newtonian deflection don't think I mean billiards balls, or if it has to be billiard balls think trillions of them simultaneously

[Rapzid]

https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...

[somat]

I did not say reflector as implied by that link but deflector, a thing put in the fluidstream to move it somewhere else. airplanes lift because you are moving air down. People get hung up about the convex side of the airfoil but what else is the fluid going to do, stay a vacuum? it is going to move in the way the deflector shaped, adding to(actually providing most of) the downward flow. There is a lot of engineering that goes into it but at the end of the day an airfoil is the shape that moves enough enough air downward with the least drag. The only reason it is a thick teardrop shape is it has to be strong enough to support itself and the airplane. otherwise the ideal shape would be super thin shaped like the upper surface of the wing bending slightly from the cord(aspect directly into the stream) to the trailing edge(a few degrees of slope).

[Rapzid]

IDK man, what I said was correct; it's wrong. GLHF.