Why we don’t understand heavier-than-air flight • Hacker News Board

Re: Why we don’t understand heavier-than-air flight

#21

Post by meheleventyone » Sat, Jun 26, 2021, 9:59 AM UTC

garmaine wrote: post #19

parineum wrote: post #14

Earlier quoted context omitted.

Knowing Newtonian physics doesn't mean we understand all things that move. The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope. C…

I don’t think this is true. There are many physical ststems for which we know the underlying physics very well, but the equations can’t be simply solved, and numerical simulation is more costly than just building the damn thing and testing it. Wing lift under turbulent conditions is one of those things. So we use wind tunnels. Not because we don’t understand lift—we do—but because it’s just easier. This is getting le…

The edges really show for smaller craft. At low Reynolds numbers lift gets funky. A lot of flight research is around making very small things fly efficiently.

Re: Why we don’t understand heavier-than-air flight

#22

Post by jessedhillon » Sat, Jun 26, 2021, 10:01 AM UTC

parineum wrote: post #14

Cogito wrote: post #7

The continued assertion that "we don't understand heavier-than-air flight" is a weird one. The article even skates around this, saying (essentially) "well maybe we do understand it, but chaos theory!" If you're in the sky and you want to stay there, you have to counteract gravity. Heavier-than-air flight does this by pushing down on air. Want to stay in the sky? Push down on enough air, fast enough, and you will stay…

Knowing Newtonian physics doesn't mean we understand all things that move. The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope. C…

Almost nobody understands software then, by this definition.

Re: Why we don’t understand heavier-than-air flight

#23

Post by cryptica » Sat, Jun 26, 2021, 10:08 AM UTC

jobigoud wrote: post #4

tempestn wrote: post #3

Earlier quoted context omitted.

In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, a…

Air moves faster on the upper side , creating a pressure differential.

I thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing.

If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.

Re: Why we don’t understand heavier-than-air flight

#24

Post by IshKebab » Sat, Jun 26, 2021, 10:08 AM UTC

jobigoud wrote: post #4

tempestn wrote: post #3

Earlier quoted context omitted.

In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, a…

Air moves faster on the upper side , creating a pressure differential.

A common and wrong explanation. The pressure differential is created by the fact that the air is being pushed into the lower side of the wing.

It's much simpler than that anyway. The wing forces the air downward, so the plane must be forced up.

Re: Why we don’t understand heavier-than-air flight

#25

Post by belter » Sat, Jun 26, 2021, 10:08 AM UTC

jobigoud wrote: post #4

tempestn wrote: post #3

Earlier quoted context omitted.

In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, a…

Air moves faster on the upper side , creating a pressure differential.

Its complex...The example normally given is, the wing is shaped a little flat in the under side and curved on the top. So that would explain the flow as you mentioned. However when an airplane flies upside down, its not sucked into the ground ;-)

It seems nobody really knows:

"No One Can Explain Why Planes Stay in the Air"

https://www.scientificamerican.com/article/no-one-can-explai...

Edit: Added brief from article above:

------------------------------------------------------------

- On a strictly mathematical level, engineers know how to design planes that will stay aloft. But equations don't explain why aerodynamic lift occurs.

- There are two competing theories that illuminate the forces and factors of lift. Both are incomplete explanations.

- Aerodynamicists have recently tried to close the gaps in understanding. Still, no consensus exists.

------------------------------------------------------------

Re: Why we don’t understand heavier-than-air flight

#26

Post by raverbashing » Sat, Jun 26, 2021, 10:09 AM UTC

Yeah, the usual explanation of "air goes faster over the top because they have to meet at the end" is BS. Especially the 2nd part. A video about the subject https://www.youtube.com/watch?v=QKCK4lJLQHU

But the explanation I can come up with is: lift is a force due to low-pressure regions caused by laminar flow over a surface. It is essentially "form drag" (caused not by the profile facing air directly but by the aft part) but the tricky part is that it is not directly parallel to the flow of air, but also depends on the orientation of the wing.

Re: Why we don’t understand heavier-than-air flight

#27

Post by graderjs » Sat, Jun 26, 2021, 10:12 AM UTC

Isn't it because planes are continually falling (because gravity), and this leads to two things:

1) wings increase the surface area pushing down (gravity) on the air below, which pushes back (air pressure), and

2) as wings are falling toward ground (gravity), they create vortices above the wing, which lowers the pressure, increasing the push up effect of the air below, and

at a certain speed, the vortices are stabilized into low pressure regions above the wings, and in a certain "envelope" region, of speed, plane shape, air pressure, all of these forces are equalized to give you level flight, so long as the dial you turn to get into the envelope region, "speed", keeps up.

That's how I understand it. Happy to hear a physicist / aerospace engineer guide me in how to think about this clearly.

Re: Why we don’t understand heavier-than-air flight

#28

Post by tomsto » Sat, Jun 26, 2021, 10:14 AM UTC

rcxdude wrote: post #17

NickNameNick wrote: post #12

Earlier quoted context omitted.

Why does the air move faster on the upper side of the wing? It's not because there's a magic force that requires air particles parted be the leading edge to rejoin thier partner at the trailing edge. The air particles on the upper surface reach the trailing edge much sooner than the ones under the wing.

Because the pressure on the top is lower :) (this is half-serious: the whole problem with these explanations is that cause and effect for all of these variables is not straightforward: you can see from the navier-stokes equations they are all dependent on each other).

Kind of. Actually the real ‘cause’ in my understanding is 1) the curved geometry of the suction (upper) side of the aerofoil and 2) the fact that the flow remains attached to it. Everything else - you can actually approximate the curved surface to a circle and apply equations of circular motion to a parcel of air to satisfy yourself with why the flow is accelerating. And Newton’s 3rd law explains how lift is generated on the wing. In my view there’s no need to use Navier-Stokes to explain how an aerofoil works, if you simplify the geometry to make a special case.

Most of the lift comes from the suction side.

Actually, if you really want to test an explanation, try to apply the same reasoning to explain how a sailing boat can sail upwind (or at least up to about 45 degrees off).

Re: Why we don’t understand heavier-than-air flight

#29

Post by tomsto » Sat, Jun 26, 2021, 10:16 AM UTC

cryptica wrote: post #23

jobigoud wrote: post #4

Earlier quoted context omitted.

Air moves faster on the upper side , creating a pressure differential.

I thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing. If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.

This is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)

Re: Why we don’t understand heavier-than-air flight

#30

Post by AstralStorm » Sat, Jun 26, 2021, 10:22 AM UTC

tomsto wrote: post #29

cryptica wrote: post #23

Earlier quoted context omitted.

I thought it was mostly because of the slight upward angle of the wing which creates air compression under the wing and suction above the wing. If you try to move a flat object through water, it creates pressure at the front and suction at the back. If you tilt it diagonally (and move it right to left), you get pressure in the bottom right and suction in the top right.

This is true of a symmetrical aerofoil (e.g. most helicopters) but not for an asymmetrical aerofoil (most fixed wing aircraft). It is true that a slightly positive angle of attack generates more lift than none (because the pressure/lower side starts making a contribution)

Correct. Still incomplete. Angle of attack causes a vortex at the trailing edge which has nothing to do with raw air speed and everything to do with fluid dynamics (which involves speed but is much more complex)

Short version is that you created a hole (lower pressure area) in air which it now tries to fill. Air and gasses have finite limited velocity known as speed of sound, which is why you get these pressure differentials while the wing is moving. With a flat wing, they're rather small and low pressure vortex is located behind the wing. In an angled wing, some of it is located below the wing and the air trying to fill the low pressure area exerts a lift force on the wing. (It's unlike a balloon. Bernoulli has very limited impact, unlike essentially wind.)