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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#51

Post by Tyr42 »

simonebrunozzi wrote: post #27
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

> can also sail faster than the wind at times. Really? Can you elaborate?

Yeah.

There are two kinds of sailboats. There's the big square sailed ones which always have to move downwind. Then there are the triangular sailed ones, which can move in any direction except for right into the wind. (Maybe a 30 degree on each side dead zone).

The square sailed boats can't move faster than the wind, as they just rely on the air pushing on the sail to move with the wind.

That's also how the triangular sailed boats move when moving directly downwind. They too can't outrun the wind.

However, when moving sideways to the wind, the wind passes over the sail, which looks like a wing sticking out of the water. Like a wing, there's a "lift" force generated, although it's not up but sideways. The boats also have a fin sticking into the water, which prevents the boat from slipping directly along with that "lift" force, and instead move forward.

Re: The mysteries of aerodynamic lift

#52

Post by luckyscs »

WalterBright wrote: post #38
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

> can also sail faster than the wind at times. Really? Can you elaborate?

Take a drafting triangle that has 30-60-90 degree corners. Place it between two objects, and squeeze the triangle between them. It'll move to the side faster than the two objects move together. Or you can just think of it like squirting toothpaste. The wind pressure on the sail and the water pressure on the keel form the two "objects" being pushed together and the sailboat "squirts" out the side. Edit: The angle betw…

It's like our brains aren't meant to handle resolving that not only is it pushed through, it's sucked into a thin and ever moving void. It's pushed and pulled at the same time, in otherwords, part of the continuum.

Re: The mysteries of aerodynamic lift

#53

Post by luckyscs »

WalterBright wrote: post #38
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

> can also sail faster than the wind at times. Really? Can you elaborate?

Take a drafting triangle that has 30-60-90 degree corners. Place it between two objects, and squeeze the triangle between them. It'll move to the side faster than the two objects move together. Or you can just think of it like squirting toothpaste. The wind pressure on the sail and the water pressure on the keel form the two "objects" being pushed together and the sailboat "squirts" out the side. Edit: The angle betw…

It's like our brains aren't meant to handle resolving that not only is it pushed through, it's sucked into a thin and ever moving but thin void. It's pushed and pulled at the same time.

Re: The mysteries of aerodynamic lift

#54

Post by Rury »

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

>Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question

Bernoulli's principle is often misunderstood. The principle itself doesn't say anything as to why different speed effects are observed around airfoils. It only states that within a steady state of fluid flow, increases in speed are associated with a drop in static pressure or a decrease in potential energy. That's it.

Again, as to why air flows faster over on one side of an airfoil, or similarly a sail isn't explained by the principle. However we can measure the airflow around airfoils and see that it is flowing faster over one side. Hence, we can accurately model forces exhibited on a wing using Bernoulli's principle under the appropriate conditions...

Re: The mysteries of aerodynamic lift

#55

Post by solarkraft »

eganist wrote: post #41
opwieurposiu wrote: post #40

A wing is a device that pumps air downward, which in turn pushes the wing upward, by newton's third law. For a large plane, the wing will be pumping many tons of air per second. Start with a cube of still air, with zero mean velocity. Fly a plane through it, and that cube will have a mean downward velocity. http://www.aviation-history.com/theory/lift.htm

Wait a second. Are you saying if I have a cube suspended in space, and I put e.g. a drone inside it that just flies around in a circle inside the cube, the cube will start to move down? What then? Will the drone crash into the ceiling of the cube because it's independent from the cube itself? My brain disagrees with this, but I'm a hacker, not a physicist.

If you just track that cube of air and it's not somehow fixed ... yes, because a downward force is acting on it (with corresponding flow), to keep the drone up.

A better analogy one of my favorite kids' shows (I think it was the German Wissen macht Ah) made was putting a heavy boat into a pot of water on a scale - of course the scale reflects the weight of the boat, it has to go somewhere.

Re: The mysteries of aerodynamic lift

#56

Post by baroomba »

jayrot wrote: post #28
mech1234 wrote: post #8

I suspect if you made a wing out of a flat piece of material, tilted at the appropriate angle of attack, it would be sufficient to fly a plane. It just wouldn't be optimized at all. Really, you need full Navier-Stokes behavior to explain all the forces acting on the wing. Bernoulli doesn't generalize to a full vector field, it's a simplified version of Navier-Stokes. Calling in the big guns doesn't make for an easy d…

When you were a kid, did you not ever put your hand out the window on the freeway? You can actually feel the pressure differential lifting your hand up once you get the right angle.

Same, intuitive way I think of it since learning the school and pop-culture explanations are basically wrong—and for reasons that are unclear to me, because why make up some unintuitive BS when the intuitive and obvious explanation is closer to correct.

Stick hand out window, tilt hand, feel wind push hand up. Wind hits bottom of hand, pushes it up. That's the main thing, and everyone already gets that if they've ever, like, experienced wind. Play with it a little and you can feel your hand respond a bit differently based on the kind of "shadow" it's casting in the wind. That's the rest of it, more or less. There, airplanes explained, certainly way better and closer to correct than "well you see the top of the wing is longer than the bottom, so Bernoulli's principle is the reason airplanes can fly..."

Re: The mysteries of aerodynamic lift

#57

Post by keanzu »

Retric wrote: post #35
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

The billiard ball model works fine when you include the impact of other billboard balls on each other resulting in vortexes etc. It’s simply computationally expensive to do so. Anyway, absolutely flat wings generate lift as long as the angle of attack is non zero. But, by changing the wings shape they get more efficient. The reasons for that are complex differential equations that don’t really have simple plain Engli…

> Aka simple 2d diagrams don’t result in wingtip devices

Why not? You have a flat plate with high pressure below and low pressure above. Air escapes around the end of the plate at the wingtip from the high pressure to the low pressure area, reducing net force. So you put some sort of barrier to impede the flow - a wingtip device.

Re: The mysteries of aerodynamic lift

#58

Post by munificent »

simonebrunozzi wrote: post #27
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

> can also sail faster than the wind at times. Really? Can you elaborate?

Imagine a sailboat pointed 90° relative to the wind so that the wind is coming right at its side. The sail is curved so that it takes that wind and redirects it towards the rear of the boat, giving it forward thrust.

The boat starts moving forward. But, because the wind is perpendicular to the boat, even when its moving the wind is still coming in at the same velocity, so it's still producing thrust. If you can get the boat efficient enough, it can harness enough of that wind energy to reach a velocity along the sailboat's line of motion higher than the velocity of the wind relative to the ground.

Note that boats can only do this when sailing at least somewhat offcenter from the wind direction. When sailing directly downwind, the faster the boat goes, the slower the wind is relative to the boat, leeching away thrust.

Re: The mysteries of aerodynamic lift

#59

Post by starpilot »

keanzu wrote: post #57
Retric wrote: post #35

Earlier quoted context omitted.

The billiard ball model works fine when you include the impact of other billboard balls on each other resulting in vortexes etc. It’s simply computationally expensive to do so. Anyway, absolutely flat wings generate lift as long as the angle of attack is non zero. But, by changing the wings shape they get more efficient. The reasons for that are complex differential equations that don’t really have simple plain Engli…

> Aka simple 2d diagrams don’t result in wingtip devices Why not? You have a flat plate with high pressure below and low pressure above. Air escapes around the end of the plate at the wingtip from the high pressure to the low pressure area, reducing net force. So you put some sort of barrier to impede the flow - a wingtip device.

2d wing does not have a wing "tip." It is infinite.

Re: The mysteries of aerodynamic lift

#60

Post by munificent »

Tyr42 wrote: post #51
simonebrunozzi wrote: post #27

Earlier quoted context omitted.

> can also sail faster than the wind at times. Really? Can you elaborate?

Yeah. There are two kinds of sailboats. There's the big square sailed ones which always have to move downwind. Then there are the triangular sailed ones, which can move in any direction except for right into the wind. (Maybe a 30 degree on each side dead zone). The square sailed boats can't move faster than the wind, as they just rely on the air pushing on the sail to move with the wind. That's also how the triangula…

> There's the big square sailed ones which always have to move downwind.

Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.

The main reasons square-rigged ships fell out of fashion, as far as I know, are mostly logistically. Square-rigged sails make sense when you have a really big ship. A single huge fore-and-aft sail would be too hard to handle. Square-rigged ships break that sail area down into a larger number of smaller individually manageable pieces.

But larger vessels are almost all powered now. For a smaller sailing vessel, it's easier to manage the simpler Bermuda sail plan, and its more efficient. I think maybe square-rigged ships have an efficiency advantage when going straight downwind, but most sailboats simply carry a spinnaker to cover that case.

(Caveat: I'm not a sailor, I've just read some textbooks.)

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