How wings and sails work

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ent228
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How wings and sails work

Post by ent228 » Thu Aug 14, 2014 9:36 pm

I realised a few weeks ago that I had no real idea of why boats actually go to windward, so I did some research.

Found a nice treatment by Holger Babinsky, Professor of Aerodynamics at Cambridge, however it's 10 years old so may be out of date.

Look at this video, http://www.cam.ac.uk/research/news/how- ... eally-work

and then read the link below, you will have to download it to read it

https://drive.google.com/file/d/0B0JABu ... p=sharing

an interesting read that deals with a lot of nice stories that I was told..........

roger
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Re: How wings and sails work

Post by roger » Fri Aug 15, 2014 9:36 am

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trebor
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Re: How wings and sails work

Post by trebor » Fri Aug 15, 2014 5:28 pm

Thank you both, very useful.
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ent228
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Re: How wings and sails work

Post by ent228 » Sat Aug 16, 2014 9:52 pm

Thanks for the link to the WB sails page, packed full and needs much further study.

The Babinsky paper is nice because it narrows the source of lift down to curves in the streamlines, no Bernouli or venturi, no Kutta-Zhukobsky circulation theory of lift, no unequal path or equal transit explanations are now needed so all those sailing instructors need to put out a new story.

I'm pleased to find this because I could never equate the old stories with thin wings like sails....as they did not seem to fit the unequal path theory and yet they still produce lift.

So now I know why the portion of sails behind the point of maximum camber/draft needs to be straighter as you don't want lift angled backwards caused by curved streamlines. The lift being created at right angles to a tangent to the curve of the streamlines.

And the next question is why should a fluid flow around a curved surface anyway? Babinsky says it's something to do with friction, I sense Coanda effect comming into it somewhere.

Some more study comming up

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jpa_wfsc
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Re: How wings and sails work

Post by jpa_wfsc » Sun Aug 24, 2014 9:15 pm

Its simple realy - and best explained by putting a dinghy on its side on a lawn, and get some tennis balls. Roll the balls accros the sail.

What stops a ball dropped on the sail from hitting the ground? The force exerted on the ball by the sail.

Every force is reacted (Newton). So - the sail pushes the ball, the ball must be pushing the sail.

Air is made up of lots of balls....

This method also shows how where in a sail it is curved affects airflow over the sail - it is easy to 'hook' the leach with too much kicker and see that the balls get trapped in the sail. You need a flat after section to the sail to allow the air to flow off and away.

Try it for yourself some time!
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Re: How wings and sails work

Post by Rupert » Tue Aug 26, 2014 12:31 pm

jpa_wfsc wrote:Its simple realy - and best explained by putting a dinghy on its side on a lawn, and get some tennis balls. Roll the balls accros the sail.

What stops a ball dropped on the sail from hitting the ground? The force exerted on the ball by the sail.

Every force is reacted (Newton). So - the sail pushes the ball, the ball must be pushing the sail.

Air is made up of lots of balls....

This method also shows how where in a sail it is curved affects airflow over the sail - it is easy to 'hook' the leach with too much kicker and see that the balls get trapped in the sail. You need a flat after section to the sail to allow the air to flow off and away.

Try it for yourself some time!
I'm glad air particles are smaller than tennis balls, though - that would hurt...

I was trying to think of a comment containing the words "a load of balls", but inspiration failed me!
Rupert

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Re: How wings and sails work

Post by Rod » Sat Aug 30, 2014 8:54 pm

This reminds me of reading an airplane forum several years ago where the same discussion raged - suction (Bernoulli) vs. pressure - with both sides going at it tooth and nail. Some of the problems I have with Holger's PowerPoint presentation is that his drawings feature no AOA (angle of attack) yet it is obvious his smoke experiments were done with the foil at some angle of attack. Most foils, even sails work at an angle of attack. One should not ignore the Kutta condition, the Kutta condition, from what I can discern, is a fudge factor that tries to explain why the flow in front of the foil anticipates the approach of the foil and bends toward the leading edge (the Kutta condition introduces a circular flow around the foil that does bend the flow - how this happens in real life, I don't know). Holger's experiments do prove there is no flow equalization at the leech of the sail but I assume, at some point downwind, flow equalization does occur. Whether this has any impact on the lift of a foil that has long gone past is unknown (reading quantum physics there seems to be effects that occur over very long distances).

Let me state that I am not an aerodynamicist. I have been a sailmaker for part of my working life and for a longer part of my working life I worked in hi-tech where if you didn't have the expertise, you tried to ask the right questions.

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Re: How wings and sails work

Post by ent228 » Sat Aug 30, 2014 9:47 pm

Here is an interesting commentary on false theories of lift by Claes Johnson,"Incorrect or trivial theories of flight." Kutta and Zhukovsky are being taken to task.

I never new the K-Z theory was so old, possibly flawed and still used.....

https://www.youtube.com/watch?v=LNLXdlXKvL8

The talk also explains why I never now have to sharpen the trailing edge of my foils, just make them uniform

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trebor
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Re: How wings and sails work

Post by trebor » Sat Aug 30, 2014 10:47 pm

presumably 50 years from now, people will be discounting Claes Johnson.
What do the particles of air being pushed forward from leading edge of foil have ?
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Re: How wings and sails work

Post by Rod » Sun Aug 31, 2014 12:43 am

Thanks for the link. Though the Claes Johnson video says the Kutta theory is incorrect I didn't get any alternative explanation from that video. Claes Johnson did admit that the foil designs of the 20th century use the mathematics of Kutta condition.

Here is a quote I lifted from an online forum from "Vortexman" who doesn't see how you can toss the Kutta condition aside.
"This is done by imposing the 'Kutta condition', which specifies that the flow departs at the trailing edge, and how it does so. This is not really a mathematical artifice. In reality, it is a way to impose the effects of viscosity on a non-viscous solution. It works because the main effect of viscosity on an 'almost inviscid' flow, which is what the flow around an unstalled wing is, is to force the flow to depart at leading edge, rather than upstream of it. Although this is expressed mathematically, it is an empirically determined way to make the potential flow equations solvable, and to make the solution physically meaningful, in the sense of predicting lift."


Not that I understand all of this, the point being there are some very smart aerodynamicists who, whether they agree that the Kutta condition is part of the correct theory for lift, agree you need to include it if you want to mathematically explain how a foil works.



Rod
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