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Forums > Windsurfing General

lift myth debunked

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Created by decrepit > 9 months ago, 25 Jan 2012
decrepit
WA, 12365 posts
25 Jan 2012 7:51PM
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the myth being, "that the flow the particles on both sides of the foil, that arrive together at the leading edge leave together at the trailing edge
http://www.newscientist.com/blogs/nstv/2012/01/physics-in-a-minute-how-wings-really-create-lift.html

joe windsurf
1481 posts
25 Jan 2012 7:55PM
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yeah butt ...

this foil is at an angle
are we as windsurfers not pushing the foil straight through ??

decrepit
WA, 12365 posts
25 Jan 2012 8:12PM
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Not unless you're going dead downwind.
A symmetrical foil with no angle of attack will produce no lift. If you want to resist the sideways force of the sail, fin lift is needed, so is an angle of attack. Same goes for the sail.

racerX
462 posts
25 Jan 2012 8:16PM
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That's old news... I think it been clearly understood since the early part of the last century. Its just that many text books erroneously over simply what actually happens.

Also the airflow does not actually split at the leading edge either. Some of the airllow that would have flowed under the airfoil actually flows over the top of the foil. You can see that in the video in the area between the leading edge and the leading stagnation point. So not only is there a downwash from the trailing edge but there is an upwash as the air approaches the foil.

Apparently for tappered airfoils like sails this upwash increases as you reach the tip of the foil, this is one of the reasons why you need some sail twist to maintain an optimum angle of attack.

This is the best explanation that I found for all this stuff.

www.av8n.com/how/htm/airfoils.html

elmo
WA, 8763 posts
25 Jan 2012 8:25PM
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Methunks there is some Confucius with the term "lift" when applied to a wing and a windsurfing fin.

With my back yard physics (I use a hammer I'll hurt myself) A fin generates lift because the faster it travels through water the more solid the water appears, the fin moves in the direction of least resistance which is upwards

Mark _australia
WA, 22727 posts
25 Jan 2012 8:50PM
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elmo said...

Methunks there is some Confucius with the term "lift" when applied to a wing and a windsurfing fin.

With my back yard physics (I use a hammer I'll hurt myself) A fin generates lift because the faster it travels through water the more solid the water appears, the fin moves in the direction of least resistance which is upwards


Yeah I reckon too - makes sense

And the high pressure on the leeward side causes lateral lift, eventuallycausing the board to rail up.
Classic overfinned situation

racerX
462 posts
25 Jan 2012 8:56PM
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Mark _australia said...

elmo said...

Methunks there is some Confucius with the term "lift" when applied to a wing and a windsurfing fin.

With my back yard physics (I use a hammer I'll hurt myself) A fin generates lift because the faster it travels through water the more solid the water appears, the fin moves in the direction of least resistance which is upwards


Yeah I reckon too - makes sense

And the high pressure on the leeward side causes lateral lift, eventuallycausing the board to rail up.
Classic overfinned situation


The drag component is obviously important in lifting the tail of the board, but the fin still acts as an airfoil. Consider for example what happens when you spin out, if it was just the resistance that was keeping the board straight why does the board continue to spin out when the fin presents even more surface area to the oncoming stream of water?


decrepit
WA, 12365 posts
25 Jan 2012 9:44PM
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I think some people don't understand what foil lift refers to.
Foil lift is in a direction at 90deg to the foil, not necessarily upwards.

elmo
WA, 8763 posts
25 Jan 2012 10:48PM
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There is a reason why I sell gaskets

TrevNewman
VIC, 237 posts
26 Jan 2012 12:20PM
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There is only one thing we can do, write a letter to Mythbusters and lets get this sorted because it has always confused me, I will get onto it tonight.

pweedas
WA, 4642 posts
26 Jan 2012 11:24AM
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racerX said...

Mark _australia said...

elmo said...

Methunks there is some Confucius with the term "lift" when applied to a wing and a windsurfing fin.

With my back yard physics (I use a hammer I'll hurt myself) A fin generates lift because the faster it travels through water the more solid the water appears, the fin moves in the direction of least resistance which is upwards


Yeah I reckon too - makes sense

And the high pressure on the leeward side causes lateral lift, eventuallycausing the board to rail up.
Classic overfinned situation


The drag component is obviously important in lifting the tail of the board, but the fin still acts as an airfoil. Consider for example what happens when you spin out, if it was just the resistance that was keeping the board straight why does the board continue to spin out when the fin presents even more surface area to the oncoming stream of water?



Because the fin is an aerofoil operating in a fluid. Once the aerofoil is in a stalled condition, which it is when it's sliding out, it loses almost all lift until such time as a smooth fluid flow is re-established over it's surface.
To do this you have to return the angle of attack to zero, (kick the nose downwind) whereby the flow stream resumes a smooth flow condition, and then you can load it up again.
If you can't re-establish a smooth flow across it then the lift it generates is limited to the small amount it can generate operating in a parachute mode. i.e. turbulent flow.
This is why a bit of weed, jellyfish, or even an air bubble can make the board slide out.
Anything which disrupts a smooth fluid flow will result in the whole aerofoil (fin) stalling and losing almost all lift.
Stalling does not always mean a low fluid speed, although it is more commonly associated with it due to it corresponding with the higher angle of attack needed to generate the required lift at the lower speed.
It can happen at any speed. Even at top speed. The stalled condition simply means the fluid flow is no longer smooth.

Glitch
QLD, 291 posts
29 Jan 2012 11:12AM
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Any time that a foil is generating lift (with a low pressure on the top and a high pressure underneath) there is a reverse flow at the trailing edge as the high pressure tries to equalize with the low. The closer to a stall the more pronounced the reverse flow is. Because of this there is no way the two flows could reach each other uniformly.
Many years ago we did an experiment to settle an argument on what actually happens to the airflow when it stalls. The 1st picture shows the wing as it starts to increase the angle of attack. With the tufts on the tailing edge flowing opposite way and the 2nd in the stalled condition. For anyone interested the 3rd photo is the tail when the aircraft is stalled (that is why the stick shakes). I don't have any good pics when in level flight but there was always a reverse in the airflow.








fullmoon
WA, 314 posts
29 Jan 2012 9:44AM
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Thank god they dont have tufts on comercial aircraft. I found those pics extremely disturbing.
Or mabey they could just glue them down?[}:)]
Skydiving never really appealed to me until now.

evets
WA, 685 posts
29 Jan 2012 9:49AM
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Brilliant images Glitch they have really helped me understand more about what is happeing with flkow. . Why is the uneven flow more pronounced towards the fuselage?

evets
WA, 685 posts
29 Jan 2012 9:51AM
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fullmoon said...

Thank god they dont have tufts on comercial aircraft. I found those pics extremely disturbing.
Or mabey they could just glue them down?[}:)]
Skydiving never really appealed to me until now.


You have a great point fullmoon. I always thought the flow was even (laminar?) across the whole wing! My ignorance would have been frightening for me if I saw these tufts on a 747!

Glitch
QLD, 291 posts
29 Jan 2012 12:05PM
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Wings are deliberately built with a twist on most aircraft so that the wing will stall close to the fuselage 1st while the outer section will still have some lift. This reduces the chance of one wing stalling before the other. When that happens life can get exciting!

fullmoon
WA, 314 posts
29 Jan 2012 2:04PM
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STOPIT Glitch STOPIT NOW!!!!!!!

Glitch said...

Wings are deliberately built with a twist on most aircraft so that the wing will stall close to the fuselage 1st while the outer section will still have some lift. This reduces the chance of one wing stalling before the other. When that happens life can get exciting!



decrepit
WA, 12365 posts
30 Jan 2012 8:39PM
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Glitch said...

>>>>>>>>
Many years ago we did an experiment to settle an argument on what actually happens to the airflow when it stalls. The 1st picture shows the wing as it starts to increase the angle of attack. With the tufts on the tailing edge flowing opposite way and the 2nd in the stalled condition. For anyone interested the 3rd photo is the tail when the aircraft is stalled (that is why the stick shakes). I don't have any good pics when in level flight but there was always a reverse in the airflow.

>>>>>>



Thanks Glitch, great experiment, not very often I start a thread with such positive results.

TrevNewman
VIC, 237 posts
31 Jan 2012 1:23AM
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The folks on the mythbuster forum have enlightened me about something called 'Equal Transit Fallacy' so the myth has indeed been debunked

www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html
www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html
www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

I am never getting on airplane ever again if no one knows how the f@$k they work, although these dudes seem to have an idea:

www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

landyacht
WA, 5921 posts
30 Jan 2012 10:53PM
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elmo said...

There is a reason why I sell gaskets

same reason i mow lawns,and put wheels on my boats

elmo
WA, 8763 posts
30 Jan 2012 11:37PM
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eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

fjdoug
ACT, 548 posts
31 Jan 2012 10:23AM
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I reckon a wing works like a skimboard, expose it to flow at an angle and it gets pushed up.

dinsdale
WA, 1227 posts
31 Jan 2012 4:43PM
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fjdoug said...

I reckon a wing works like a skimboard, expose it to flow at an angle and it gets pushed up.

Exactly correct!! Any pilot knows it's all about angle of attack.

sausage
QLD, 4873 posts
31 Jan 2012 10:14PM
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dinsdale said...

fjdoug said...

I reckon a wing works like a skimboard, expose it to flow at an angle and it gets pushed up.

Exactly correct!! Any pilot knows it's all about angle of attack.




I thought skimboards worked on ground effect, but hey what would I know - I only draw things.

Kel71
QLD, 65 posts
31 Jan 2012 10:19PM
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When do you get time to draw things Snags! Your always on the net!
And No, I'm not stalking you.

sausage
QLD, 4873 posts
31 Jan 2012 10:34PM
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What a thesis - I got lost after the acknowledgements. What ever happened to that flexible fin that was spruked on SB about 2 years ago (forgotten the name)?
Kel71 said...

When do you get time to draw things Snags! Your always on the net!
And No, I'm not stalking you.


Kel, the net is just my thinking time - as a designer it is very easy to sit at your desk looking into space (or a computer screen) and everyone just thinks you're thinking (I think).

yoyo
WA, 1646 posts
1 Feb 2012 3:06PM
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Don't confuse wings in air which are at 20 degree plus stall angles and fins that operate at 1-3 degrees in water.

TrevNewman
VIC, 237 posts
1 Feb 2012 9:44PM
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yoyo said...

Don't confuse wings in air which are at 20 degree plus stall angles and fins that operate at 1-3 degrees in water.




Yeah they have been giving me a bit of tude on the mythbuster forum, one of em said this link explains it. There were no really colorful block diagrams and it looked like it had numbers involved so I gave up pretty quick.

www.grc.nasa.gov/WWW/K-12/airplane/right2.html

KazeRonin
WA, 11 posts
2 Feb 2012 11:31AM
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If you look at the classic airfoil shape of the wing of an aircraft in cross section - they arent symmetrical - the bottom surface is flatter, and the top surface more rounded - meaning that a higher pressure is created under the wing, during flight, than over it - thus creating "lift". However if you look at a fin of a windsurfer - it is a symmetrical foil - in cross section - both sides are the same. Movement of a foil/airfoil through water is no different than air - the only thing that changes is density.
What puzzles me is that - as our windsurfer fins are symmetrical, they will both create an equal pressure on both sides as speed increases through water - so the definition of "lift" that we use re. our windsurfer fins is I believe a misnomer.
If you put two symmetrical airfoils of appropriate size on an aircraft, as wings - it wouldnt fly, as the pressure on both side of the airfoil would be the same - no lift!
My thought are, that the equal pressure generated over the symmetrical foil (windsurf fin), might translate into an upward force that helps unstick the board from the drag that we have at pre planing speeds - my two bobs worth.

r2908
NSW, 214 posts
2 Feb 2012 3:26PM
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KazeRonin said...

If you look at the classic airfoil shape of the wing of an aircraft in cross section - they arent symmetrical - the bottom surface is flatter, and the top surface more rounded - meaning that a higher pressure is created under the wing, during flight, than over it - thus creating "lift". However if you look at a fin of a windsurfer - it is a symmetrical foil - in cross section - both sides are the same. Movement of a foil/airfoil through water is no different than air - the only thing that changes is density.
What puzzles me is that - as our windsurfer fins are symmetrical, they will both create an equal pressure on both sides as speed increases through water - so the definition of "lift" that we use re. our windsurfer fins is I believe a misnomer.
If you put two symmetrical airfoils of appropriate size on an aircraft, as wings - it wouldnt fly, as the pressure on both side of the airfoil would be the same - no lift!
My thought are, that the equal pressure generated over the symmetrical foil (windsurf fin), might translate into an upward force that helps unstick the board from the drag that we have at pre planing speeds - my two bobs worth.


this is correct with the wing. its high school stuff really... but why its the leading edge blunt, i understand the fin being blunt for lift, but the wing?? i feel there's alot more to it, is anyone out there an aeronautical engineer ? and can answer this?.. probably not,

nebbian
WA, 6277 posts
2 Feb 2012 12:30PM
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r2908 said...

KazeRonin said...

If you look at the classic airfoil shape of the wing of an aircraft in cross section - they arent symmetrical - the bottom surface is flatter, and the top surface more rounded - meaning that a higher pressure is created under the wing, during flight, than over it - thus creating "lift". However if you look at a fin of a windsurfer - it is a symmetrical foil - in cross section - both sides are the same. Movement of a foil/airfoil through water is no different than air - the only thing that changes is density.
What puzzles me is that - as our windsurfer fins are symmetrical, they will both create an equal pressure on both sides as speed increases through water - so the definition of "lift" that we use re. our windsurfer fins is I believe a misnomer.
If you put two symmetrical airfoils of appropriate size on an aircraft, as wings - it wouldnt fly, as the pressure on both side of the airfoil would be the same - no lift!
My thought are, that the equal pressure generated over the symmetrical foil (windsurf fin), might translate into an upward force that helps unstick the board from the drag that we have at pre planing speeds - my two bobs worth.


this is correct with the wing. its high school stuff really... but why its the leading edge blunt, i understand the fin being blunt for lift, but the wing?? i feel there's alot more to it, is anyone out there an aeronautical engineer ? and can answer this?.. probably not,




Both of you are under a very common misconception about how wings work.

If this misconception were true, then aeroplanes would not be able to fly upside down, which they can do.

Don't worry, you're not alone! There are more people in the world who believe the lie than who know the truth.

Read TrevNewman's post above.



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"lift myth debunked" started by decrepit