Oldie but helps to visualize how our sails may works...


As a first reflection after watching I comes with idea that
a) our sails could be equipped one day with sort of "flaps" that could increase chamber at low speeds....maybe already done ....b) how nice would be to see similar test showing our modern sails in this practical wind tunnel c) slotting leading edge could be next and easy to implement - I imagine as small additional attachment in front of our mast -made of plastic, carbon fiber !

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Macroscien said..
Oldie but helps to visualize how our sails may works...


As a first reflection after watching I comes with idea that
a) our sails could be equipped one day with sort of "flaps" that could increase chamber at low speeds....maybe already done ....b) how nice would be to see similar test showing our modern sails in this practical wind tunnel c) slotting leading edge could be next and easy to implement - I imagine as small additional attachment in front of our mast -made of plastic, carbon fiber !

That's not really that relevant to how a sail works because the only reason there is air flowing over the the sail from front to back or mast to clew in the first place is because of the air that is hitting the sail at a perpendicular angle or somewhere there abouts which powers the sail making it go forward. That would change the airflow shape over the sail dramatically. The sail is what is doing the powering of the windsurfer whereas with an aeroplane wing it's the engine and the wing creates the lift. The only way you could make a comparison is if the board was powered and it was no longer the sail supplying the power. That then takes away the whole essence of windsurfing.

Here's another one that shows the relative speeds of the streams.



All that stuff applies to fins and hulls as well.

a) You can increase camber at low speeds with adjustable outhaul. I'm pretty sure that works better than a flap would. I could be wrong.

c) Leading edge slots/slats increase drag. They make sense if you have a high speed low drag foil and you want it to work at low speeds. Willy Messerschmitt put automatic leading edge slats on his fighters so they could get away with a low drag high speed wing and actually land and take off at relatively slow speeds.
Indeed you could get more lift from a smaller sail but you wont go as fast with it unless the slat is collapsed.

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mqldnacra said..

Macroscien said..
Oldie but helps to visualize how our sails may works...


As a first reflection after watching I comes with idea that
a) our sails could be equipped one day with sort of "flaps" that could increase chamber at low speeds....maybe already done ....b) how nice would be to see similar test showing our modern sails in this practical wind tunnel c) slotting leading edge could be next and easy to implement - I imagine as small additional attachment in front of our mast -made of plastic, carbon fiber !

That's not really that relevant to how a sail works because the only reason there is air flowing over the the sail from front to back or mast to clew in the first place is because of the air that is hitting the sail at a perpendicular angle or somewhere there abouts which powers the sail making it go forward. That would change the airflow shape over the sail dramatically. The sail is what is doing the powering of the windsurfer whereas with an aeroplane wing it's the engine and the wing creates the lift. The only way you could make a comparison is if the board was powered and it was no longer the sail supplying the power. That then takes away the whole essence of windsurfing.

Yeh, my first thought was this is only relevant to apparent wind... the video doesn't demonstrate effect of the true wind angle, nor does it take into account leech twist. The video is also demonstrating lift rather than the forward momentum a sail promotes.

...still, an interesting physics lesson :)

That may looks like boring back to school , but I found interesting in this wetty, windless weather outside window to watch.BTW clear quite popular misconceptions too :

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qldnacra said..

Macroscien said..
Oldie but helps to visualize how our sails may works...

That's not really that relevant to how a sail works ...

Yes it is. It doesn't matter how the air flow is generated. All air foils work with apparent wind.

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NotWal said..

qldnacra said..

Macroscien said..
Oldie but helps to visualize how our sails may works...

That's not really that relevant to how a sail works ...

Yes it is. It doesn't matter how the air flow is generated. All air foils work with apparent wind.

It also shows that a wide luff and cambers disturb airflow far less than a non-cam sail.

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2035 said..
...

It also shows that a wide luff and cambers disturb airflow far less than a non-cam sail.

That would be interesting to see modelled.

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NotWal said..

qldnacra said..

Macroscien said..
Oldie but helps to visualize how our sails may works...

That's not really that relevant to how a sail works ...

Yes it is. It doesn't matter how the air flow is generated. All air foils work with apparent wind.

Yes, it is exactly how a sail works. You redirect air to the opposite direction you want to travel. I'm surprised by naysayers.

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2035 said..
The video is also demonstrating lift rather than the forward momentum a sail promotes.

...still, an interesting physics lesson :)

Just turn everything by 90 degrees.

Sailing, the entire thing where you go forward, works because there is lift (at 90 degrees) redirected by friction against water, land or ice.

Take your fin off, place your board on super slippery ice so there's no friction, and you're not going to get much forward momentum no more!

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evlPanda said..

2035 said..

The video is also demonstrating lift rather than the forward momentum a sail promotes.

...still, an interesting physics lesson :)

Just turn everything by 90 degrees.

Sailing, the entire thing where you go forward, works because there is lift (at 90 degrees) redirected by friction against water, land or ice.

Take your fin off, place your board on super slippery ice so there's no friction, and you're not going to get much forward momentum no more!

I'm sure the turbulence (and the amount of it) would change if the wind wasn't coming from directly front on (i.e. apparent Vs actual). I agree, there are similar principals at play and yes, the fin (and other factors) create the forward drive, but it's not a like-for-like comparison.

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qldnacra said..

Macroscien said..
Oldie but helps to visualize how our sails may works...


As a first reflection after watching I comes with idea that
a) our sails could be equipped one day with sort of "flaps" that could increase chamber at low speeds....maybe already done ....b) how nice would be to see similar test showing our modern sails in this practical wind tunnel c) slotting leading edge could be next and easy to implement - I imagine as small additional attachment in front of our mast -made of plastic, carbon fiber !

That's not really that relevant to how a sail works because the only reason there is air flowing over the the sail from front to back or mast to clew in the first place is because of the air that is hitting the sail at a perpendicular angle or somewhere there abouts which powers the sail making it go forward. That would change the airflow shape over the sail dramatically. The sail is what is doing the powering of the windsurfer whereas with an aeroplane wing it's the engine and the wing creates the lift. The only way you could make a comparison is if the board was powered and it was no longer the sail supplying the power. That then takes away the whole essence of windsurfing.

a powered aeroplane wing works the same as a sailplane, hang-glider, windsurfer, B747, Cessna, Antoine Albeau...

and I agree, on the fly adjustable outhauls and downhauls like the olympic RSX sailors use have similar effects to flaps;

flatten the sail for less lift and reduce the by-product of lift; drag

make the sail deeper/increase camber for increased lift at slower speeds, allows an aeroplane to fly slower without stalling.

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2035 said..
I'm sure the turbulence (and the amount of it) would change if the wind wasn't coming from directly front on (i.e. apparent Vs actual). I agree, there are similar principals at play and yes, the fin (and other factors) create the forward drive, but it's not a like-for-like comparison.

Sails work exactly the same as a wing. Both are airfoils. Honestly there is absolutely no difference. Sails are wings are sails. They work by redirecting the air flow.

You get more turbulence overall as you increase the angle of attack, be it landing an airplane or oversheeting a sail, and vice versa.

You also get more turbulence at high angles of attack with cambered foils than non cambered foils, be they a wing, a sail, a piece of paper or a car. That's more slip than lift, or lack thereof. Most people recognise it as "aerodynamics", like when you think of a car. Low camber cars like a Diablo have less turbulence than high-cambered cars like a Beetle. Cars are wings too.

I remember my old Hobie. Had two sails instead of one. The one in the front helps power main sail.From the video I am guessing right now that two sails configuration helps only at high angle of attack, but for our speed one will be better ( less resistance).
Unless we are doing also max speeds going down wind with high angle to the wind (?)

I don't know if I am telling folks here things they already knew.

Remember that famous Cadbury chocolate advert? The professor placed a hard-boiled egg into a glass with a piece of burning paper? Was the egg sucked in, or was it pushed?

Sails work the same way. A slight negative pressure is created by the different speed of airflow over the convex and concave sides of the sail. It takes a bit longer to travel over the convex side, hence a slight reduction in pressure. So the sail is either pushed from the concave side, or being sucked into from the convex side. Take your pick.

If one knows about "vectors", and in this case, "vector addition". The combination of the forward speed of the sail and the wind itself produce the "apparent wind" which will be faster than the original "true wind". The "apparent wind" will not be coming from the same direction as the true wind, but at an angle as defined by the "vectors". The faster you travel, the stronger will the apparent wind becomes, while all the time its angle becomes more and more acute. That is why you can sheet in hard and point upwind. Unfortunately, there reaches a point when the shape of the sail simply can't cope with the sharper angle of attack, and the sail will stall.

The shape of the sail also decides how high can you point into the wind. Ice boats have very flat sails. Their sails are so flat that they can't start on their own but needed a tow to get the "apparent wind" up to its "useable" range.

Obviously, the sail will get pushed side-way if there is nothing to stop it. The fin(s) are there to stop the side-way slide. Again, the foil shape of the fin(s) creates the lift needed to counter the side-way force exerted by the wind. Like all foil shapes, there comes a point the angle of attack becomes too acute and the fin will stall. This means the flow of water over the sides of the fin becomes detached and turbulence occurs. Spin-out is the result.

The answers are already there...in the design of bird wings.

in honour of Jim Drake - a pioneer of windsurfing:

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

Sounds like you guys know what your talking about..... Question for you, i think its still on topic
So if im going for a jump height record do i put a cammed sail, non-cammed freeride sail or a wave sail on me speed board

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xoff said..
Sounds like you guys know what your talking about..... Question for you, i think its still on topic
So if im going for a jump height record do i put a cammed sail, non-cammed freeride sail or a wave sail on me speed board

personally for jumping very high indeed I would attach kite instead. Some kiters are known to jump so high that never go back. Hopefully next mission to the Moon could rescue them ..

Cambered...

control the Flight and go higher
others will stall earlier and go lower

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Jupiter said..
The combination of the forward speed of the sail and the wind itself produce the "apparent wind" which will be faster than the original "true wind".

This is quite interesting and intriguing and obvious at same time observation and completely not taken into account in the upper video lectures.
How in effect this lift could produce forward motion that effect then lift again till equilibrium is reached.....

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?What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes ? lower pressure on the upper surface and higher pressure on the lower surface,? clarified Babinsky, from the Department of Engineering. ?This is why a flat surface like a sail is able to cause lift ? here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it?s the curvature that creates lift, not the distance.?

- See more at: www.cam.ac.uk/research/news/how-wings-really-work

I've also read it is mostly the simple redirection of the airflow, which is why a paper plane flies. There is also the pressure difference.

Here's a modern version of the original video from this thread. HD with commentary.

The video is good at describing apparent wind. There's a term called angle of attack, the angle between the relative airflow and the cord (centreline) of the aerofoil. A stall occurs when angle of attack is exceeded and the air on the upper edge of the wing becomes turbulent and detaches (light aircraft wing stall at around 17 deg). This turbulent air then causes a large amount of induced drag and subsequently the aircraft goes from flying to falling. This same principal can somewhat be applied to sailing whereby oversheeting (stalling) the sail is less effective than having a small angle of attack to the wind. As AOA is a function of forward velocity, being stationary will incur 0 AOA. It should also be noted that as velocity increases the component relative airflow increases. At low speeds perhaps on a reach a large portion of the airflow will be at a high angle to the sail and board, therefore it is crucial oversheeting does not occur. As speed increases however, the combination of apparent wind and true airflow alter the vector of relative wind so that the relative wind is closer inboard to the centreline of the board. If the angle between the sail and true wind were kept constant with an increase of speed the angle of attack would decrease. This explains why at high speed you can fully sheet in a sail. At high speed the characteristics of the sail come into play as the camber, chord and thickness of the aerofoil all affect how the sail feels at speed.

Obviously the type of sail and fin combination would vastly alter the specifics but the basic concept remains true. I might be able to draw up a diagram to help explain the concept but I'm currently on my break at work. Also it is a brief explanation of a complicated concept and I would love to waste more time at work explaining it but I probably shouldn't

PS. I read Jupiter,so comment after I posted and we pretty much say the same thing.

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Macroscien said..

Jupiter said..
The combination of the forward speed of the sail and the wind itself produce the "apparent wind" which will be faster than the original "true wind".

This is quite interesting and intriguing and obvious at same time observation and completely not taken into account in the upper video lectures.
How in effect this lift could produce forward motion that effect then lift again till equilibrium is reached.....

it is actually "completely taken into account"... as the streamlines represent actual apparent wind. The thing that isn't shown in these types of videos, is a wing that is rotated up to 90 degrees to the streamlines - if they did this, we would somewhat see how well the given foil works.

^ Just imagine that instead of looking across a wing you are looking down a rig.

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joe windsurf said..
in honour of Jim Drake - a pioneer of windsurfing:

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

.......... now that is gold thanks Joe

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Haggar said...

joe windsurf said..
in honour of Jim Drake - a pioneer of windsurfing:

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

.......... now that is gold thanks Joe

Much respect to Jim but he has missed (as does all physics of windsurfing force beeakdowns) sail/mast foot pressure & board forces since it too becomes a wing, maybe that's why hypersonic tried to kill me The inward force created by a wing is something that is ignored in basic aeronautics, yet all windsurfers exploit it.

Once the extra forces are factored in the loss of efficiency starts to show which is why sorry to say sailboards will never beat a scientifically designed craft such as sailrocket or more simply kites.

Jim's & high school physics both recognise the effect of board lift (& he mentions it as planing hull) resulting in the lift component being moved forward of the fin but I dont think these forces should be bundled with "lift" or "weight" in the sense that they are being used.

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mkseven said..
board forces since it too becomes a wing, maybe that's why hypersonic tried to kill me

all fast super cars divert quite a lot of air dynamics down to glue car to the road.
Maybe one day our speed boards needs similar shape to push down board into water ?
I am guessing above 50 knots even small board is happy to fly away, unless super heavy rider keep it in reins.
But wait isn't the shape of the board already airfoil with lift forcing down ?

No you are "shifting aoa on the plate", technically not a wing in the airfoil sense.

They are looking at it with deck concaves etc but the marketing is a w@nk & I dont think is entirely correct, the biggest advantage to come out of deck concaves is lowering the pivot point on mast foot but also effectively raising boom.

Fast cars have lots of hp to overcome drag from the aerodynamic aids, we dont have hp on tap like they do so need to maximise efficiency.

Thanks SO18. Now to actually do it cough cough aaah......

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evlPanda said.. Sails are wings are sails.

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xoff said..
Thanks SO18. Now to actually do it cough cough aaah......

evlPanda said.. Sails are wings are sails.

I wonder if somebody theoretically jump from aircraft/ballon on full windsurfing gear may have a chance of survival ? What could be terminal velocity at optimal/perfect angle of descent.

Yes finally Macroscien. Someone else on the site from Amsterdam too
Yes of course you'd survive. What? james bond did..... in a rubber ducky.
Who's up for it? Who's the biggest nutter on the forum? ....... Macro looks like your up mate

Had a bit of trouble with footage for this one 2.05(nothing on vid after 3.00)

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xoff said..
Yes finally Macroscien. Someone else on the site from Amsterdam too
Yes of course you'd survive. What? james bond did..... in a rubber ducky.
Who's up for it? Who's the biggest nutter on the forum? ....... Macro looks like your up mate

Had a bit of trouble with footage for this one 2.05(nothing on vid after 3.00)

Fantastic xoff !! That is it . But we don't know if that was real guy of dummy strapped on .
Next, he used only board surface for air resistance not sail that is much bigger but almost impossible to control without the fin as we know.All he need is sort of air pocket in the place of fin and should be fun run down.