racerX said...
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I think dale cook, gives a pretty good demonstration of how much lift you need to support 100kg.

It does look like Dale has negated gravity with 100kg of lift early in the jump before he loses apparent wind. In mid air he can sheet in a lot harder than he could if he was to maintain equilibrium in normal sailing. You can't sheet in a 4.2 reaching in 50 knots but in the air it should be no problem. (Not that I've tried it)

Never done a forward loop either but I hear that some serious sheeting in is done in that manoeuvre, more than you could use in normal sailing. So I suppose a windsurfer sail can generate 100 kg but not when constrained to an equilibrium sailing stance.

very funny Choco I like that

Barn, very entertaining.

By deduction, it looks like you are arguing that total vertical aerodynamic lift is insignificant (15% of weight and all from the rig?), and that the rest of the weight is on the tail of the board.

This is probably close to the case for your sailing experience.

Your analysis completely ignores the effect of nose aerodynamics when flying the fin.

Some homework for you: typical GPS proven case of board speed = 30kts, wind speed = 20kts at 90 degrees of direction of travel. Board length 240, 40cm in the water, the rest in air feeling the apparent wind. Mean width of 55cm. AOA of 5 deg (combination of rocker and board cant). Assume camber is ~0 since the deck and bottom are both essentially flat. Treat it as an oblique wing in heavy ground effect so its actually very efficient due to the vortex lift. Surprised? The number I get is already much bigger than your 15%. Then take that number and see how it is balanced by rig pressure. And then you will see that the rig is actually generating a lot of vertical lift too...

Mythbusters experiment:
Stick your board out the car at 60 to 70kph near the ground, and tilt it up at 3-4 degrees (the magic carpet angle). Can you hold it with down with your hands at its Lifting Centre, without having to do a handstand out of the car?

I will buy you a superman t-shirt if you can...

7 is the magic number

barn said...
This would mean the apparent wind at board height would be 31.6knots at 18.43 degrees off the nose.

bout right I reckon

barn said...
2. Also, at what Yaw angle should I hold the board in the car experiment? And do I have to drag the last 40cm along the ground? And in light of the re-evaluated apparent wind, what speed should Buster drive at?

ballpark here is 60kph. Lift will be front on in this case but it should be pretty similar if the wind is coming more from the side. You can try yawing the board to match the apparent wind angle you calculated! No need to drag the board- but better if you can! Maybe strap on some sandals. Ask these guys which the best ones are:

barn said...
3. When you say there is 5 degrees of rocker and board cant, does cant refer to pitch or roll?

both... average angle of attack of the board in the direction of the apparent wind is about 5 degrees due to rocker. Also the board has a slight camber from side to side (flatter on the bottom) so this will increase the lift from the side component of apparent wind when its sitting at zero roll.

barn said...
4. Finally, what is the nose lift on a speed board?. And what is the angle of attack of the 40cm wetted flat section, in the 30knot speed scenario, is it the 7-10 degrees as Wineman sugests? Or zero, because we get the lift elsewhere?

tail flat sits at much lower angle- maybe 3-4 degrees. Protractor + picture = answer.

barn said...
(I don't yet see why getting lift from the air is better than getting lift from the water?)

Answering with 2 questions: 1)What direction is the drag component in the case of water and in case of air? Therefore which one has lower drag?
2) What is the L/D of a surface using the low pressure side for lift, in heavy ground effect (ie the nose in the air) compared to a flat plate being dragged over the water?

barn said...
I Have started my homework already, these are just some questions I have run into.

Remember we are looking for a ballpark figure

Anita, O Ye of Little Faith

slowboat said...

barn said...
(I don't yet see why getting lift from the air is better than getting lift from the water?)

Answering with 2 questions: 1)What direction is the drag component in the case of water and in case of air? Therefore which one has lower drag?
2) What is the L/D of a surface using the low pressure side for lift, in heavy ground effect (ie the nose in the air) compared to a flat plate being dragged over the water?

Well the drag component (friction) in water is more I would agree, but the induced drag is less. Because the board has less angle of attack, there is less lift in the backwards direction.

The thing about ground effect is it works best when the air can escape out the back of the foil. In our case, the back of the foil is dammed, because it is in contact with the water. This means the air must be pushed out the sides, and pushing air out the side at 30ish knots is pushing air forwards. This is all drag, and you don't have this problem when dragging a flat plate over water..

Below I have detailed the difference between flight in the airstream, and flight under ground effect. The pitch of the wing in ground effect is more horizontal. The final diagram shows how dragging the back of the foil will hinder the ground effect.



Now, because the nose of the board is at a high angle of attack, there may be no benefit from any ground effect, as the nose would be acting at an extreme angle. Any lift will have a high component of backwards bias. Ergo, drag.. It might be better to eliminate lift by putting speed holes in the nose?.

All this is detailed in this rather easy to read report.

http://www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf

Bonominator said...

Think of the sail more as a hanglider wing than just a source of drive, and literally hang off the boom if you can find that sweetspot.

Have you not been reading? I explained that you cannot gain 'hang glider' like lift from a windsurfing sail. The lift vectors are all in the wrong direction.

You will be lucky if the sail can create enough vertical lift to support your Christmas Lunch.

Hanging the weight off the boom just translates the weight from your legs to the mast track. Which goes into the board.

Now we are deciding how the board is generating its lift.


Anybody got a GPS and a weight vest I can borrow, I get the felling I need to record some Alphas or some sh1t before people will listen haha

In true Mythbuster logic, I ran a simulation of the car experiment and found some points that raise some issues in this current OH&S climate..

Here is the simulation... (you can tell I'm good at getting sidetracked from my Homework)



So I have devised a more practical experiment, using a sheet of plywood on the roof to raise the ground effect zone into the clean air above the Barnmobile.. The board is hinged at 40cm to simulate contact with the water, and some weights can be hung from the mast track 'zone', this will measure the lift.

It may also be necessary to measure the lift at the hinge, but will assess that later.



Are there are any queries in this experiment design? As I have all this junk under the shed..

barn said...
(I don't yet see why getting lift from the air is better than getting lift from the water?)

Planing surfaces only have about 1/3 to 1/2 the lift/drag ratio of sections that have both sides contribuing to lift

Well the drag component (friction) in water is more I would agree, but the induced drag is less. Because the board has less angle of attack, there is less lift in the backwards direction.

A board is also a very low aspect planform ~ 0.125 etc therefore high vortex/induced drag

Does anybody think that a board pointing lengthways into the wind is as efficient as a true wing?

Probably not, but a lot more effiecient than a 0.125 AR planning flat plate

Barn have you been to SP?

yoyo said...

A board is also a very low aspect planform ~ 0.125 etc therefore high vortex/induced drag

Does anybody think that a board pointing lengthways into the wind is as efficient as a true wing?

Probably not, but a lot more effiecient than a 0.125 AR planning flat plate

The definition I'm using of induced drag is a simplified version. It's simply the lift at 90degrees to the foil, if the foil is at a high angle of attack then the lift is in the backwards direction. It's not drag as such, just lift that acts against the forward motion. This no doubt translates to increased tip vortexes.

Barn have you been to SP?

Yes, I got in trouble for laughing at this guy who had Two hooks on his spreader bar.. Apparently he was somebody high up in the Church Of Speedsailing..

And I annoyed Kato quite a bit.

Didn't get any good speeds, and my GPS tracks were fairly erratic..

barn said...

Bonominator said...

Think of the sail more as a hanglider wing than just a source of drive, and literally hang off the boom if you can find that sweetspot.

Have you not been reading? I explained that you cannot gain 'hang glider' like lift from a windsurfing sail. The lift vectors are all in the wrong direction.

You will be lucky if the sail can create enough vertical lift to support your Christmas Lunch.

Ha ha Barn. Actually I didn't read any of the second page posts. I'm saying that even a Christmas lunch-worth of lift can make the difference between 45 knots and 46 knots, which is where I'm struggling to advance! I guess what I've been doing for the last 25 years is wrong and if I follow your advice, adjust my stance a little, I may just add a knot to my disappointing averages.
Seriously though, unless

kato said...


Its an interesting discussion Barn. Just remember any pract tests with the car will have to take the disturbed air flow into account.

That was the thinking behind the plywood splitter, mounted on some high roof racks it should be possible to get the board in some clean air.

My money is still of Slowly as some skills just carn,t be explained by science.

Bet on me and you might be able to replace that 1993 Bic Saxo you cruise around on.

Just to clear some things up, as I think there has been a bit of nitpicking as far as definitions and whatnot, my first point was that you cannot generate enough lift from the sail to take the weight off the board.. I understand there can be some, but there is a difference between claiming 100% and a few kilos. No matter how important those kilos are.

I believe I proved that, and everyone seems to agree (from the silence). Although I am the only one who has put a 'Ballpark' on that lift, at 10-20kg...

Putting my money where my mouth is.


The second point raised is how the board generates the remaining lift, Slowy has pointed out that the lift comes from the 'Heavy Ground Effect' from the first 2m of the board. An interesting point, but I am also skeptical this is a source for much lift. I'm guessing in the ballpark of 5kg at the mast track.


The centre of lift should be around the mastbase, acording to the specs on a Ground Effect foil http://www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf


I ain't really concerned about the efficiency of a foil in Air or water, as I doubt there is much lift at the nose anyway.

Anyway, so far I think I understand how the airflow is supposed to interact with the nose of the board. I dunno if my pictures are helpful but sometimes words are confusing.


This is a board doing 30knots with a 10knot breeze at nose height, giving us 31 knots of apparent wind at 18 degrees. The airflow above is in white, and below is in yellow. I feel these lines are being fairly aerodynamically generous, there would be more turbulence IRL.

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

Side note, if there is such a thing as Heavy Ground Effect under a windsurf board, has anybody ever tried watersking with a windsurf board, if the HGE Hypothesis is correct it should be possible get True Flight. No?

Kinda like a Hoverboard? Except they don't work on water (unless you've got power)

A fin angled at 45 will also give a bit of lift I guess

To the pic above..what type of fin??

Somehow this thread reminds me of this: