Flying Wing.
Part 1: 22 minutes each.


Part 2: (it will be interesting if a "wing fence" will appear in future foil wings).



Kite Hydrofoil:
5 & 7 minutes.
Part 1:

Part 2:

In our foiling sport, the massive planform of the strut (mast) provides huge yaw stability, to the point where yaw in flight is a non-factor.

When I am out foiling at a boat speed of 14 mph or higher, I cannot yaw my foil with my feet. I try to, and I watch the tail of water coming off the strut. Pushing my feet asymmetrically does not show any appreciable change in this flow. It just does not yaw.

Because I believe yaw is a non-factor in our windfoiling/kitefoiling, wing fences will not have much, if any, effect or benefit. The strut does all the work to lock in the yaw. That said, fences might help reduce span-wise flow in swept wings. It would be great if somebody experimented with this.

Yes, so how do the racers go upwind? Not by yaw, I believe. Instead they roll the foil to an angle where the front wing provides enough side force to power the system upwind. In the old days of formula racing, it was all fin generating side forces from yaw. Now it's mostly front wings generating side forces from roll.

Segler thanks for your reply. It make sense that a higher speed the mast would be self correcting for yaw.
So at a speed slower than 14 mph/12 knots; asymmetric foot pressure will allow for yaw control?
Assuming that the wing is good enough to fly at that speed.


The depth of the mast would make a difference, the deeper the mast, the stronger yaw correcting force.

At what mast depth would asymmetric foot pressure force out weigh the Newtonian force of the mast for yaw?
Ignore the blade vs delta wing shape and also assume the speed is good enough to control mast height.

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segler said..
In our foiling sport, the massive planform of the strut (mast) provides huge yaw stability, to the point where yaw in flight is a non-factor.

When I am out foiling at a boat speed of 14 mph or higher, I cannot yaw my foil with my feet. I try to, and I watch the tail of water coming off the strut. Pushing my feet asymmetrically does not show any appreciable change in this flow. It just does not yaw.

Because I believe yaw is a non-factor in our windfoiling/kitefoiling, wing fences will not have much, if any, effect or benefit. The strut does all the work to lock in the yaw. That said, fences might help reduce span-wise flow in swept wings. It would be great if somebody experimented with this.

Yes, so how do the racers go upwind? Not by yaw, I believe. Instead they roll the foil to an angle where the front wing provides enough side force to power the system upwind. In the old days of formula racing, it was all fin generating side forces from yaw. Now it's mostly front wings generating side forces from roll.

Professional racers ride their boards flat (horizontal) these days. For kitesurfers definately yes, they go upwind by pushing the wing, for windfoilers really not sure. About the physics. I really only bank the board when I'm overpowered, and even then, less banking and a more upright stance allows for more upwind angle.

At my weight of 195 lb, any good wing of about 1000 cm2 or higher will fly just fine at 14 mph. Of course, planform and aspect ratio affect this. I fly my 1120 cm2 AFS wing and 1100 cm2 Bellacera wing at 14 mph quite readily. My 779 cm2 AFS wing and 940 cm2 LP wing need a little more speed.

I have no idea about the question of depth of strut. One would have to get out there and experiment a lot to figure that one out.

I have yet to see one single foil racer crank upwind with a level board. Level meaning zero degrees of roll, excactly parallel to the water surface. They ALL are tilting their boards some 10-20 degrees to windward to generate the sideways lift needed to go upwind. It does not take much roll angle.

Same for the jibe, rolling the board to leeward. No roll, no turn.

Maybe, not sure, a few degrees look like a lot if you're riding a 91cm wide board 10-20 degrees would equal the upwind rail sitting about 15-30 cm lower than the center, and 30-60cm lower than the downwind rail, that seems unreasonable to me (100cm mast, you need some clearance to the water aswell, plus in slalom they ride more roll angle with shorter masts than they do in courseracing.). I think if we're talking roll for upwind, we are talking 10 degrees at most, but closer to 5, it might be more when the goal is to increase control. Its definately not the same physics as in kiteboarding, their roll angle is 70-80 degrees, but their upwind angles are the same (higher speeds ofcourse, but similar angles). I am not convinced. How do you explain pro's riding higher roll angles on downwind slalom courses compared to upwind?

To me, roll angle increases control, not angle upwind. Riding the board more banked than is needed decreases upwind angle. I can ride the board banked over towindward and with a lower angle upwind, or force the board a flat as possible, put myself and my sail as upright as possinle and swivel the board upwind (yaw-axis) so to speak to increase my angle. We're talking especially in lighter winds, in heavier winds I push the windward rail down to increase control, although I have a feeling keeping the board more flat would also be more efficient in terms of raw power. Rolling the board to me feels like I am putting power into keeping the rail down (just dissipating it so to say) instead of into going higher upwind.

But then again, I am an ok foiler, not a physicist and also definately not an expert on hydro/aerodynamics or wings. Most of my knowledge when it comes to foiling is practical and achieved through experiences, not theoretically based.

Interesting discussion!

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WhiteofHeart said..
Maybe, not sure, a few degrees look like a lot if you're riding a 91cm wide board 10-20 degrees would equal the upwind rail sitting about 15-30 cm lower than the center, and 30-60cm lower than the downwind rail, that seems unreasonable to me (100cm mast, you need some clearance to the water aswell, plus in slalom they ride more roll angle with shorter masts than they do in courseracing.). I think if we're talking roll for upwind, we are talking 10 degrees at most, but closer to 5, it might be more when the goal is to increase control. Its definately not the same physics as in kiteboarding, their roll angle is 70-80 degrees, but their upwind angles are the same (higher speeds ofcourse, but similar angles). I am not convinced. How do you explain pro's riding higher roll angles on downwind slalom courses compared to upwind?

To me, roll angle increases control, not angle upwind. Riding the board more banked than is needed decreases upwind angle. I can ride the board banked over towindward and with a lower angle upwind, or force the board a flat as possible, put myself and my sail as upright as possinle and swivel the board upwind (yaw-axis) so to speak to increase my angle. We're talking especially in lighter winds, in heavier winds I push the windward rail down to increase control, although I have a feeling keeping the board more flat would also be more efficient in terms of raw power. Rolling the board to me feels like I am putting power into keeping the rail down (just dissipating it so to say) instead of into going higher upwind.

But then again, I am an ok foiler, not a physicist and also definately not an expert on hydro/aerodynamics or wings. Most of my knowledge when it comes to foiling is practical and achieved through experiences, not theoretically based.

Interesting discussion!

The more i do it the more i feel this too. Was sailing along behind one of the better upwind sailors the other day with the board close enough to bolt upright, and was surprised to see i was holding his angle and speed. At some points i was getting a better angle than he was.

like you say i think lean is more about control than upwind performance. It works for the kite foils because they have a power source that they can keep close to vertical when its light, and are able to manipulate the board and foil easier since they don't have their power source linked to the board.

When you heel the board to windward you change the component of the lift vector of the foil into both a vertical vector and a vector to windward. This is why it works, a component of your lift vector is actually forcing you to windward, which means you can fly the foil higher = less drag as you aren't relying on the foil mast so much to resist the sideways component from the sail. It's a much more efficient way to get to windward but it only works if you have excess lift power, which is why kites can use it so effectively....and they out point us no problems, particularly < 15kts. It also increases your righting moment but that's another discussion entirely.

The reason it does not work well when it's light is you do not have an excess of lift power coming from the main foil and for optimum VMG your target board speed will be much lower. This is due to the fact that the apparent wind angle is so acute due to the boardspeed/windspeed ratio being so high.

For the record on a 91cm wind board, the side to side vertical differential measured at the rail is as follows.

-5 deg = 80mm
-10 deg = 158mm
-20 deg = 311mm

I reckon in 6kts i'm probably 5 deg, 10-12kts 10-15 deg ish. And above that at my weight (70kg) i'm basically overpowered all the time upwind so I reckon 20 easy. Moths run 26ish degrees. There's a lot more to angle upwind than the windward heel of the board, although no doubt it plays a part.

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CJW said..
When you heel the board to windward you change the component of the lift vector of the foil into both a vertical vector and a vector to windward. This is why it works, a component of your lift vector is actually forcing you to windward, which means you can fly the foil higher = less drag as you aren't relying on the foil mast so much to resist the sideways component from the sail. It's a much more efficient way to get to windward but it only works if you have excess lift power, which is why kites can use it so effectively....and they out point us no problems, particularly < 15kts. It also increases your righting moment but that's another discussion entirely.

The reason it does not work well when it's light is you do not have an excess of lift power coming from the main foil and for optimum VMG your target board speed will be much lower. This is due to the fact that the apparent wind angle is so acute due to the boardspeed/windspeed ratio being so high.

For the record on a 91cm wind board, the side to side vertical differential measured at the rail is as follows.

-5 deg = 80mm
-10 deg = 158mm
-20 deg = 311mm

I reckon in 6kts i'm probably 5 deg, 10-12kts 10-15 deg ish. And above that at my weight (70kg) i'm basically overpowered all the time upwind so I reckon 20 easy. Moths run 26ish degrees. There's a lot more to angle upwind than the windward heel of the board, although no doubt it plays a part.

I get that explanation, thats what I thought aswell, but in practice that doesnt feel like the most efficient way. Banking the board feels depowereing in any condition, in very gusty weather, running into a 20 knot gust with 9.0 for example, I can roll the board towindward, keep the same angle and control the gust with no real felt benefit, or I can keep the board level and swivel the board upwind over the yaw-axis. This way, making most out of the lower appearant wind angle in the gust. Rolling the board doesnt steer it upwind in this situation.

Except for Kiran Badloe and Luuc van Opzeeland I'm easily the highest pointing rider at my spot, Kiran does the same thing as me but even more extreme, theres where I get it all from, because I always used to push hard on the windward rail!

I think part of the equation of why a flatter board works is because both you and the sail are more upright, meaning there is more power in the sail and you can transform that power into angle more efficiently.

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WhiteofHeart said..
Except for Kiran Badloe and Luuc van Opzeeland ...

Slightly off-topic but since you mentioned those two: do they pump as much upwind as some videos might suggest or is that more in the low wind side of things. Luuc was amazing grinding it upwind at Engadinwind by pumping.

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Paducah said..

WhiteofHeart said..
Except for Kiran Badloe and Luuc van Opzeeland ...

Slightly off-topic but since you mentioned those two: do they pump as much upwind as some videos might suggest or is that more in the low wind side of things. Luuc was amazing grinding it upwind at Engadinwind by pumping.

When we sail together they dont, but they often dont have to since they're already faster to begin with ahahah. Although if they end up in your dirty air somehow they'll do anything to get out of it.

Upwind the difference isnt even that big; downwind they're twice as fast, both higher boardpeed and deeper angle, even without pumping.

To stay on topic, I think the truth is somewhere in the middle, if you roll your board towindward to much your stance becomes inefficient which costs angle. If you ride too flat it might be you dont get anywhere, I find that hard to measure to be honest, as I dont really pay attention to my exact roll angle at all. I ride it how it feels nice, and upwind I actively try to lift the windward rail, but could still be at -5 degrees or so, no idea.

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segler said..


Same for the jibe, rolling the board to leeward. No roll, no turn.

I'm not sure that rolling the board actually causes the turn. I think once you are rolled it you have the chance to apply more rear foot pressure and that causes the turn. If you've flown a real or RC plane you'll find that rolling to one side with the aileron doesn't really turn it until you apply up elevator (same as rear foot pressure) to maintain height.

When I first started windfoiling I would crash to windward or leeward without it turning sharply in that direction.

Of course this is true. If you have the board dead level and press on the back foot, all you do is go up. You can try to turn upwind using the mast to add yaw, but that does not work. At least for me it doesn't. If the board is tilted to windward and you press the back foot, the wing increases its lift horizontally upwind as the sine of the heel angle.

Again, watch the course race videos. Not one single racer is foiling upwind on a dead level board. They are ALL heeled to windward.

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segler said..
Of course this is true. If you have the board dead level and press on the back foot, all you do is go up. You can try to turn upwind using the mast to add yaw, but that does not work. At least for me it doesn't. If the board is tilted to windward and you press the back foot, the wing increases its lift horizontally upwind as the sine of the heel angle.

Again, watch the course race videos. Not one single racer is foiling upwind on a dead level board. They are ALL heeled to windward.

I have to say, a certain degree of angle is always there indeed. But if roll was the (only) thing making you go upwind, why would a wider board go upwind faster, and why would at a certain point increasing roll decrease angle?

To expand on the wider board a little more, you're further out compared to the mast, increasing the lever. Relative to the wings not much changes right?? The difference in upwind angle is huge between an 85cm board and a formula, even with the same sail and foil.

I think White is right about angle. There must be an optimum angle for a given sail size, rider weight, and speed. If you exceed that angle by heeling even more, you start to lose lift. Not only do you need that upwind sine angle horizontal lift, but you still do need vertical flight lift to keep the board above water.

This coming summer in USA I plan to have great fun experimenting with all this with my L6 formula board, Moses Race foil, and formula sails.

That is, if I can get back home (far eastern Gorge) at all. Being "stuck" in Florida is OK, but it's not home. My daughter, who is a doctor, says, absolutely do NOT get onto an airplane. So, we might rent a minivan, buy some pads and sleeping bags, and drive the 3,700 miles home, sleeping in the van to avoid hotels. It will take a week.

Hi Everyone!

Just want to add that there are efficient modes upwind and downwind and forcing a windward heel upwind is slow generally except maybe in overpowered conditions. The optimal angles depend on board, sail, and rider center of gravity as well as the relative position and angles of foil.

We force some yaw angle on the board upwind, perhaps without realizing it, with back foot pressure. Just like with conventional boards upwind, there is (was) a 2-3 deg AoA on the fin. It didn't feel like yaw, however its what makes the board go upwind... Its similar with foils and there is a component of lift from the mast that has a contribution. This can be simulated in XLFR5 to determine the right tuning for wing and fuselage angles....

Br,Chris

I tend to agree and disagree with that. Lets talk upwind: I agree that you need to be overpowered for it to be effective (both foil and sail) but this happens relatively early on the foil due to the high apparent windspeed upwind, remember in 12kts of wind upwind you're seeing apparent wind of 30kts over the deck. I urge someone to show me a picture of any of the pro riders in well powered conditions upwind, 12-15kts, without at least 10 degrees of board angle.

Now in light wind conditions I agree it's not as efficient but this is simply due to a lack of foil power. If you foot off to generate the speed needed to have this excess power your VMG is worse. I'd also add that there is still some heel but it's obviously a lot less. I also disagree that the yaw angle of the foil approaches anywhere close to what you see on a slalom board; basically down to the fact that the fin area (foil mast area) is just way way larger so for the same lift you don't need anywhere near the same AoA. There's also the fact that if you add any yaw angle with windward heel it forces the nose down, IE negative lift. You only get lift from the foil mast when you're yawed into the wind (which you have to be to be effective in in this situation) if you have leeward heel of the board, which is literally impossible on a powered race foil setup. Which is why you rail a slalom or formula board in this way coincidentally.

Would anyone disagree that the newer generation race foils with the front wings further forward are faster upwind? More power forward = you can heel the board to windward far more easily as you have power to burn; you trade vertical lift, which you have an abundance of, for resistance to sideways force. You can also foil higher = less drag, the whole thing is a massive feedback loop with a large number of variables. I also believe that rider weight would make a difference to what you apply and when, at my light weight (70kg) I would generally be having to bleed that foil power (heel the board to windward) much earlier than someone who is say 90kg.

Well I suppose you could always revert to Jim Drake's diagram to estimate the ballpark of the heel angle. If you sail with the board horizontal you need 233 lbs of vertical lift provided by the foil and 61.6 lbs of horizontal provided by the mast (fin) . 294.6 lbs of lift being generated under water. What's the lift to drag of the foil? A wild guess 20 :1 . And the mast? another wild guess but it's not going to be as good as the foil, it's symmetric , not optimised for working in one direction, and it's breaking the surface. Let's say it's 10:1 . So that's 233/20 + 61.6/10 = 11.65 + 6.16 = 17.81 lbs of drag. But if you incline the foil at artan 61.6/233 = 14.8 degrees you can do exactly the same job generating the lift of the hypotenuse. That's sqrt(61.6 ^2 + 233^2) = 241lbs of lift from a foil inclined at 14.8 degrees. And because it's all from the foil having a lift to drag of 20 to 1 that's 12.05 lbs of drag. There's still a bit of drag from the mast though, but because it's not generating lift it'll just be the friction drag. If it was operating in the drag bucket, ( probably wasn't) friction drag = induced drag so let's pretend it still has 6.16 /2 =3.08 lbs of drag when not generating lift..

Total drag of inclined foil = 12.05 + 3.08 = 15.13lbs. That's a saving of 15% drag by inclining at 14.8 degrees. Of course you need to have the foil operating in the sweet spot where it's comfortable generating a bit more lift,, if it's only just foiling asking it to do a little more work might increase induced drag more than is gained by full inclination to 14 degrees.

I pulled all the drag numbers out of a hat just to demonstrate the way the sums work. But the angle of lean to relieve the mast of doing any lifting is only dependant on the forces in Jim's diagram

Ian... pounds!!...i'm disappointed, what are they doing to you over there in WA

I never actually ran the numbers but it entirely makes sense just from a first principles perspective. You also get more righting moment over the foil when you heel it to windward and even more if you can fly it high, an added bonus for us light weights. The moth class have done a bonkers amount of analysis on this sort of stuff and whilst we aren't a moth we are somewhat close in terms of the dynamics and they are the kings of windward heel.

Sorry for hijack - but it is too do with wing theory .

if I put the board on the car with the wing still attached and drive down the road at say 80 ks - will it damage board or racks or car due to lift ( in this case downwards) ?

Hi Normster!

Due to big differences in the density between water and air, your foil is pretty useless as an airplane :). If it's okay to go 80 with an attached foil on top of your car? Not sure, it probably depends on the size of your foil and the angle of attack. And the obstacles on your way to the beach. Beware of bridges and balconies!

BR, David

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normster said..
Sorry for hijack - but it is too do with wing theory .

if I put the board on the car with the wing still attached and drive down the road at say 80 ks - will it damage board or racks or car due to lift ( in this case downwards) ?

The air density is low enough to keep stresses well below that in water. But cars are shaped the way they are, low bonnets, clearance between bonnet and hard engine bits, sloping windscreen, so they slip under pedestrians, bouncing them with minimal injuries over the top. Putting a foil up there sort of defeats the purpose. The board's bad enough up there, I always take the wing off.

Thanks - didn't realize density would alter things - i would generally take
off - cheers

Thanks - didn't realize density would alter things - i would generally take
off - cheers

Different question - newbie to foiling

is ther an optimum height for straight line foiling - if mast is 90 cms, what is optimum height of board in straight line sailing ?

Optimum for what? If you're purely talking speed/racing, generally the higher you can fly the better as the higher you fly the lower the mast drag = faster. Obviously though you have swells etc to contend with which would require a different height to dead flat water and if you have a lot of windward heel you can't fly as high as you pierce the main foil tip in swells which ventelates the foil.

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normster said..
Different question - newbie to foiling

is ther an optimum height for straight line foiling - if mast is 90 cms, what is optimum height of board in straight line sailing ?

As high as it will comfortably go is a good rule of thumb. To be honest I dont actively adjust rideheight in flight, the foil does that itself seems like.

Thanks - just freeriding as I'm new to foiling - so their is no natural height? You can control the height to be say half mast out of the water or close to full mast ?

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normster said..
Thanks - just freeriding as I'm new to foiling - so their is no natural height? You can control the height to be say half mast out of the water or close to full mast ?

I feel like there is a natural height, which is about half way up for the average foil / ~40cm of the mast left in the water. With a higher mast you'll naturally ride higher above the water, with a lower mast you'll naturally ride lower.

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segler said..
In our foiling sport, the massive planform of the strut (mast) provides huge yaw stability, to the point where yaw in flight is a non-factor.

When I am out foiling at a boat speed of 14 mph or higher, I cannot yaw my foil with my feet. I try to, and I watch the tail of water coming off the strut. Pushing my feet asymmetrically does not show any appreciable change in this flow. It just does not yaw.

Because I believe yaw is a non-factor in our windfoiling/kitefoiling, wing fences will not have much, if any, effect or benefit. The strut does all the work to lock in the yaw. That said, fences might help reduce span-wise flow in swept wings. It would be great if somebody experimented with this.

Yes, so how do the racers go upwind? Not by yaw, I believe. Instead they roll the foil to an angle where the front wing provides enough side force to power the system upwind. In the old days of formula racing, it was all fin generating side forces from yaw. Now it's mostly front wings generating side forces from roll.

1)How you yaw board whan you are foiling/both feet in footstraps and board is100% flat,what you do whit your legs or sail?
2)Can you yaw board when board is heel to windward?
3)Is it possible to turn downwind when board is heel to windward,and how you do that?