Standup Zone Forum
The Foil Zone => Foil SUP => Topic started by: surfcowboy on July 23, 2017, 09:54:45 AM
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https://youtu.be/rilSsFga2wA
I submit this as evidence of these wings not working the way many think they do.
If these wings were generating lift like an airplane wing, this Nash would be flying dead level in the water. These guys are only lifting with a high AOA and speed. Now some will say they aren't going fast enough but really 15kph, 10 mph and that big wing should be flying like a bird, am I wrong here?
Beasho, Pono, and others with the math, are we looking at why flat foils should (and in kiteboarding seem to) work?
If we can't vary the pressure of water like we do air, then traditional wings clearly must work differently or not at all.
I'm about to start a build and I think I need to go flat and see what happens. Anyone else done any work like this?
I'm thinking of the discussion of whether my paipo was foiling or skimming/planing and from what I can see here, it's the same thing as long as your mast is long enough to keep you "planing" below the surface.
Am I off track here?
I do agree that shape and curve, dihedral etc would affect controllability and performance but I'm now very suspicious of worrying about the profile of the wing after watching hours of video of people doing this.
Let's hear it and try to keep it on topic and not personal. I have no info here but I'm seeing some things that don't add up. If wings work in water then someone should be flying one dead flat at speed. Real world experience is welcome as well, are you guys flat or angled?
The only boards I see flat/horizontal are dudes on waves and that gives you an extreme AOA to the surface of the water.
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a flat plate will lift if given enough angle of attack how ever it comes down to how much power is needed to do so which equates to efficiency. sharp or thin edges also lead to other characteristics, just think nose shapes on race boards or rails on surf boards and the effects they have, there is a reason winds have a round leading edge and a thin trailing edge. You do not want to tell the water which way you to go rather you want to just help it along and persuade it which way you would like it to go and let it kinda make up its on mind .
very similar to the wife ;)
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The fundamental problem in theorizing about lift is that the conventional explanations of how wings work is a simplification. If you do the math, or just observe the many ways that wings work and how lift is generated the simple theories start to fall apart. The full explanation requires a working knowledge of Euler's equations and doesn't really simplify. So there will always be holes in how the explanations work, and uncomfortable inconsistencies. I sort of understand the full explanation. I say sort of, because I can't express my understanding clearly, which means I really don't get it. I'm not alone though. In trying to dig out a useful explanation for Euler's approach to lift I found this somewhat frustrating NASA paper on lift that handily pokes holes in the general explanations without replacing them with something useful. https://www.grc.nasa.gov/www/K-12/airplane/wrong1.html
That said, the general equations and the common explanations for lift work just fine. They're a useful tool. just like quantum mechanics is a useful tool. I appreciate the name quantum mechanics because it's a more honest way of describing what we actually know about WHY it works--not a clue. All we really have is a set of rules about HOW it works, and some half-baked, internally consistent theories about why. The Euler approach to understanding lift isn't nearly as hopeless as quantum theory, but I can't translate it into useful language.
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If these wings were generating lift like an airplane wing, this Naish would be flying dead level in the water. These guys are only lifting with a high AOA and speed.
Looks are deceiving. I've been kiting lately with my SUP foil and I'm amazed by the range of AOA it flies at compared to my high performance kite foil.
The SUP foil wings are impressive to look at. Even the stabilizer, is a thick upside down wing. Not the flatish symmetric stabilizer seen on many other foils, where just AOA is used to make downward lift.
I can climb and fly very nose high and not stall and I can cruise super slow dead level.
I've very impressed. So I say go with thick asymmetric wings and get more usable fun range from your foil.
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Spoke to a buddy today who works at an aircraft/aerospace company today ( he also used to work at a company that did a lot of hydrodynamics and flow testing) and he actually offered that as you say, the wing profiles should act similarly in water to air. Although the reasons are slightly different.
UK, you are correct I believe that lift can be achieved but I hadn't considered the turbulence. (Reading about Reynolds numbers today.)
DW, I'm starting fairly traditional for sure, just based on everyone's experiences. I'll branch out from there. But I reallly want to try to get a fuselage design that I can bolt different wings onto. I think there's a lot to be done in this area but I do need a good working baseline to start from.
One other thing we haven't considered or at least I've not seen it mentioned here is the size we are working with. Hang gliders are using giant wings whereas we are talking a fraction of the size. I know obviously the water is denser and so we get more bang for the buck, so to speak, but has anyone seen people testing the limits of size vs maneuverability? I'm somewhat fascinated, academically at least, at what people were doing when they started upsizing kite foils to slow down and surf.
Anyone know, for instance, how big Alex and GoFoil went when they were figuring it out? Still intrigued by the testing process and wondering if we've seen the biggest foil yet. Heading off now to draw some more.
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Hey SC great topic and good on you for digging deeper and experimenting a bit. I plan to do the same. Air and water both act as a fluid, so it doesn't matter to the wing which medium it is operating in...still a wing. As you mentioned, the difference is the density of the fluid....that will ultimately determine the wing area required to support the weight during "flight". i think the complicating fact for wave foiling is the dynamic motion through the medium with a constantly varying Angle of attack etc. An airfoil section is going to be more productive than a flat plate both in lift and drag produced. Interesting thread! I need to get in the water soon and quit thinking so much!
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Hey SC great topic and good on you for digging deeper and experimenting a bit. I plan to do the same. Air and water both act as a fluid, so it doesn't matter to the wing which medium it is operating in...still a wing.
Yes and no. Bernoulli's principle applies directly to incompressible fluids since there isn't any energy change resulting from compressing the fluid, but people often visualize lift as occurring because of a density change--a partial vacuum on top of the wind resulting from the pressure change, with the higher pressure under the wing pushing the wing up. A useful mental picture in air, but water doesn't change in density to any appreciable degree. And in fact, even in air, the density of the air above a wing is higher for many foils than density below (pressure and density are lower in the boundary layer, but higher above it), and the density change ranges fairly far away from the wing. A fully representative equation treats the affected distance as infinite--sort of a practical implementation of butterfly effect. But a faulty mental picture doesn't change the fact that wings work more or less the same in air or water.
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Oooo..that's a good point about incompressability of water as compared to air. Maybe more useful to think of acceleration of mass and Newtons opposite and equal reaction to that. It's fun to think about. I flew paragliders for many years (stopped when too many friends got hurt or killed) and there was much discussion about the physics of those things.....here we were flying a wing that virtually disappeared when stalled and with no structure to support a negative angle of attack....amazing what they could do though...
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Thx, OPSD, and Pono, that's the part that I'm working out in my head, the Bernoulli stuff. I've heard the same about the mental image being flawed, need to get onto YouTube for some explainers.
Worked last night on some foil drawings and design. Thinking that I'll use the Seabreeze design for Keel and mounting plate and have a dual box mount on the way in the mail now. Keel ( I think we should leave the term "mast" to the Windsurfers) will be about 24" just to have some leeway with foil breeches and yet to not put me too high. (Note to potential foil pilots, even 18" feels crazy high!) I may cut that to 18" when I get into it.
I'm starting with a Clark-Y with no anhedral or dihedral at about 290 sq. in. to see where that takes me since I'm not expecting to be able to tip the tips out of the water with hard cranked turns anytime soon and I want to just experience things.
I have a few plans for anhedral once I get the sizing dialed better, or maybe I'll just buy one if they seem far better than the homebuilt.
I still have a feeling we're quite a ways from "done" with foil design.
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It's interesting to look at the eFoils in operation and see how much AOA that seem to use. With the majority of riders the boards are nose high and while they are faster than paddling, aren't particularly fast. The exception is some of the riders using the lyft system, where the board is quite flat and hauls ass. The kid sitting on the foil and the guy with the little kid are both mind-blowing now that I've tried to ride a GoFoil. I think this sport is going to separate the sheep from the goats.
I don't know precisely what that means--my friend Tony Garmey says that about challenging race tracks like Watkins Glen.
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I hadn't thought of those but yes, a lot of angle there.
I need to check the lyft stuff.
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My two cents (and that's all I could get for it)-
I've spent the past 17 years riding and competing on the sit down hydrofoil (Airchair or Sky Ski). For the past 15 years have been developing wings for XtremeFoils and have had the opportunity to learn a lot of what doesn't work (and does). For the sit down hydrofoil, it is all about Angle of Attack on the rear wing. The front wing wing design is very typical to what is being done on the surf foils (chord being 3/4ish forward) and a flat bottom- however, there is shape to the wing (curved up, curved down, and even a mustache). No matter what we did, the rear wing had to be in a position of facing down (shimmed). We did try and front wing that was symmetrical (same top shape as bottom) and it wouldn't fly that showed the Bernoulli's principle does effect the wing- just not as much as the AOA. Guys are now surfing behind boats at 9 mph's without the rope on the sit down hydrofoils. Wings that fly at 28 mph's behind a boat and get out of the water 20+ feet also can surf at 9 mph's... kinda crazy if you ask me.
On the Crazy Foil set up, it has thicker wings which create more drag but am finding lift at 6-7 mph's that is needed on the river. Behind the boat can surf at 9.5 mph's with a huge AOA.
Ben
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My guess, and it's only that, is that the stabilizer angle is giving you that big range. But I bet when it goes bad at the upper end that stuff flies everywhere. Generally, the only hydrofoils that have good range are surface piercing. Fully submerged wings increase lift enormously with higher velocity unless they have some kind of spoiler system.
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Thx Sam, good to hear the experience and you sit down guys have a ton of hours on this stuff.
That is a crazy speed range. I wouldn't have thought that you could go that far but as I think of it, I have read some kite guys who think that everyone other than racers will be riding big, thick, low aspect foils for everything. They are saying that the thin foils may eventually fall out of favor since the thick ones are showing the ability to handle all speeds. You just backed that up. Interesting.
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The sit down world adjusts for speed ranges by the amount of lift- the faster needs more lift and less slow. When Rush Randle pulled me into waves on the North Shore on the sit down, lift had to be taken out when I let go of the rope.
Learned something new today- I put the 28 inch wing in the rear position and 22 inch front wing (5 inches wide each and fairly thick). Mounted it to an old junior wakeboard and found some success!! Even let go of the rope a bit. The angle of attack was the same on the near foil not attached to the board in the picture. The closer Foil is the one that has been able to wake foil surf without the rope and when I added a wing to the bottom found lift at 6-7 mph's and foiled a river wave. However, the Crazy set up on the wakeboard today performed awesome- but I had boots attached for better stabilization. It has more lift than the other set up. Interesting stuff!!
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heres some other foils . not mine , in South Africa where theyre doing a bit of AOA and foil
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What a cool design. But do you have info on why such a large foil on a small board?
Seems almost like a stepped hull board where you ride up on another surface once at speed but in this case the sq. inches are really similar.
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I'm not sure about the why's of this design, l it does seem big but it might be more of a full planing foil rather than when a submerged or surface piercing. I thought it was. A cool design tho.
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Yes, I checked the thread and he's doing some cool stuff. Wild all the paipo stuff that's out there.
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Is there any theory on zero AOA or do you need foil for lift.
Here's one from a guy named Bruno in France.
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duno about theory but my old foil had so much lift you would end up flying at negative aoa to keep the thing in the water. the problem was it had too much lift so there was a huge wave drag hump to over come at sub foiling speed, ended up cutting the trailing edges straight across and removing 10+mm of material from the underside going to a semi symmetrical airfoil with no big concave 'hook' in the aft sections. that got rid of heaps of drag while only sacrificing maybe half a knot of take off speed. last pic is what it looked like in the end
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While lift is the goal, too much or constant high lift requires so much effort to counteract it. Similar to what you've done Container, less lift may result in slower initial lift but will result in a more manageable ride.
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Is there any theory on zero AOA or do you need foil for lift.
The theory was settled nearly 100 years ago.
A Cambered Airfoil, Flat Bottom Curved Top, generates lift at 0 degrees angle of attack. A symmetric foil, Double Foil, generates Zero Lift at Zero degrees. The lift profiles are otherwise identical - See the blue lines in the first diagram. The Camber induces lift, through "Circulation", approximately 8 degrees sooner (to keep it simple). A cambered foil also has the benefit of generating through a larger range of Angles of Attack before stall for equivalent speed.
For example: High Cambered Airfoil generates lift from -8 degrees to +12 degrees: Spanning 20 degrees - ORANGE ARROW
vs. a Symmetric Foil that will generate lift from 0 degrees to +16 degrees. Spanning just 16 degrees - PURPLE ARROW. This is the distance from where the blue line crosses the X-Axis to the Stall Angle.
I am now convincing myself why the thick Cambered Go-Foil works so well in the surf:
Because we wobble up and down and the angle of attack (called pitch) changes so much when dropping down a face, carving down the line or pumping to make a section - We need a heavily Cambered foil. This produces the greatest 1) Potential Lift 2) Range of Pitch before Stall
Here are some pictures describing what is going on.
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Beasho, I love you. Thank you for this post.
So in the end, basically anything works, but the more cambered the foil, within limits, the more flexibility you have with angle. And yes, of course it makes sense that on a big long keel/mast where you can wildly vary the AOA by your movements, you'd want a thick, cambered foil.
When U.K. told me to start at 1 1/2" for a thickness I was surprised but as I see more of these types, I get why you'd want that.
I have a feeeling that once I get one made, I'll start messing around and seeing where you can go. I think there are many solutions to foiling yet to be seen.
Again, thank you for the science, it's sadly so rare these days.
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Is there any theory on zero AOA or do you need foil for lift.
The theory was settled nearly 100 years ago.
But that theory was based on static air and not moving water wasn't it ?
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In engineering terms it's all called fluid. Water, air... effectively the same.
Lift = 1/2 Density x Velocity^2
Density of water is 800 X that of air which is why the tiny wings work.
There's a slight difference due to incompressibilty of water or when nearing the speed of sound and scaling (called Reynolds number think a bee flying vs 747) but otherwise for LIFT start with this stuff.
Otherwise for what you/we are doing zig zagging starting and stopping on a wave I agree no one knows what works best yet.
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Surffoils, I also verified this with an aerospace engineer buddy of mine here as well. There are differences in practices for sure when you get into the water (speed, lift per sq inches etc, but the ideas are the same.
I think that clearly there are many ways to solve the problem here but the best way to find out is to do a few wings and vary the angles.
I've also got some weird ideas that I'll try to draw while I'm traveling and send you. Some of the paipo ideas I think will translate pretty well to standing up. The low drag ideas are interesting for paddle in foiling.
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There doesn't seem to be a lack of lift , we've got so much lift we're literally standing on it to hold it down, I think control is the goal to aim for ?
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That's the part I have to get into the water to see, but yes, control is key. We need to get UK to talk about his Tacuma vs his home built foil. That's why I'm considering starting with a flat wing and a curved one to feel the difference. Then add in a flat foil with no foil profile and we will learn a lot in a short time.
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i have tried both go foil models and i found the feel to be very similar to my various foils, the maliko in my opinion is unstable in the roll axis but super stable in pitch and a great L/D ratio, like a slipperier version of my current foil. the kai foil rolls easily but has alot of reserve stability thanks to the crescent shape and needs a fair bit more speed than mine and the maliko to fly. its also interesting to note the foil section differences between the two. i'll try do a sketch from memory comparing them
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the maliko in my opinion is unstable in the roll axis
Do you think that's because of the negative dihedral of the tail stabilizer and the fact that that main wing has no dihedral? I guess the downside of positive dihedral is that the wing might pierce the surface, but it seems like it would make it more stable in roll. Come to think of it, I suppose the stabilizer needs negative dihedral to initiate a turn, but I think just a little positive dihedral on the main wing would go a long way.
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because the main wing is flat so has zero reserve stability. my foil has dihedral and its slightly more stable than the maliko but still less so than the kai foil. the tips poking out thru the waves is also a fairly big problem.
it seems counter intuitive but anhedral is more stable. my tail foil can go in either way, lifting with dihedral or downforce with anhedral and that makes a huge difference to all round control but thats more cause of the lift/downforce than what way the wings are angled.
the downforce just steadies up the whole operation because of a sort of 'triangulation' of forces whereas with it also providing lift you really are just balanced on top of the mast trying to keep the thing level. i reckon its alot faster and less drag though so i prefer it this way
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Keep testing: Aeronautic theory states
- Dihedral is More Stable in flight.
- A dead Flat Wing is Most Efficient from a lift to drag ratio: Maliko
- Anhedral, curved down wings like a gull, is inherently UNSTABLE. Unstable is coincident with and produces maneuverability. Go-Foil KAI Model
Fighter aircraft can be so unstable that a human can no longer fly them. BUT a computer can fly them. This combination gives the benefit of flight with wacky maneuverability. With our "Go-Foil" wings we are the computer starting from Kai's processing power and moving down from there.
However modern theory doesn't play on the boundary between Air and Water that is 800X denser and foam which is somewhere inbetween.
Watching the videos you will see why Dihedral doesn't work. When the rider is turning, as suggested, the tips get exposed. The primary benefit of Anhedral with underwater wings is (maybe) that the entire wing stays submerged during maneuvers.
https://en.wikipedia.org/wiki/Dihedral_(aeronautics)
Anhedral:
Military fighter aircraft often have near zero or even anhedral angle reducing dihedral effect and hence reducing the stability of the spiral mode. This increases manoeuvrability which is desirable in fighter-type aircraft.[/list][/list]
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Beasho, I think it's the degree of dihedral or upturn thats too great.
From a flat foil there's reduced lift as the foil is tilted. By adding an upturned tip of 9 degrees to 50% of the each wing length you get a greater spread of lift as the foil tilts out to about 40 deg.
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From a flat foil there's reduced lift as the foil is tilted.
TRUE
But a Flat Foil still has more horizontal lifting force than a Dihedral foil of equal area, hence more vertical lift, when tilted at the same angle.
A bit about my background: I have an Aeronautical Engineering Degree from Cornell '92. Same school as Bill Nye the Science guy except he was just a Mechanical Engineering. So while I never played engineer on TV I did get my arse kicked and was graded on this stuff.
I was a pretty good designer. From an original design perspective I would never have spec'd a Kai Go-Foil.
The beauty of Design was that the original
Theory almost NEVER, EVER, EVER, NEVER plays out in the real world. Nature always throws you a curve ball.
This is why every Chuck Yeager has his Jack Ridley https://en.wikipedia.org/wiki/Jack_Ridley_(pilot) (https://en.wikipedia.org/wiki/Jack_Ridley_(pilot)), and Kai Lenny has his Alex Aguera.
I wax on about the theory because to break the rules you have to understand them first.
PS: When you stumble upon something that REALLY doesn't make sense but your theory fits reality you get to name your discovery like Bernoulli. Because it is NOT common sense.
As Feynman said: "If you think you understand quantum mechanics, you don't understand quantum mechanics."
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Here is an example of the Go-Foil on the edge.
Note Keahi turning hard but the foil is still engaged, producing lift and not stalling. The Right Wing Tip appears to be at its limit. The tip appears to be cavitating, on the verge of stall or all of the above.
The Anhedral is keeping the tip in the water, which otherwise would have breached and caused all sorts of chaos.
Note too: When we turn, and turn hard, the speed of the outside tip can be MUCH, MUCH greater than the inside of the turn (Maybe 2X??). Airplanes DO NOT have as great a speed differential, and yet they crash when flying on the verge of stall and turn too hard. We had 2 people die in Half Moon Bay this April turning downwind and not realizing how slow they were flying relative to the column of air.
This may be another advantage that the Go-Foils have over airplane wings and why the thick wing is necessary to keep the inside wing from stalling during turns.
https://youtu.be/3WzVKy-tP8M?t=107 (https://youtu.be/3WzVKy-tP8M?t=107)
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Light Bulb Moment:
"Pumping a Board Left to Right should produce Lift and Forward motion similar to Pumping the Board up and Down."
- Beasho
You heard it here first.
Like a skateboard generating forward thrust from pumping left to right the foil board should be able to achieve the same forward thrust from pumping left to right. The harmonics, or resonant frequency, favors up and down motion but should work left to right if you can turn fast enough.
There is a gravity kick from up/down but the physics are the same.
A Few Quick Searches Later and OMG! :o This guy figured it out. He explains Pumping on a Skateboard and unknowingly explains Pumping on a Surfboard, Accelerating down the line, and explains Pumping on a Foil board. Its all about reducing the Radius of Curvature by pumping your legs.
Ahhhhhhhhhh! Creek and all those Crazy wiggle-waggle pumpers have been vindicated by some Khan Academy YouTube guy with fluorescent chalk.
https://youtu.be/YgUmAwq1WG4?t=299 (https://youtu.be/YgUmAwq1WG4?t=299)
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Beasho, as a guy who translates complex things (computer code) into simple ones that can be sold, I'm going to keep this going and try to lay out some testing concepts here with input from more of the guys who are testing in the real world as well.
If everyone will indulge us and continue to pitch in while also remembering that sometimes what we feel as humans can be affected by a lot of factors including conditions and our physical and mental perceptions.
One of the things I love about surffoils and container is that they didn't worry as much about theory and just built stuff to try. I'm trying to set a jumping off point for my own build/s using not the dogma of the commercial foils, but the engineering wisdom that's magically here (crazy how many creative and smart folks are on this forum) as well as the "let's try it" side as well.
So. For the fuselage foils, I offer these. Copy and correct please, where you see inaccuracies.
1. Flat wing, with a foil might be a place to start. Might suffer from issues turning, but should have good lift and be relatively stable compared to anhedral.
2. Flat wing, no foil, should be tested just for testing to solve arguments and for curiosity. Predicting similar results to above, but we can see. I believe this is the Clearwater design, can someone with a kit confirm?
3. Wing with foil and anhedral, should be less stable but should perform better in turns and waves. Thick profile tapering to ends should help with lift when tilted.
4. Wing with foil and dihedral, should be most stable and have good lift, but there are concerns about tip exposure during turns. (Should this even be tried? Worth the effort?)
5. Downward curved wing with foil but dihedral tips, are we hitting a sweet spot here or over-thinking things into a mess?
I don't think I could survive making 5 wings but I'm curious about this and would like to try to make a few over time or split the load with someone else in easy shipping/swap range. Considering a keel and fuselage that could accept these for testing, nothing crazy but glass, plywood, and carbon so it'd be cheap and relatively fast to build and bolt on to.
Then... we get into the surffoils discs and other alternative shapes that I think could really be cool as well lol. Maybe I won't build a new board this year, just lam up wings all winter? ;)
Hope this inspires thought, feel free to use the numbers and comment and we will aggregate the results.
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And then Beasho tossed another grenade into the pool lol. I love that idea! But man, who's got those chops? (Kai?)
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I'm going to keep this going and try to lay out some testing concepts here with input from more of the guys who are testing in the real world as well.
I agree with everything. How about trying adding the possibility of moving from GREEN to BLUE.
Figure out how we can take off in the Green profile, at low speed, and then retract the flap to shift to Blue profile with lower Coefficient of Lift but with less drag and therefore a HIGHER Speed profile. Effectively a 2 gear foil.
LIFT must stay constant. The foil has to lift ~ 200 lbs of board and rider whether going 10 mph or 20 mph. The only way to modulate this is to Change Angle of Attack. Otherwise lift increases 4X and you fly out of the water. A flap would massively expand the speed envelope.
I want to surf big waves and the Flap may be necessary.
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Here's another one.
Who ever heard of a Gurney Flap?
This could be a cheap way to modify an existing foil for low speed or learning conditions. Just bolt a vertical Fence off the trailing edge.
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this site just got real interesting real quick! what about a flexi foil? super massive camber in the aft sections that loads up and straightens out when you are flying?
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Love how everyone is thinking now this stuf has been going through my mind a lot. Yet to prove the 3D printed/composite wings but think it could be a way way for a group of us to do a lot of testing and prototypes of the same design and then make everything work on a universal Ali mast and fuz. Maybe we set up a group on fusion if a few of you are now playing with it.
I think a form of altitude management similar to the moths is going to be the next big step as the hole breaching thing is one of the largest problems with sustaining flight for beginners at least.
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This could be a cheap way to modify an existing foil for low speed or learning conditions. Just bolt a vertical Fence off the trailing edge.
How about having a hole in the front of the rig that passively diverts a small amount of water through a manifold and out of holes spread across the back of the foil? Not sure if they should be pointed downward or upward for the effect to work. It might be possible design a proportional valve in the manifold that modulated orifice size based on flow rate, and thus at high speeds it effectively "spoils" the lift. I am not sure, but I think it would only take a very small flow rate of liquid to make a pronounced effect on the lift.
I could imagine a totally passive system that is tuned to provide something closer to constant lift across a wider speed range without the need for angle of attack and drag at low speeds.
Maybe this has been demonstrated already and named - I don't have the background to know what to search for.
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This could be a cheap way to modify an existing foil for low speed or learning conditions. Just bolt a vertical Fence off the trailing edge.
How about having a hole in the front of the rig that passively diverts a small amount of water through a manifold and out of holes spread across the back of the foil? Not sure if they should be pointed downward or upward for the effect to work. It might be possible design a proportional valve in the manifold that modulated orifice size based on flow rate, and thus at high speeds it effectively "spoils" the lift. I am not sure, but I think it would only take a very small flow rate of liquid to make a pronounced effect on the lift.
I could imagine a totally passive system that is tuned to provide something closer to constant lift across a wider speed range without the need for angle of attack and drag at low speeds.
Maybe this has been demonstrated already and named - I don't have the background to know what to search for.
Oh you mean like a hoop foil ?
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this is the closest thing I could find to what I was thinking of. But I was thinking of ejecting water through the holes in the trailing edge of the foil rather than air. Maybe it only works with air.
https://www.google.com/patents/US3221698 (https://www.google.com/patents/US3221698)
Here's a different idea for passive height control of a foil. Not what I was thinking, but kind of similar in that it uses a water inlet (pitot tube) on the leading edge for actuation.
http://www.foils.org/taig.pdf (http://www.foils.org/taig.pdf)
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As we are fairly borderline on lift already I think it would be more appropriate to have height management rather than just scrubbing lift off by introduceing air anywhere as not only will you lose lift but allso efficiency and if you have experiaced a foil cavitating on anything it's not a good feeling!
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Oh wow, 'borderline on lift' ?
I see the amazing performance by Kai and all the pro makers and riders to show the opposite.
They're amazing !
They can do full airs , 360's and the downwinding is really flying on a puff of wave chop.
I see the currently available foils as absolute masters of lift.
Can you explain how that's borderline ?
Maybe it's the wave power difference around the world.
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Oh wow, 'borderline on lift' ?
I see the amazing performance by Kai and all the pro makers and riders to show the opposite.
They're amazing !
They can do full airs , 360's and the downwinding is really flying on a puff of wave chop.
I see the currently available foils as absolute masters of lift.
Can you explain how that's borderline ?
Maybe it's the wave power difference around the world.
Not so much on proper waves but for open ocean and for small unbroken waves were I have been foiling the drag/lift ratio is marginal so once your up you really don't want to be eating any energy.
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I was thinking about wave surfing where getting in early might call for good low speed lift, but as the wave develops and speed increases it might be desirable to spoil the lift. It may well be that the best way to do this is varying angle of attack through body position - but it is also fun to think about other solutions.
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I'm with you there Jondrums, the increase in energy from take off to full speed is about 2 seconds due to gravity. Lift on takeoff isn't what you want as you're trying to move forward not lift.
As long as you've got enough height in the wave , gravity pulls you down and forward as the foil dictates. With a steep enough wave that drop,due to gravity, is constant and gives constant lift.
The period of high lift is much longer when you're actually foiling which is what we're after. Low lift and better control are the goals for surf foiling.
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That last line was confusing... :o :P ::)
What I mean, and you can see it in any of the foil videos is that within a sec or two the foil is out of the low lift/speed range on anything more than a minor ripple.
It transitions very quickly to the high riding speed and lift and there fore doesn't need exceptional low speed lift. By reducing the overall lift coefficient then the lift will be more manageable at higher speeds.
Rather than looking to specialise on low speed lift which is rarely encountered on waves, low lift at higher speeds is a better focus and a lot of the foil makers are doing that already with more sweep, lower A/R and moving the foil apex back.