Author Topic: Wing lift or AOA lift?  (Read 11951 times)

surfcowboy

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Wing lift or AOA lift?
« on: July 23, 2017, 09:54:45 AM »
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.

supuk

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Re: Wing lift or AOA lift?
« Reply #1 on: July 23, 2017, 10:15:11 AM »
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 ;)

PonoBill

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Re: Wing lift or AOA lift?
« Reply #2 on: July 23, 2017, 11:37:29 AM »
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.
« Last Edit: July 23, 2017, 11:40:33 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

Dwight (DW)

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Re: Wing lift or AOA lift?
« Reply #3 on: July 23, 2017, 02:04:44 PM »

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.

surfcowboy

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Re: Wing lift or AOA lift?
« Reply #4 on: July 23, 2017, 04:53:48 PM »
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.

oldfartsuperdad

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Re: Wing lift or AOA lift?
« Reply #5 on: July 24, 2017, 08:46:59 AM »
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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PonoBill

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Re: Wing lift or AOA lift?
« Reply #6 on: July 24, 2017, 09:07:40 AM »
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.
« Last Edit: July 24, 2017, 09:14:44 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

oldfartsuperdad

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Re: Wing lift or AOA lift?
« Reply #7 on: July 24, 2017, 11:30:12 AM »
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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surfcowboy

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Re: Wing lift or AOA lift?
« Reply #8 on: July 24, 2017, 11:41:40 AM »
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.

PonoBill

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Re: Wing lift or AOA lift?
« Reply #9 on: July 24, 2017, 11:52:43 AM »
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.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

surfcowboy

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Re: Wing lift or AOA lift?
« Reply #10 on: July 24, 2017, 06:26:08 PM »
I hadn't thought of those but yes, a lot of angle there.

I need to check the lyft stuff.

Sam the Surfer

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Re: Wing lift or AOA lift?
« Reply #11 on: July 25, 2017, 09:48:32 AM »
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

PonoBill

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Re: Wing lift or AOA lift?
« Reply #12 on: July 25, 2017, 05:44:30 PM »
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.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

surfcowboy

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Re: Wing lift or AOA lift?
« Reply #13 on: July 25, 2017, 08:30:18 PM »
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.

Sam the Surfer

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Re: Wing lift or AOA lift?
« Reply #14 on: July 26, 2017, 12:28:33 PM »
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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