40 years ago I remember a pilot telling me he wasn't worried about the turbulence in bushfire plumes upsetting the vertical hold of his IR scanner because " The F27 has a high wing loading".

en.wikipedia.org/wiki/Wing_loading

"Effect on stability Wing loading also affects gust response, the degree to which the aircraft is affected by turbulence and variations in air density. A small wing has less area on which a gust can act, both of which serve to smooth the ride. For high-speed, low-level flight (such as a fast low-level bombing run in an attack aircraft), a small, thin, highly loaded wing is preferable: aircraft with a low wing loading are often subject to a rough, punishing ride in this flight regime."

I'm sure my top speed is limited by control and not drag. (I know, more TOW and less theory).

The lift equation is
L = 1/2 d V^2 Area times Lift Coefficient.
d = density of water = 1000 kg/ cubic metre
V = velocity let's use 10m/sec about 20 knots
Area = 0.1 for a 1000sq cm wing
Lift = mg = 980 newtons for a 100kg rider and gear.

Plug all those in the lift equation and solve for the Lift coefficient. I get Cl = 0.2
Here's the Cl vs angle of attack curve for a typical asymmetric foil that you'll find everywhere on the internet. (the graph that is)


For Cl = 0.2 it looks like the angle of attack of our 1000 sq cm wing at 20 knots is minus 3 degrees!
(Never mind we'll go with that, all angles are relative)
Now if you get a perturbation of 1 degree on this -3 degree angle of attack, to -2 degrees, ( you might pass through an eddy under the wave, a gust might hit the sail and dip your balance, you might loose concentration,) then looking at the graph the lift coefficient has jumped from the initial 0.2 to 0.35 our 100 kg of lift has turned into 175 kg of lift. Better shift weight forward very quickly.
If you do the same calculation for a 500 sq cm foil you'll get CL = 0.4. From the graph you can see the smaller foil is running at a - 1.3 degree angle of attack. If you hit the same hypothetical eddy below the swell there will be that same 1 degree change in angle of attack ie. now at - 0.3 degrees ( we did the calculation for the same speed remember). That will increase the Cl from 0.4 to 0.5. Which proportionally increases lift from 100kg to 125 kg. A lot easier to correct for an extra 25kg of lift than an extra 75 kg.

Sorry about the arithmetic. I just needed to do the sums to convince myself that a smaller wing is easier in gustier, windier and choppier conditions even though you may not be going any faster.

Of course if you let your smaller wing run faster you'll eventually get to the same point.

Is your top speed limited by drag or control?

Great foilers have their speed controlled by drag.
Bad foilers......me....are limited by lack of skill and control.

Definitely control for me. Definitely drag for Bruce Peterson.

I didn't follow any of that text, but going off the title, its mostly drag. But I do find that when my sail is not rigged optimally, I find that I cannot fully sheet in and probably lose a knot

Definitely control for me.

The graph is quite interesting, but would have to be looked at together with drag as a function of AoA. The article at www.ripublication.com/ijaer17/ijaerv12n21_90.pdf gives a quite different curve:
The lift at 0 degrees is much closer to 0, which makes more sense to me than the Wikipedia curve for a cambered airfoil. A CL of 0.2 is at an angle between 5 and 10 degrees, and a 1 degree change as in your example has less of an effect.
For a given amount of forward force from the sail to compensate for the drag, a smaller foil will be faster. With regards to turbulence in the water, you need to look at the "apparent water angle", the vector sum of foil speed and water movement. The faster you go, the lower the change in angles will be, so the small foil reacts less to the water state. That's definitely the case if you actually go faster on it. But maybe it's even true if you foil at the same speed, but a higher AoA, because you are higher up on the CL curve, so the relative change in CL from a given AoA change is smaller.

I'm limited by fear
I find this good for my health

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boardsurfr said..
Definitely control for me.

The graph is quite interesting, but would have to be looked at together with drag as a function of AoA. The article at www.ripublication.com/ijaer17/ijaerv12n21_90.pdf gives a quite different curve:
The lift at 0 degrees is much closer to 0, which makes more sense to me than the Wikipedia curve for a cambered airfoil. A CL of 0.2 is at an angle between 5 and 10 degrees, and a 1 degree change as in your example has less of an effect.
For a given amount of forward force from the sail to compensate for the drag, a smaller foil will be faster. With regards to turbulence in the water, you need to look at the "apparent water angle", the vector sum of foil speed and water movement. The faster you go, the lower the change in angles will be, so the small foil reacts less to the water state. That's definitely the case if you actually go faster on it. But maybe it's even true if you foil at the same speed, but a higher AoA, because you are higher up on the CL curve, so the relative change in CL from a given AoA change is smaller.

I dunno? I wondered about the negative AoA on the curve I had found but yours I'd say is even more puzzling. An 8 degree angle of attack on a 1000sq cm at 20 knots!

Maybe I've misapplied the lift equation? But a Cl of 0.2 at 20 knots seems about right. Using the same equation to hold up 100kg at 10 knots you'd need a Cl of 0.8 which is +3 degrees on the curve that I dredged up. Even 3 degrees is fairly noticable. Do you see about that much nose up when a 1000 sqcm foiler slows to 10 knots?

actually my top speed is limited by fear !

small balls!

Drag for me, I keep stuck around the 30 knot mark with my 900cm2 foil. Ofcourse I do crash sometimes (raketrim is so important at these high speeds to make occasional touchdowns soft and bouncy!!), but all my runs are around that mark. I'm eager to try a smaller (kiteracing) stab, currently I use a 300cm2 freeride stab, I can go down to 220cm2, I think that will make quite the difference. I have also requested a wing with an even slightly thinner profile and reduced washout, for I feel there is too much washout for the 30+ knot range in the current design (focussed for the 22-28 knot freeride/freerace speedrange).

I dont feel much for going smaller than 900, I would need to rig bigger (I set all my speedruns with 4.9 wave), and my foil is a freeride/freerace model so very comfortable and fun to jibe, jump & 360 aswell. On the same kit I can do 30 knots, but also practice my backloops in the same session, wouldnt want it another way so I'm gonna see how much I can tweak the setup to get some more out of it, but not switching to a slalom/speed type foil.

I haven't found the limit of my speed yet so I cannot answer.

For me, it's the sense of survival.
After 5 big crashes going for top speed trying to stay with a guy on a 600 sq cm foil...and 30 years my junior, I joined him with a Naish 600 foil.
Unfortunately, now ingrained in my head.....fear. Now, hooked in full speed launches really didn't hurt, and nothing compared to windsurfing...or roadracing/motocross crashes, but years of crashing have added up to timidity and caution.

Yeah, that explains my experience with the AFS F800 wing, at higher wind speeds (16+ knots) when a gust hits me it wants to lift a lot, going to the F700 wing was amazing, much easier to control when a gust hits me in-flight, and felt like I was foiling through air versus thick water with the F800 in 18+. Of course at lower speeds (16 or less) the F800 is perfect, guess all wings have a range just like sails.

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Ian K said..
I dunno? I wondered about the negative AoA on the curve I had found but yours I'd say is even more puzzling. An 8 degree angle of attack on a 1000sq cm at 20 knots!

None of the curves you and I posted were from foils actually used for windfoiling. The ones I posted are from shapes that are almost symmetrical, so they have almost no lift at 0 AoA. An asymmetrical foil will generate lift at 0 AoA from the water speed / pressure differences at the bottom and top. That also means that it must generate some lift at small negative angle of attack. That non-intuitive, so I missed that looking at your curve. Freeride wings like the Slingshot Infinities have a lot more thickness and asymmetry than the foils used in the figures I posted, so they should have much higher lift coefficients.

From what I have found, foils made for surfing have an angle of incidence (AoI) of about 2 degrees; the angle of attack would be roughly the same. Windfoiling is faster, which would allow a lower AoI and AoA. For my SS infinity 84, is seems to be close to 0 degrees (although I don't have the tools to measure that accurately). The stabilizer has a negative angle of attack, but seems quite symmetrical, so it probably has negative lift (which moves the center of lift forward).

It would be interesting to see measured lift and drag curves for windfoil wings.

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WhiteofHeart said..
Drag for me, I keep stuck around the 30 knot mark with my 900cm2 foil. Ofcourse I do crash sometimes (raketrim is so important at these high speeds to make occasional touchdowns soft and bouncy!!), but all my runs are around that mark. I'm eager to try a smaller (kiteracing) stab, currently I use a 300cm2 freeride stab, I can go down to 220cm2, I think that will make quite the difference. I have also requested a wing with an even slightly thinner profile and reduced washout, for I feel there is too much washout for the 30+ knot range in the current design (focussed for the 22-28 knot freeride/freerace speedrange).

I dont feel much for going smaller than 900, I would need to rig bigger (I set all my speedruns with 4.9 wave), and my foil is a freeride/freerace model so very comfortable and fun to jibe, jump & 360 aswell. On the same kit I can do 30 knots, but also practice my backloops in the same session, wouldnt want it another way so I'm gonna see how much I can tweak the setup to get some more out of it, but not switching to a slalom/speed type foil.

I think you can be happy enough if you have the ability to take the 900 wing to 30kts, a one wing kit keeps it simple

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boardsurfr said..
With regards to turbulence in the water, you need to look at the "apparent water angle", the vector sum of foil speed and water movement. The faster you go, the lower the change in angles will be, so the small foil reacts less to the water state.

Yes, for a given amount of turbulence the "apparent water angle" varies as the linear inverse of speed. (Sines and tans are roughly linear for small angles). The unwanted lift as you pass through turbulence is the ratio of the steady speed angle of attack and the "apparent" component of the angle of attack. The steady speed angle of attack also comes down but with the inverse square of that steady speed. ( The Cl curve is fairly straight in the range of operation ). The square of speed term will win so I think ( having never actually been that fast on a foil), that for a given amount of water turbulence, a fixed size of foil and rider weight, it will always get sketchier the faster you go.

For me, definitely an issue of control and lack of guts/madness.
AFS state a top speed of more than 24 knots on the F800 wing. Had only two sessions on it, but my top of 19.8 knots didn't feel comfortable to me.
For the F700 they claim more than 26 knots to be possible and I've tiptoed myself to a top speed of 21.5 knots. This as well felt like totally on the edge to me, with the numbers indicating the respective foils not anywhere near the point where drag would limit speed.

Sail control wouldn't play a big role here, either, as these speeds mean a relaxed cruise even for my wave and freeride sails.
If I were to make an educated guess, I'd say up to about 25 knots on freeride gear are no matter of drag. Get dedicated race stuff and about 30 knots should be a no-brainer with the right skills. Above these marks, gear set-up would come into play a lot more.
For quick speed progression of a fearless ex-pro on dedicated equipment, watch our local hero, Andy Laufer:

www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjusY2OiYLuAhUNdxoKHeDlAe0QwqsBMAF6BAgOEAg&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DqCr-xbdSd9Y&usg=AOvVaw3XB1xt0Nrgi_UwjAEQPxBK

www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjusY2OiYLuAhUNdxoKHeDlAe0QwqsBMAJ6BAgOEA0&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DbVGnnlkb7FM&usg=AOvVaw2JNZKFG_Rmvh6V66QRaB6x

www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjusY2OiYLuAhUNdxoKHeDlAe0QwqsBMAN6BAgOEBI&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DtBHxGEUvJms&usg=AOvVaw3VEIeKQl7vlgC1qdaVMr2Z

Ok, he really knows how to go fast on windsurf gear, but this shows that for most of us, drag is not the question.

I suspect that my freeride foil combination (1100 SB Freeride/330 rear/95+ fuse) is the limit at the moment. My top speed is 22 knots but only for a brief moment (5 seconds or so). 20 knots when the water isn't choppy is my average high speed and I can get there somewhat comfortably.
Because this setup is so comfortable I have not used the 800 front wing/255 rear I have much but I'm sure it would give me a couple knots more.
Ultimately FEAR will be the limitation for me. I don't see myself at 30 knots anytime soon - 24 or 25 maybe.
I know the post is about reaching for top speed but tbh I'm quite happy flying about at 15-20 knots.

yo viendo sus velocidades maximas quedo sorprendido!! apenas llego a 20kn!!! ala de 1000 SB race! y navegando al travez!!! que generalmente tiene una ola a favor de aproximadamente de 70cm a 1 metro! En que condiciones de agua logran velocidades de 25 y mas? Gracias