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Wing loading as a metric

Posted: Wed May 22, 2019 2:51 pm
by Kamikuza
...for choosing a foil for your weight and style. Thinking out loud.

So for surfing around, catching swell and using small kites, I think a big wing is great and love my 820. Peter Frank thinks it's way too big, and I think I know why he thinks that :D and why others use such smaller kites than me.

Me on a 820 = 88kg/m2 :oops:
120kg guy on a 750 = 83kg/m2
75kg on a 680 = 68kg/m2 (Axis foils)
Peter Frank from KF on his Moses = 65kg/m2 (IIRC PF said he was 65 on a 1000cm2 wing...?)

So PF actually gets the most lift out of his foil, but they're all within a range. It seems to me that 60 to 90kg/m2 is a good range for “surfing” with a kite and foil, allowing for small kites and maximum/good glide and surf.

Now, for the much smaller Axis kite foil, area 630cm2, it’s something like:
Me = 352kg/m2
120kg = 302kg/m2
75kg = 188kg/m2

That seems quite an extreme difference, even though I obviously was able to ride the K-series foil. IIRC I needed the Speed3 21 to match the 10m LEI/820 low end (was a whole lot less fun) and could use the 10m when I got 15 knots or so. Actual low end of the foil was still good, I think to the design of the foil, but the glide was very much less compared to the 820 = constant kite power needed.

Maybe wing loading doesn’t matter, but it seems like wing loading could provide a guide for wing size selection...or maybe I'm overthinking things again :D

Re: Wing loading as a metic

Posted: Wed May 22, 2019 3:02 pm
by tkaraszewski
I think the idea here is correct, and this number is used all the time for aircraft.

Your numbers are wrong though, a square meter is 10,000 square centimeters. Think about how big wing a meter square would be.

That doesn’t change the main idea, though, which works fine.

Re: Wing loading as a metic

Posted: Wed May 22, 2019 4:19 pm
by windmaker
Principe is correct but numbers are completely wrong:

Wing loading= mass divided by area.

Re: Wing loading as a metic

Posted: Wed May 22, 2019 4:47 pm
by tegirinenashi
When measuring hydrofoil wing pressure, g/cm^2 seems to be the most convenient units.

75kg rider over 1000 cm^2 wing amounts to 75 g/cm^2.

Re: Wing loading as a metic

Posted: Wed May 22, 2019 8:55 pm
by plummet
Interesting thread. I agree g/cm3 could be more relevant.

Me 80000/550=145gm
Kami 107000/820= 130gm

Post up your pie loading so we can get a data pool of results.

Re: Wing loading as a metic

Posted: Wed May 22, 2019 9:28 pm
by GregK
I'm 170 lbs on 170 sq. inch front wing so 77, 273 grams / 1, 097 cm2 = 70 g/cm2

Is lift from the stabilizer negligible ? Isn't most foilboard riding done at a large enough angle of attack ( say 5 to 10 degrees positive or up ) that the stab is generating lift as well ? Albeit its lift force is off-set slightly negative compared to the front wing so that it generates a pitch-up moment on the foilboard system ...

Re: Wing loading as a metic

Posted: Wed May 22, 2019 9:38 pm
by BWD
the stab i think still makes a downward force as it operates in waterflow that has been curved down by the main foil.
Some commentary of those who have ridden stabless reflects this, as they seem to have more lift with no stab, not less, I think.

Re: Wing loading as a metic

Posted: Wed May 22, 2019 11:16 pm
by Peter_Frank
BWD is correct, the stab decreases the overall lift a tad on normal foils.

Kami, you are correct, "wing loading" is essential.

The numbers on me are wrong though, but doesnt matter for the principle really.

I am 78 kg (meaning average weight which is 75 to 80 kg), and ride wings from 520 to 1200 cm2, and dont have any Moses foils.

Wing loading is essential if you are foilsurfing.

When kitefoiling it is partly essential yes, you are right.

But only partly and not proportional, because when heavier you use a bigger kite, thus not needed a similar bigger sized wing.

It is the same as for twintips, if you weigh 50 % more, you use a bigger board, but not 50 % bigger in area, as your kite lifts more.
Also, there is a tradeoff in terms of agility.

Meaning, at some point the heavier riders prefer a tad smaller board than the "relative" difference, for being able to toss it around fast and fun.

So yes, the "wing loading" is essential for which size used, apart from personal preferences.

I like that the wing is low drag and I can foil fast without any kite power.

Others ride with bigger wings and are more powered, thus they use the kite to ride faster.

Two very different styles... I like that the wing itself rides fast, without having any kitepull.

My wing loading for the "overall" wing 790 cm2 is around 100 g/cm2.

For my big surf wing it is 65 g/cm2.

And this is the size I feel can become boring, it you dont have any waves.

8) PF

Re: Wing loading as a metic

Posted: Thu May 23, 2019 12:02 am
by GregK
BWD wrote:
Wed May 22, 2019 9:38 pm
... it operates in waterflow that has been curved down by the main foil.
Please explain what causes the water to flow down ? I would have thought that the water under and very close to the front wing is at higher pressure than the water on top and near the wing surface, so after the front wing passes, there would be an upward force acting on the water.

And how much is the water moving ? Isn't it relatively stagnate and it's the hydrofoil that is moving ?

Re: Wing loading as a metric

Posted: Thu May 23, 2019 1:14 am
by BWD
The foil moves through the water, yes.
Moving water down is what gives the reaction force called lift that holds the rider up. Often you can see a disturbance in the water level behind a rider of the foil is near the surface, like a boat wake.
Check some videos of foils riding in shallow water over sand, you can see the wake of the foil kicking up the sand on the bottom....