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Foil Kites - 4 knots ?

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:28 pm

approximations only...

Sheet Out
SheetOut.jpg
Sheet In
SheetedIn.jpg
http://www.powerkiteforum.com/viewthrea ... #pid260574
Last edited by PugetSoundKiter on Thu Aug 25, 2016 9:35 pm, edited 1 time in total.

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:30 pm

The two attached graphs (sheeted-in & sheeted-out) illustrating the pulling force of a kite in varying airspeeds. This is based on simple calculations using the lift coefficient equation and is intended to show a few conceptual relationships only. Estimations/Inaccuracies include:
- Kite is not an ideal foil-bearing body
- Kite planform area is not uniform
- Kite chord varies and is flexible
- Inefficiencies unaccounted for
- Lift Coefficient values vary
- Air Density values vary
- And many more…

The illustrations have a color overlay indicating a notional limited usable range. There are calculated curves for 5, 10, and 15 square meter planform/projected area kites. This is different than the typical windrange-vs-kite size correlations. Hopefully this helps to visualize the magnitude of force at various airspeeds, using different size kites. What is not included is the direction of the force (vector), which helps determine how useable the force will be.

Airspeed is calculated as a function of force in the illustrations. Beach windspeed (true wind velocity) is easy to visualize with a steady kite as it is equal to the kite airspeed (apparent wind velocity). As soon as the kite moves, the apparent wind velocity changes. The kite can be moved by steering (sine-ing) and/or being set in motion from sailing. Y-axis airspeed can be visualized using three different perspectives:
(1) As beach/true wind velocity (for forces seen static flying)
(2) As kite/apparent wind velocity (for total forces seen at the kite)
(3) As sailing and/or sin-ing wind velocities (for forces the rider controls)

One of the graphs illustrates the minimum calculated forces for a kite sheeted-out (using CL=0.75) and the other graph illustrates the maximum calculated forces for a kite sheeted-in (using CL=1.7). The two graphs show different usable kite windranges depending on the sheeting of the kite. Again the specific graph numbers may not be accurate to actual kite performance but, this does provide a general idea of the range a depower kite could provide. Many other pro/con variables influence actual airspeed-vs-force curves. Kite shape, rigidity, weight, line lengths, etc… are ignored for mathematical simplicity. The above concepts are all well known, but sometime drawing a picture helps.

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:32 pm

Temperature
Attached (click on it to see full size) is a temperature plot for the 10m planform area wing. The blue (lower=less wind needed for pull) lines are for air density for very cold dry air (23°F or -5°C = 1.13Kg/m³ @ Sea Level) and the red (upper=more wind needed for pull) line represents very hot dry air (105°F or 40.5°C = 1.32Kg/m³ @ Sea Level). The biggest difference is on the Sheeted-Out plot because lift efficiency is low so, more wind is needed to make up the difference in air density. Some numbers are:
50lbs of pull sheeted-out needs 1.7mph more wind for hot air (sheeted-in needs 0.8mph).
100lbs of pull sheeted-out needs 2.4mph more wind for hot air (sheeted-in needs 1.1mph).
150lbs of pull sheeted-out needs 2.9mph more wind for hot air (sheeted-in needs 1.4mph).
200lbs of pull sheeted-out needs 3.4mph more wind for hot air (sheeted-in needs 1.6mph).
250lbs of pull sheeted-out needs 3.8mph more wind for hot air (sheeted-in needs 1.8mph).
Airspeed-vs-Pull-Temperature.jpg

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:33 pm

Altitude
Attached (click on it to see full size) is an altitude plot for the 10m planform area wing. The green (lower) lines are for air density for sea level dry air (1.2Kg/m³ @68°F/20°C) and the purple (upper) line represents dry mountain air at 10,000ft/3048m (0.826Kg/m³ @ 68°F/20°C). To show only the altitude differences, the temperatures are held same but, note that would be a warm day that far up a mountain. Some numbers are:
50lbs of pull sheeted-out needs 2.6mph more wind for mountain air (sheeted-in needs 2.1mph).
100lbs of pull sheeted-out needs 3.7mph more wind for mountain air (sheeted-in needs 3.0mph).
150lbs of pull sheeted-out needs 4.5mph more wind for mountain air (sheeted-in needs 3.7mph).
200lbs of pull sheeted-out needs 5.2mph more wind for mountain air (sheeted-in needs 4.2mph).
250lbs of pull sheeted-out needs 5.9mph more wind for mountain air (sheeted-in needs 4.8mph).
Airspeed-vs-Pull-Altitude.jpg

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:34 pm

Humidity
No plots needed for this one. Even for worst case (high temperature) Air Density only varies 0.3% between the full range of 0% and 100% humidity. Dryer air is denser (better for lift) than humid air, because water has less molecular mass than air. If it is raining, your kite will get heavier, and you will lose pull, otherwise humidity is really a non-factor.

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PugetSoundKiter
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Re: Foil Kites - 4 knots ?

Postby PugetSoundKiter » Thu Aug 25, 2016 9:34 pm

Wind Gusts
Attached (click on it to see full size) is a plot for gusts-vs-pull assuming 68°F/20°C dry sea level air at 60lbs of steady lift (17mph wind sheeted-out or 11.5mph sheeted-in). For simplicity it is assumed the wind gusts are in the same direction as the apparent wind seen by the kite (worst case). Actual gusts are generally in the true wind direction. The brown (upper) line represents when sheeted-out what gust speeds cause what additional pull. The pink (lower) line represents when sheeted-in what gust speeds cause what additional pull. This plot shows why it can be much more challenging to kite in gusty conditions. And, the lulls that accompany gusts add even more to the challenge (by reducing pull). This plot also illustrates that the effects of gusts/lulls are quite a bit more pronounced when sheeted-in. Some numbers are:
Sheeted-In
4mph gusts while sheeted-in sailing in 17mph winds adds 50lbs of pull
7mph gusts while sheeted-in sailing in 17mph winds adds 100lbs of pull
10mph gusts while sheeted-in sailing in 17mph winds adds 150lbs of pull
12mph gusts while sheeted-in sailing in 17mph winds adds 200lbs of pull
Sheeted-Out
6mph gusts while sheeted-out sailing in 17mph winds adds 50lbs of pull
11mph gusts while sheeted-out sailing in 17mph winds adds 100lbs of pull
15mph gusts while sheeted-out sailing in 17mph winds adds 150lbs of pull
19mph gusts while sheeted-out sailing in 17mph winds adds 200lbs of pull
Airspeed-vs-Pull-Gusts.jpg

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Re: Foil Kites - 4 knots ?

Postby foilholio » Fri Aug 26, 2016 12:00 am

It's a complicated problem. The AoA and kite speed for maximum lift will change with windspeed. The position in the window is a factor too. Also lower air density = less drag, so an increased speed may counter some of the effect.


ronnie wrote:Alex Aguera, who was a race director for years with the PWA, which had an 11 knots minimum speed requirement for racing.
"race director" lol does that make him an authority on the subject? It's the same old tired argument. I hear it all the time especially about humidity. Science says humidty does near nothing. My own observations are air temperature and humidity do very very little, almost not worth mentioning. I think some peoples brains get left behind when they go to exotic places.
Regis-de-giens wrote: It shows that if your windmeter can indicate 3.5 m/s at 2m elevation from the ground, you can actually have 5 m/s at the 20m elevation reached by the kite = you believe you ride in 7 knots while your kite sees 10 knots
I have observed this with wind meters, 7knot on meter 10knots on station. It varies though and the gradient usually becomes less with less wind. I almost always base my wind readings on stations.
Regis-de-giens wrote: Ps to foilholio: the fact that we can rider with more speed than the wind is not directly linked to this Force / energy discussion
I think it is. This is why lower drag kites can produce more pull by being flown than higher ones. The kitespeed and windspeed are two related but different things.
Regis-de-giens wrote:For example there is no "wall effect" or threshold or any thing related when you reach or go beyond the wind speed
You lost me with that.

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Re: Foil Kites - 4 knots ?

Postby Regis-de-giens » Fri Aug 26, 2016 9:47 am

Interesting graphs, thanks . The last one is not clear to me ;

What I miss is some examples of the shape of the Buoudary layer on the ocean in the case of a thermic wind toward coast, to understand how much we underestimate the measure of the wind compared to 20 m altitude.
foilholio wrote:
Regis-de-giens wrote: Ps to foilholio: the fact that we can rider with more speed than the wind is not directly linked to this Force / energy discussion
I think it is. This is why lower drag kites can produce more pull by being flown than higher ones. The kitespeed and windspeed are two related but different things.
Regis-de-giens wrote:For example there is no "wall effect" or threshold or any thing related when you reach or go beyond the wind speed
You lost me with that.
What i mean is that the speed of the rider and the speed of the wind have no direct relation or threshold in equations, appart from the obvious statement than the higher wind the higher speed (all other things remaining equal).

As you know with a poor gliding board like bodydrag, you will go very slow . With a medium drag like TT you can go approx to the wind speed , with a foiboard you can double or triple this speed ( with same wind), and with a race buggy on magnetic rails you could go even faster with still the same wind; for given conditions and material, final speed is just the result of the board drag ( to summeaize, details hereafter). That is what I me an by there is no threshold near the true wind speed, no physical change or equation that prevents you to go beyond it, and you can not deduce speed from the kite energy or lift equation alone.

To understand what happens, you need to think about a park and ride movement (no turning of the kite), and decouple the problem in two separated issues :
1- the pulling force, which is the vectorial summ of (kite lift + drag of the kite and part of the lines) ; it depends mainly on wind+rider speed , air mass, kite size (projected area) , kite L/D ratio.
2- the resisting force, which is the summ of the drag ( board + rider + part of the lines); it depends mainly on rider speed, water mass, weight and angle of the rider vs horizontal, board size, board L/D ratio.
Both should be equal, which imposes the rider speed as a result of solving the equation, so speed is increasing with L/D of the board with no limit.

I said no limit, but to be completely accurate, there is one. With some geometrical construction you could also deduce that there is actually a speed that will never been reached whatever the board efficiency : it is Vmax = (L/D ratio of the kite) * wind speed (true speed, on the beach). For example in 4 knots of wind with the current race foil kite designs, you will never go beyond 60 knots ... (ok we have some margin :lol: )

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Re: Foil Kites - 4 knots ?

Postby foilholio » Fri Aug 26, 2016 1:42 pm

I wasn't talking about the speed of the rider although that is a factor. I was talking about the kites speed, that is certainly related to wind speed. A kite can fly faster than the wind, much faster at a guess maybe 5+ times the wind speed. There is a limit and each kite is different. Kites with the lowest drag would have the highest speed I guess, but what makes the most power? I would guess the highest lift. Which would be at a higher speed than the wind.

Again I am talking about the kites limit. So if it was placed on long lines what is the top speed it can achieve flying through the window? I have done this with long lines and there is a limit. If you look at the equations for calculating theoretical max power of the wind for wind mills they cube the velocity of the wind . I think this is based on the blades being able to fly through a large area at higher speeds. Vs a calculation that squares the velocity to work out lift the cube function looks more accurate for kites. It explains to me why lower drag kites flown well in a sine can produce so much more power than fat crap like that twitspert loves and other shitty designers.

I found this great article through the paraglider forum http://para2000.org/book/miscellaneous/63400.html. I think it's applicable to this and to other discussions like on maximum upwind etc.
Last edited by foilholio on Tue Aug 30, 2016 4:38 am, edited 1 time in total.

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Re: Foil Kites - 4 knots ?

Postby Regis-de-giens » Fri Aug 26, 2016 3:30 pm

Please kindly refrain to transfer your personnal animosities here :remybussi:

For a park-and-ride configuration, kite speed and rider speed is the same :wink: , hence my segregation between motor force (kite) and resisting force (kiter) in our low wind ride case. (*) OK; Now if you speak of a kite crossing the wind window, i.e. not quasi-static (i.e. kite moving not park and ride), the result will interest you .... it is quite the same ! if the kiter is static on the beach , the speed of the kite when crossing the window is ... vmax = L/D ratio * wind speed, which is by the way one method to measure the L/D ratio. So your 5+ is reasonnable for a standard tube kite, while a foil race kite could even reach 10+.

With long lines, top speed of the kite is almost the same, just need to add the drag of line extensions (and the fact that line extensions will tension a bit more the rear lines, so bar should be a bit more sheeted-out with the extensions to get the same AoA for both tests.

Now regarding the use of square or cubic speed in the equations it is quite simple : lift is proportionnal to (speed)^2 ; lift is a force ; if you want to know the power, power is always calculated as Force * speed , so leading to (speed)^3. Using forces (square of speed) in equations will help you resolving the pull and speed equilibrium you can have during a ride; power in itself does not bring a lot in discussions, except if you integrate along the whole waterstart phase, then you get the total energy (and back to square of wind speed). kite Energy could then be compared to the energy required to get out of water (i.e. lift 70 kg up to 1 meter high) and accelerate at a certain planning speed (say 5 knots) , so useful only for transient phases , like waterstart limits in low winds.

(*) I need to make a clarification for full accuracy: speed of the kite is the same as rider toward a fixed point ; but for the lift calculation you need to add the true wind speed before using the square function, as the wind speed the kite sees is higher than kite speed toward a fixed point.


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