## Foil Kites - 4 knots ?

For all foil kite riders

Peter_Frank
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### Re: Foil Kites - 4 knots ?

Kamikuza wrote:
plummet wrote:What is the actual increase/decrease in kinetic energy from changing temperatures?

First up we have to use the ideal gas law to calculate the mass of the air

http://www.engineeringtoolbox.com/humid ... d_677.html

Lets use a 0 and 30 Degrees C for calculations.
0 deg c = 1.29kg/m2
30 Deg c= 1.17kg/m2

Then we throw that into the kinetic energy formula

https://en.wikipedia.org/wiki/Kinetic_energy

1/2 mass x velocity squared.

Lets use 4 knots for wind speed.

The increase in kinetic energy dropping from to 30 degrees to 0 Degrees = 11%.
That doesn't sound like much. But it could be significant in ultralight winds.

What is the increase in kinetic energy going from 4 to 5 knots or wind speed all other aspects being equal?
55% increase.

Wind speed increase is far more significant that temperature variations.
Wrong equation. http://large.stanford.edu/courses/2010/ph240/sleiter2/

Power in the wind = 1/2pv^3

Maybe air density doesn't matter so much after all . . . But for sure, there is a big difference summer and not summer. Definitely less consistent, recently.
Nope, Plummet is right

It can be easy to confuse Work and Power and Energy and Force.

But the power is not relevant, only if you have a wind turbine.

It is the total lift (or linepull/force) from the kite, which is proportional to the kinetic energy in an amount of air, so it is in fact proportional thus the same (like Plummet writes) as the lift/pull from a kite, so it is

Lift = area * liftcoefficient * airdensity/2 * airspeed^2 = area * liftcoefficient * kineticenergyoftheair.

Where the kinetic energy of the air is the same as the dynamic pressure thus = airdensity/2 * airspeed^2.

So Plummets calculation is correct, and air temperature has a very minor effect, compared to the actual windspeed
Which you also state Kami, but it is only squared to the windspeed and not the power of 3 like power.

But it CAN be felt slightly, eventhough not much difference from 25 to 35 degree celcius....

Peter

PullStrings
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### Re: Foil Kites - 4 knots ?

Peter_Frank wrote: Plummets calculation is correct, and air temperature has a very minor effect, compared to the actual windspeed
Strongly disagree ( sorry if i stole your line )

Air temp versus water temp is what makes more power...same power...less power...with the same windspeed

Warm light hot wind over cool cold water make the wind the densest and therefore the most powerful...and that's when you will get extraordinary claims

Warm light wind over warm water gives you a wind that is basically neutral and therefore the most accurate when reporting low wind claims...identical with cold cool light wind over cold cool water

Cold cool light wind over hot warm water tend to flow lower / closer to the water therefore at kite level there is lot less juice...often referred as sucker light wind..might as well pack it up

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### Re: Foil Kites - 4 knots ?

PullStrings wrote: Warm light hot wind over cool cold water make the wind the densest and therefore the most powerful...and that's when you will get extraordinary claims
I wonder what impact evaporation has on this.

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### Re: Foil Kites - 4 knots ?

PullStrings wrote:
Peter_Frank wrote: Plummets calculation is correct, and air temperature has a very minor effect, compared to the actual windspeed
Strongly disagree ( sorry if i stole your line )

Air temp versus water temp is what makes more power...same power...less power...with the same windspeed

Warm light hot wind over cool cold water make the wind the densest and therefore the most powerful...and that's when you will get extraordinary claims

Warm light wind over warm water gives you a wind that is basically neutral and therefore the most accurate when reporting low wind claims...identical with cold cool light wind over cold cool water

Cold cool light wind over hot warm water tend to flow lower / closer to the water therefore at kite level there is lot less juice...often referred as sucker light wind..might as well pack it up
Hmmmm. Your assertions are interesting. But The air density will not change unless there is a change of temperature, pressure or composition. So ocean temperature will have zero effect on the air density unless it changes one of the above 3 factors.

So what other effects are at play? perhaps there is a wind gradient when you have a cold sea and warm air? So the kite sees more wind than what is measured on the wind meter at ground level.

Cold water/warm wind may coincide with high barometric pressure giving slight increase in air density?.

What we haven't discussed the effects of humidity. Changes in humuidity change the composition of the air (slightly) by increasing/decreasing water vapor in the air. As water vapor (h20) is lighter than air the higher the humidity the lower the air density. But by how much? i haven't done the maths to calculate that one.

Perhaps a cold sea, warm air combo coincides with high barometric pressure and dry low humidity air to combine to give a slightly increase density????

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### Re: Foil Kites - 4 knots ?

every 10 degrees difference in c (or about 20F) gives you something like 3% more dense air. Depends on the temperature but when it's 100% humidity at most you have something like 2% of the air H2O, compared to bone dry. You can assume that a N2 molecule weighs twice as much as H2O (ignore the H2 it doesn't weigh much and O weighs about as much as N), so going from dead bone dry at 75F/21C go completely saturated 100% humid only decreases the power of the air by 1% but typically the difference will be fractions of a percent. And of course there is a huge difference when you change altitude 1 mile up you have 20% less dense air. When people talk about feeling how "dense the air is" most of the time I find it's because of a wind gradient not the humidity or temperature. You see them having trouble relaunching but once the kite is in the air, it's going gangbusters. It makes you feel as if the air is extra thick that day. They can see the air isn't going that fast on the ground, but then it's in the air and pulls so strong, how can it be? One explanation is that the air is like molasses that could explain why it feels so light on the ground but it pulls so hard. The other (and more reasonable explanation) is a wind gradient. Cold water and hot air is a good recipe for a steep wind gradient and makes for a great day kiting.

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### Re: Foil Kites - 4 knots ?

Gradients are the norm on land or close to it, but they are rare on the ocean. Still though I have seen some crazy gradients on open ocean 10knots head level with 15knots at 11oclock and 20knots at 12oclock. Makes riding with the kite directly overhead possible! In higher winds 40+knots 10to 15knots difference is actually pretty common. The conundrum in such strong winds is the kite has much less pull near the water, but body weight can control it better with it overhead.

Kamikuza
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### Re: Foil Kites - 4 knots ?

Peter_Frank wrote:But the power is not relevant, only if you have a wind turbine.
"Kite-Powered Generators"

The question was power in the wind...

foilholio
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### Re: Foil Kites - 4 knots ?

I don't think that is directly applicable to kites, but it is interesting how you would calculate it for a kite. It is obviously greater than just a square of the wind speed because a kite can fly faster than the wind.

ronnie
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### Re: Foil Kites - 4 knots ?

Alex Aguera, who was a race director for years with the PWA, which had an 11 knots minimum speed requirement for racing.

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### Re: Foil Kites - 4 knots ?

My two cents ...

Just as a (re)introduction, wind meters have their own and significant measure errors that prevent from a serious comparision between 4 and 5 knots masured by different people and windmeters; when one of my windmeter indicates 4 knot, the other gives 5 + ... that should explain a major part of people "fighting" on wind actual speed in very light wind.

I agree and it is quite well-known that the pull provided by the kite is proportional to the density on one side , and to the square of the speed on the other side; hence you can see that at 5 knots, an error of -0.5 knots on the speed measurement (-10%) will lead to a decrease of almost 20% of the lift of the kite (0.9*0.9=0.81).

Temperature : the variation of the temperature of the dry air from -10 degree to +35 degree is 14%. Significant but in the second order, corresponding to the impact on only 0.3 knot wind speed difference (and on top I am not sure a lot of you ride on the water at both -10 degree and +35 degrees
temperature vs ro.jpg (51.9 KiB) Viewed 608 times
Humidity of the air has even less impact, volumetric mass varying maximum 2% for a given temperature, corresponding to +/- 0.05 knot ...
So, measurement error is dominant compared to temnperature (while humidity is negligeable)
temperature comes at the second order compared to measurement error ...

When you speak about gradient that should actually be another major key : due to the boundary layer effect (speed of wind is zero along the sea surface), the actual sped of the wind is higher at higher altitude ; not a lot of data available on this in the case of a thermical wind of 6/10 knots on the sea, but I found this example:
art088_figure3_Profil-vertical-vitesse-vent-terrain-plat-fonction-rugosite-site.jpg (32.8 KiB) Viewed 608 times
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... one obvious sign of this is your light wind foil kite being at its flying limit of low wind but still lines tensioned, and when he backstalls up to the sea surface, there is even not enough wind to keep line tensionned... end of session.

So, measurement error is dominant compared to temperature (while humidity is negligeable)

Ps to foilholio: the fact that we can rider with more speed than the wind is not directly linked to this Force / energy discussion : it is just a matter of getting enough pull from the kite to balance the drag of your board (which has nothing to do with the wind speed); For example there is no "wall effect" or threshold or any thing related when you reach or go beyond the wind speed. That also explains that there is no physical low wind limit like 4 knots or less, you just need a kite that keeps flying in you wind target, and increase surface area up to get enough lift. Single skins kites like Ozone XXlight paraglider (22 m2 I think) could be a "4 or 3 knot" killer for example with an adapted board.