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Foil Kites - 4 knots ?

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Kamikuza
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Re: Foil Kites - 4 knots ?

Postby Kamikuza » Sat Aug 06, 2016 2:14 am

It's been over 35 degrees C this past month, and humid as. Noticeable difference in what a kite's performance actually is, especially the bottom end. I think for the foil, air density matters a lot more, due to needing a lot less raw power out of the kite . . . so a m/s more or less has a greater effect :o

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Re: Foil Kites - 4 knots ?

Postby foilholio » Sat Aug 06, 2016 2:43 am

I don't really notice a difference, if anything it is worse in the cold because of wetsuit etc. I get my best low ends in warm weather.

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Re: Foil Kites - 4 knots ?

Postby Kamikuza » Sat Aug 06, 2016 3:00 am

foilholio wrote:I don't really notice a difference, if anything it is worse in the cold because of wetsuit etc. I get my best low ends in warm weather.
Over 30 degrees of difference in air temp. I don't know water temp at the lake, but the past couple of weeks the water has become a lot warmer and I suspect that it not sucking heat out of the air is part of the problem.

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Re: Foil Kites - 4 knots ?

Postby foilholio » Sat Aug 06, 2016 4:28 am

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

5 to 35C is 1.26905 to 1.145kg/m2 or 10% less. I would think if some fundamental change in the airs ability to impart force changed, like density, that then would show on a wind meter as a change in speed, because the internal inefficiencies of the wind meter would be more affected. There is also wind meters that merely measure the force of the wind and relate that with a speed, obviously if the wind has more force but not necessarily more speed it will register a higher speed. Come to think of it measuring the winds speed is kind of stupid, the power is what we want to know.

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Re: Foil Kites - 4 knots ?

Postby kitexpert » Mon Aug 08, 2016 9:07 pm

foilholio wrote: There is also wind meters that merely measure the force of the wind and relate that with a speed,
Also? There is very few windmeters which do not measure the force of the wind. This force usually rotates the rotor of the meter or moves directly some pressure indicator. This means also less turbulent wind gives higher speed values like denser air does too. In practice this is not a bad thing.

I agree with you about differencies in temperature vs wind strenght. They are small. Could there be some rising convections or more turbulence when air is hot, over 30 degrees, perhaps. On the other hand in the winter low wind speeds are often very stable, not at all gusty. And from the extremities -25C to +35C there is some difference in density too. I have sometimes kited in -27C, but not recent years any more.

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Re: Foil Kites - 4 knots ?

Postby plummet » Mon Aug 08, 2016 9:53 pm

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.

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Re: Foil Kites - 4 knots ?

Postby PullStrings » Mon Aug 08, 2016 10:24 pm

foilholio wrote:I get my best low ends in warm weather.
Agree. Once it had wind 4 knots sideshore and i was doing 3 meter jumps. Air temp 34C and water temp 20C !!! from an overnight "upwelling". The afternoon before the water was 30C. !!!
Air was so dense it felt unreal riding the glassy ocean. That was many years ago using a SP3 21 + Flydoor XL.

To quote Kamikuza...it was magical

Image

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Re: Foil Kites - 4 knots ?

Postby foilholio » Tue Aug 09, 2016 2:31 am

PullStrings wrote:To quote Kamikuza...it was magical
Japanese mushroom magical?

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Re: Foil Kites - 4 knots ?

Postby Kamikuza » Tue Aug 09, 2016 3:02 am

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.

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Re: Foil Kites - 4 knots ?

Postby Kamikuza » Tue Aug 09, 2016 3:03 am

foilholio wrote:
PullStrings wrote:To quote Kamikuza...it was magical
Japanese mushroom magical?
Nothing is that magical :cool2:


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