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Hot Air Balloon Effect in Foil Kites

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edt
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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Tue Sep 06, 2016 4:08 pm

Regis-de-giens wrote:Edt, there is a bug in this reasoning
Ok so you want to have the kite warm slightly faster fine. That's 2 grams. Calculate it the math is simple.

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Re: Hot Air Balloon Effect in Foil Kites

Postby Regis-de-giens » Tue Sep 06, 2016 4:34 pm

You have not catch my point, maybe I was unclear sorry ; 'faster' implies that equilibrium temperature inside the kite will be higher ; indeed temperature equilibrium inside the kite is mainly the result of 2 thermical flows (*) :
- the fresh air coming from outside through the kite inlet
- the warming "rate" inside the kite due to sun radiation against a dark absorbing envelop (balanced a bit by the refreshing of the envelop by the surrounding 'cold' air that slides along the kite)
=> Result is mathematically a warmer air inside, no doubt and experienced when you deflate the kite. Hard to say how much without dedicated sensors when deflating for example.

Please explain your 2 grams estimation ; my 100/200 grams estimation is based on the impact of approx 10 degree temp elevation on the density of air of the total kite volume.

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Re: Hot Air Balloon Effect in Foil Kites

Postby Starsky » Tue Sep 06, 2016 4:48 pm

hot air effect. In the kite or the forum members?

Who really cares. Make em all black. It been my preferred colour of kite for ages. Easier to clean the goose shit off. Wait maybe we should weight the average goose shit! Take about 10% of that multiply by the density of goose shit per meter of lawn, not figure the specific gravity of goose shit, offset for the added rigidity it might give the canopy when it dries, account for absorption if it gets wet. Pretty hard to tell, but maybe a couple percent less lift due to simple shit. That would bean foilholio would need another .0365 knots to get planing..... downwind of course, but you know, a solid broad reach.

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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Tue Sep 06, 2016 5:10 pm

Regis-de-giens wrote:You have not catch my point, maybe I was unclear sorry ; 'faster' implies that equilibrium temperature inside the kite will be higher ; indeed temperature equilibrium inside the kite is mainly the result of 2 thermical flows (*) :
- the fresh air coming from outside through the kite inlet
- the warming "rate" inside the kite due to sun radiation against a dark absorbing envelop (balanced a bit by the refreshing of the envelop by the surrounding 'cold' air that slides along the kite)
=> Result is mathematically a warmer air inside, no doubt and experienced when you deflate the kite. Hard to say how much without dedicated sensors when deflating for example.

Please explain your 2 grams estimation ; my 100/200 grams estimation is based on the impact of approx 10 degree temp elevation on the density of air of the total kite volume.
2 grams is the difference between including the color of the fabric of the kite or not. Just giving you a hard time because I thought you didn't run the numbers. Let's see if we can agree on what they mean tho . . .

if you say it's 200 grams I calculate your kite contains between 4,000 and 5,000 liters of air, so I assume it's a 30 meter kite. Less than 100 grams seems more sensible to me. Let me know how you calculate the volume of air in these kites. Also a 10C difference is huge, that's the difference between 80F(27c) and 98F(37c). I was assuming a much more reasonable difference of 5c. Don't forget that the kite not only absorbs heat it also radiates it away.

If my estimates for the volume of the kite are off and my estimates for the temperature difference are off then yeah it could be due to warming of the kite.

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Re: Hot Air Balloon Effect in Foil Kites

Postby PullStrings » Tue Sep 06, 2016 5:20 pm

Lmrutledge wrote:Boy, you can tell who has no wind!!!!! U r kiddin wright??
I can assure you that kookholio is BORED

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Re: Hot Air Balloon Effect in Foil Kites

Postby foilholio » Tue Sep 06, 2016 11:24 pm

edt wrote:Think about this possibility foilho:

The air in your foil warms up at exactly the same rate as the air surrounding the ocean when the sun comes out. So you notice the kite getting warmer and at exactly the same time it pulls harder. However, when the sun comes out and warms up the air you get a huge thermal effect, which if you were able to see it in the air would look like a giant slice of cheese, where the lower air has almost no increased wind speed from the warming but as you go higher up the air is faster and faster, also as you go out to sea it gets stronger, and as time goes on this wedge rolls in. When the sun leaves, this effect reverses and the wind gradient dies and the kite itself gets cooler . . . this all happens at exactly the same time. So you think, kite gets warmer, pulls harder must be the hot in the air the kite, right? But the effect could be entirely due to the wind gradient too (obviously that's what I think it really is). I have often noticed this effect and believed it due to the wind gradient. I'm always talking about this wind gradient, that's because where I kite it is a huge effect so I notice it when others don't. When the sun comes out on a cloud covered day there are pockets of fast wind right where the sun's rays hit.

The problem with ascribing the increased lift to the kite being hotter is you can calculate it with the gas law pv=nrt, measure in kelvin let's say it's a 5C difference on a 24C day, you get a 1.7% difference, in a 400 liter foil kite that's something like 10 grams or less than half an ounce. You are not going to notice it. The physics is simple so I don't think there's a mistake in it. I think kitexpert came up with a similar calculation.
Ok some problems with your reasoning. First the effect is greater on land, so remove the ocean from your argument which is the majority of it. Second the temperature difference can be much greater than 5c, yet to measure it but I know with black plastic heating to 80C+ on a 20C day is possible. Third 400L is well below the volume of the larger kites this is really noticeable on, by a factor of at least 10.

But I see you and regis have hashed all that out.

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Re: Hot Air Balloon Effect in Foil Kites

Postby Regis-de-giens » Wed Sep 07, 2016 8:08 am

Here is my calculation:
Image
So assuming between 5 & 10 degrees Celcius warming which seems reasonable, weight saving is 100 & 200 gr = 5% & 10% weight saving

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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Wed Sep 07, 2016 12:08 pm

5500 plus 2000 gives me a total weight of 7500 so 200 difference is a bit less than 3 percent what am I missing

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Re: Hot Air Balloon Effect in Foil Kites

Postby foilholio » Wed Sep 07, 2016 1:19 pm

He includes the weight of the air. Which doesn't add weight just mass to the kite .

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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Wed Sep 07, 2016 2:36 pm

foilholio wrote:He includes the weight of the air. Which doesn't add weight just mass to the kite .
Got it. I didn't think of it that way. If you turn a kite, you can of course feel all of the air in the kite that's why the foils turn so slow so I naturally assumed you want to measure as a percentage of the total mass.


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