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Hot Air Balloon Effect in Foil Kites

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foilholio
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Hot Air Balloon Effect in Foil Kites

Postby foilholio » Tue Sep 06, 2016 12:03 am

So I have been wondering about this for a while. It is obvious to me something is going on. The bigger the kite, puffier it is, the colder the day and the brighter the sunshine the more this is noticeable. Just the other day I took a kite for a short 10 min test flight on a clear day, I felt the air temperature inside packing up and it was noticeably warmer, I would guess 5-10 degrees. Then browsing one of the french forums I saw this
phpBB [video]

So I think if I can get some estimates on kite volumes and I take some temperature readings this effect can finally be quantified.

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Re: Hot Air Balloon Effect in Foil Kites

Postby kitexpert » Tue Sep 06, 2016 11:45 am

I know the basics of physics and magnitude of things well enough not to bother with that kind of effects. But of course I can estimate/calculate and at the same time show couple of different approaches for that kind of question:

1. Practical one: You have that 67m3 sized balloon which gives 2,2kg to 6,2kg of lift according the video. Values sound reasonable. Biggest of foil kites have ca. 4m3 of volume, so 1/17 of that. Theoretically we get 130g-360g lift for the 20m kite, and 45g-130g for a 10m kite.

These values would be possible of course for the black kite in sunny conditions, and if kite moves (like they usually do) airflow of course cools the surface. In reality perhaps 1/5th or 1/10th of added lift would remain, if even that. Then effect is few grams, for sure not more than a fraction of percent of the total mass of the foil kite.

There is also some air flow through the kite when it flies, which ruins the rest of "hot air balloon" hypothesis for foil kite.

2. Physical/mathematical one: You calculate how much air density decreases when temperature rises (pV=nRT, ρ=m/V etc.), or just find needed data. Then it is easy to calculate the resulting theoretical lift (after calculating the V of the kite.)
foilholio wrote:So I have been wondering about this for a while. It is obvious to me something is going on
Trust me, there is nothing going on with "hot balloon effect" and kites. That black solar balloon is however a nice work.

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Re: Hot Air Balloon Effect in Foil Kites

Postby foilholio » Tue Sep 06, 2016 12:31 pm

There is definitely something going on. Your small grasp of physic seems to miss that despite the smaller volume increased heating is possible, because of the lower volume and wider area of the kite.

Anyway try not be such a downer. Interesting your 4m^3 estimate, I'll take it with a grain of salt though like anything you say.

Regis-de-giens
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Re: Hot Air Balloon Effect in Foil Kites

Postby Regis-de-giens » Tue Sep 06, 2016 2:12 pm

I wanted Benoit Tremblay to test one of its next light weight foilkite with a black or dark color to see the impact of air warming ; My estimations were indeed in the range of 10% weight saving (*), not crucial but not negligible ; he was doubtful, so I had forwarded the above video last week to him, with (poor) hope that this could change his mind;

I think it is worth trying (for science progress at least), now that 15m kites are around 2kg. Far easier than Helium anyway :) . For snowkite this might be interesting as well.

(*) linked to both wind speed that cools the kite surface and fresh air flowrate by the inlets (=leak through the seams) ... so not easy to estimate accurately.

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Re: Hot Air Balloon Effect in Foil Kites

Postby TheRussian » Tue Sep 06, 2016 2:20 pm

Nice thought - next time I use my Chrono, I'm going to light some of these and stuff them in the vents

Image

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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Tue Sep 06, 2016 2:55 pm

Whenever someone claims it's the strong cold wind from cold water or lift from hot air or strong dry air the real answer is always the same.

difference between cold water and hot air creates a wind gradient so the wind up high is stronger

sunny days create a wind gradient

cold fronts create a wind gradient

the real answer is always the wind shear or wind gradient.

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Re: Hot Air Balloon Effect in Foil Kites

Postby foilholio » Tue Sep 06, 2016 3:21 pm

No this is definitely a hot air effect EDT. I notice it with slack lines. Once a kite gets warm enough it becomes much harder to drop on slack lines. Makes surfing in light wind awesome.
Regis-de-giens wrote:I wanted Benoit Tremblay to test one of its next light weight foilkite with a black or dark color to see the impact of air warming ; My estimations were indeed in the range of 10% weight saving (*), not crucial but not negligible ; he was doubtful, so I had forwarded the above video last week to him, with (poor) hope that this could change his mind;

I think it is worth trying (for science progress at least), now that 15m kites are around 2kg. Far easier than Helium anyway :) . For snowkite this might be interesting as well.

(*) linked to both wind speed that cools the kite surface and fresh air flowrate by the inlets (=leak through the seams) ... so not easy to estimate accurately.
Well in snow the heating should be almost double from reflected light plus cold air should heat better. I am interested in how much better a black kite heats up over other colors.

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edt
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Re: Hot Air Balloon Effect in Foil Kites

Postby edt » Tue Sep 06, 2016 3:34 pm

foilholio wrote:No this is definitely a hot air effect EDT. I notice it with slack lines. Once a kite gets warm enough it becomes much harder to drop on slack lines. Makes surfing in light wind awesome.
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.

Lmrutledge
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Re: Hot Air Balloon Effect in Foil Kites

Postby Lmrutledge » Tue Sep 06, 2016 3:58 pm

Boy, you can tell who has no wind!!!!! U r kiddin wright??

Regis-de-giens
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Re: Hot Air Balloon Effect in Foil Kites

Postby Regis-de-giens » Tue Sep 06, 2016 4:01 pm

Edt, there is a bug in this reasoning : indeed , the air in the kite warms far faster (enclosed + in a dark enclosure that catches sun radiations without reflection than on sea surface). Then Archimede simply tells you that you get a vertical lift due to the difference in volumetric mass of traped hot air vs normal air around the kite. That is what the video shows.

Wind gradient (generation and effect) worth a dedicated thread in my opinion; it is mainly (almost exclusively) the consequence of the boundary layer effect (that you suffer in any kind of flowrate): friction against the sea surface makes the air particules stick to the sea surface, so zero speed at ground and speed increases with the elevation (whatever the cause of wind which could be thermical or meteo wind). Windward obstacles like mountains obviously creates another type of gradient as well.


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