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Trapped air in double skin foils?

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Herman
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Re: Trapped air in double skin foils?

Postby Herman » Thu Jan 07, 2021 12:14 pm

Lots of really interesting thinking and description in this thread. Is there anybody brave enough or with enough time on their hands to write the conclusions? Perhaps in another day or five.
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Re: Trapped air in double skin foils?

Postby tomtom » Thu Jan 07, 2021 1:44 pm

there isnt any, just opinions :)

Anyway my is that for low wind some flywheel stored energy is good. For anything above 12 knots intertia less peaks are best. But i suppose in lowest wind it is synergistic effect of better aerodynamic performance and stored energy

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Re: Trapped air in double skin foils?

Postby joriws » Thu Jan 07, 2021 1:57 pm

tomtom wrote:
Thu Jan 07, 2021 1:44 pm
Anyway my is that for low wind some flywheel stored energy is good. For anything above 12 knots intertia less peaks are best. But i suppose in lowest wind it is synergistic effect of better aerodynamic performance and stored energy
Exactly. If Peak has less inertia/kinetic energy and more drag in terms of lift (L/D ratio) aka more drag for same lift or at same drag less lift, it means that Peak4 consumes stored inertia quicker because drag force is greater and also there is many times less kinetic energy. So if both kites have equal lift = equal force produced on lines, on force impulse Peak's impulse end sooner. In high wind it does not matter because a lot of true wind energy is available. But in low winds it matters.

I quickly calculated a month ago that on stored kinetic energy levels on closed cell foil kite you can lift rider mass potential energy alone 60cm (or was it more) when Peak4 was at 10cm level. Sure kinetic energy of kite is not lifting rider up but I used that to give concrete visualization of energy levels. Kinetic energy overcomes airfoil lift drag based on force/energy vector directions.

True answer goes if high inertia kite can be accelerated for exponential lift force as less inertial kite. Less inertia quicker to steer, faster to accelerate, but in Peak4 case also higher DRAG (just concluded from lower L/D) to counter acceleration. So which kite inertia types lift/drag/kinetic_energy -balance point gives better force impulse for riding in low wind winds. And kiter skill has a lot to do with it too.

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Re: Trapped air in double skin foils?

Postby Peter_Frank » Thu Jan 07, 2021 5:08 pm

Agree with everything joriws.

It is all about the balance point as you write in the last sentences, and what we are discussing from both real life experiences and theory to explain :thumb:

There are numerous factors affecting this, is my take on it, and the reason why we havent a clear view on this, as it seems in the small sizes Peak4s are pretty close to double skins same size.
The latter are heavier (both weight and weight+trapped air) but better L/D.

Factors could be: line length, kite size, being able to keep riding versus being able to start, kite L/D, rider technique - just to name a few.


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Re: Trapped air in double skin foils?

Postby joriws » Fri Jan 08, 2021 3:56 pm

Peter_Frank wrote:
Thu Jan 07, 2021 5:08 pm
There are numerous factors affecting this, is my take on it, and the reason why we havent a clear view on this, as it seems in the small sizes Peak4s are pretty close to double skins same size.
The latter are heavier (both weight and weight+trapped air) but better L/D.

Factors could be: line length, kite size, being able to keep riding versus being able to start, kite L/D, rider technique - just to name a few.
I hope Adventure logs could open up his experience a bit more and help us form our own views what is relevant. I mean he has videos on Youtube about Soul 6m and he has tested Peak4 13m, has Sonic3 11m and Soul10m. So I guess he is at the best position of all of us and state something on low end differences of various inertia levels, I mean if AL could think of if riding is possible due to high or low inertia etc. That if Peak4 is "best" kite for foiling is matter of personal preference. I have Peak4 but I don't have objections on riding closed cell kites with HF too.

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Re: Trapped air in double skin foils?

Postby Peter_Frank » Fri Jan 08, 2021 6:50 pm

Hmmm, personally I am not in doubt whatsoever, that UL "other" kites higher aspect and more efficient, but a tad heavier, can start earlier than Peak's, in the bigger sizes.

But on cost of the ability to get to shore safe, as Peaks hangs a lot longer :thumb:

But in the small sizes it is a lot more complicated, and these are the ones I fínd interesting.

But good to have all "sizes" in play for debate, and correct, only in the bigger sizes trapped air has a significant part, agree :D

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Re: Trapped air in double skin foils?

Postby PrfctChaos » Sat Jan 09, 2021 2:30 am

For the smaller sizes, trapped air volume is likely less important because it has a relatively much smaller volume. For example, going from a 6m to a 15m, means area has gone up by factor of 2.5, but volume has gone up by factor of almpst 4.

Small kites are also heavier per square meter compared to large kites.

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Re: Trapped air in double skin foils?

Postby tomtom » Sat Jan 09, 2021 11:19 am

most important there arent almost any small closed cell kites to compare. There are some Concept airs and Kitech but thats it /maybe 10 pcs globaly/
6m isnt small kite anymore. So there are not so much to choose from in really small kites.

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Re: Trapped air in double skin foils?

Postby Herman » Sat Jan 09, 2021 1:07 pm

I think for low power applications the choice of kite that can be used is relatively wide. A while ago I acquired a couple of old kites to use with a bug. A 5m Montana fairly fast and spikey , and a 10m Access a steady as she goes kite. I was surprised by the wind speed overlap of theses kites for the bug. The offset of the two wind ranges for the bug is just a few knots but the small fast kite is much for fun to fly and in some ways feels safer as it is easier to correct a position if needed. I realise this is mostly aerodynamics but mention it out of interest regarding the comment around small foils. I love this old Montana!

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Re: Trapped air in double skin foils?

Postby Peter_Frank » Sat Jan 09, 2021 3:25 pm

Herman wrote:
Sat Jan 09, 2021 1:07 pm
I think for low power applications the choice of kite that can be used is relatively wide. A while ago I acquired a couple of old kites to use with a bug. A 5m Montana fairly fast and spikey , and a 10m Access a steady as she goes kite. I was surprised by the wind speed overlap of theses kites for the bug. The offset of the two wind ranges for the bug is just a few knots but the small fast kite is much for fun to fly and in some ways feels safer as it is easier to correct a position if needed. I realise this is mostly aerodynamics but mention it out of interest regarding the comment around small foils. I love this old Montana!

Agree fully, this is why the say Peak4 in 3 to 6 m2, can start (hydrofoil, not buggy now) in as low as respectively 14 and 9 knots of wind when average weight on a kitefoil, and be a lot more fun than 9 and 18 m2 foilkites which could ride in the same winds without feeling powerwise too big.

A bit out of context as you say yes, but quite important and the reason why many of us ride these sizes - the gain of bigger kites in lower end for freeride/waves is super small, whereas the downsides in turning and drift and acceleration - overall clumsiness, is huge :wink:

I have been out with a Peak4 in 5 m2, in the lower end admitted, while a racekiter used his 21 m2 - and we both had fun, crazy range :rollgrin:

8) Peter
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