Trapped air in double skin foils?

[jumptheshark]

Were not talking about boat speed and VMG or powered megaloops and handle passes. That's not what this thread is about. Its about the start up power available in winds well under 10 knots and the trade off between weight, inertia, shape, drag and duration. In that setting we begin to clearly see the variables laid bare.

I posit that flying an appropriately lightweight/inertialess kite up has more potential power than flying it down for attenuating the power spike and getting you on foil. I generally loop Peaks starting with an upstroke.

Fly a standard kite down and the inertia limits you to a one shot deal, You either have enough power and get up, or you sink as soon as the kite begins to slow. If you fail, its a reset for another attempt. Compare that to Horst working tiny ultra light kites over 20-30 seconds of less absolute power attenuated for longer to produce - high success rate.

Fly a light kite up and you get: added boost from the wind gradient as it climbs. At the apex turn it is in the highest wind, The angle of pull while climbing better suits lifting rider weight over the foil, lastly an upstroke can always be followed directly by a downstroke.

I think that kiters who feel the downstroke is "way more powerful" than the upstroke, are on heavy or high inertia kites.

Just like windsurfing, where a really efficient waterstart technique was key to getting on smaller more performance gear. The intro of ultra light fast kites provides the same opportunity for skilled kiters to get going in relatively low winds on smaller faster kites.

[Peter_Frank]

Acceleration due to gravity is same for heavy or light objects. The only way the motion is influenced on the down stroke is the fluid dynamics, is it not. More kinetic and potential energy is in the heavy object, but if you extract it you will require more energy to make it fly on the upstroke via a system that is bound to be less than 100% efficient. Guessing extra weight is bad.........but doing the maths will give me a headache plus it is early in the morning and these thoughts could be garbage!!!!!!

You are right, the acceleration is the same when flown towards the ground, no matter what the weight is, light or heavy, as the added force from gravity, has to overcome the added mass, so it cancels out, correct.
The end velocity will be higher for the heavier kite (not air but overall weight), but I doubt any kite comes near the end velocity when looped down.

What we feel when we dive a heavy kite, is it feels like a lot more power because of its momentum keeping the power for a while - it is very clear and evident, one is not in doubt at all.
But on the way up it goes south as it will fly slower and slower fighting gravity, as soon as the initial momentum from the downloop has been "used", and as you say, when having a system with loss, extra weight is bad.

The real advantage is like jumptheshark just wrote, when you can fly a kite up without having gravity slow it down to eventually a full stop, you keep line tension/power for much longer, but also pull UP, which is key for light wind starts.

I am pretty sure this is the very reason why these light kites, low mass, works so much better in light wind.

Peter

[Herman]

Peter
I think we are in agreement. The way I would think about it is that on the down stroke the extra weight (Mg) provides a force to overcome drag enabling it to reach higher velocity. On the upstroke this weight acts with the drag to slow the kite which is the real killer for a wet kite.

Regards Herman.

PS also agree that terminal velocity will be higher for heavier object than that of a lighter object of the same size and shape.

[Peter_Frank]

but some would claim it being drag when not closed cells.

Peter

It’s not a claim it’s a fact. Turbulent air is drag. It’s called aerodynamics.

Not true, I dont think this is right.

The air in the open front cells on a single skin, will together with part of, or all of the stagnant air behind (also on a LEI with a thick LE) act just as the mass of the air inside a double skin kite IMO.

Meaning it adds to the momentum besides the weight of the kite, so you can gather energy that can get the kite to fly a bit even without wind/pull.

Yes, a round LE or an open cell in a SS will have more drag, true.
But it can NOT be this air itself being drag only - it adds to the overall momentum which is quite essential, so they work the same way, just a lot less air in total, compared to double skins.

This total mass which increases overall momentum, and which we cant really agree whether an advantage or disadvantage, is important regarding its distribution though.
As if most of it is air inside or behind the LE, meaning same weight as the surrounding air, it means the disadvantage regarding being able to fly the kite up, is not present, thus they work fine in light wind.

Whereas they dont accelerate as good, but for doubleskins outweighed by less drag quite fast.

So not possible to compare a heavier kite if the canopy is heavier, with a kite with more air mass instead of canopy - eventhough they are equal in terms of acceleration.


Still interesting with the "momentum" paradox, as can it be used to gain even more energy for starting?
I dont believe it can in our kitefoil light wind scenario, but the principle should be as in these two scenarios:

You have two wind mills/turbines.
One with superlight carbon wings, and vacuum inside.
The other with concrete inside, but same wings.
And we say the bearing is "perfect".

Blowing in the wind, the light one get wings up to speed superfast.
The heavy one will accelerate slowly, but come up to same speed eventually.

Wind suddenly stops - what will happen now?

The light one will stop almost immediately, not producing any energy really, close to zero.

Whereas the heavy one will rotate for a long time, and loads of energy will be produced, before it stops.
Air inside will also help with this, does not have to be something heavier than the surrounding air.

This is the principle we are discussing if we can use for light wind hydrofoil starts, or not?

Interesting, and maybe a difference whether we talk about light wind starts and jumps, as in the latter scenario the re-direction, if not losing too much for this acceleration (turn), could end up giving you more height maybe.

Peter

[Regis-de-giens]

Agree with tomtom and evan on neutral buyancy of air entrapped, leading to only a difference in momentum while turning due to entrapped air. So mostly more inertia for the changes in directions.
Beeing a lover of marginal wind, there is no doubt that every gram saved is great for light winds. I noticed that the weight of even UL clothes is enough to "use " the increased speed when kite dives down. If too heavy you cannot work the kite properly in multi-loops for the waterstart.
There is however one big advantage of entraped air : the internal pressure provide an additional stiffness that allows more efficient foil profile, like less "twisting" of wingsfor exemple. On the contrary on a SS (so, without internal pressure ) you need to impose more Angle of Attack on wings (to avoid their collapse), and you loose some efficiency (drag) and thus less speed and pull during the travel of the kite, very useful in lightwind.

[Regis-de-giens]

I also agree with Peter as usual.
Just about the comparision with mill/trubine : no imo we do not use this priciple during loops or waterstarts. Momentum is (to me, in the sense of kinetic eneegy of the kite itself) a pain in kitesurf :
1- you can not really use it because the mecanical link with the kiter is loft (lines) and the kite will not turn or go faster (itmay just keep its max speed a bit longuer but max speed will remain lower due to the mass to accelerate (except pure dowstroke of course but for another reason than accumuted energy)
2- it is in détriment of stability when it reaches the end of window while you nee to redirect the kite.

So, kinetic inertia could (in theory ) compensate for a very sudden lull in the middle of waterstart, but not give you higher peak pull to get your ass out of the water ...

... IMO...

[Herman]

The heavy and light turbine seems to demonstrate that energy could be stored. You could even go further and say two of same light turbines but one connected to a frictionless flywheel driven through a 100% efficient fluid coupling. Unfortunately I just do not believe that this is comparable to the mechanics of the real kite scenario. Kites are more what goes up must come down, but it doesn’t come down with more energy than it took to take it up. But you could transfer energy into a flywheel but kites do not have flywheels...............sort of.........

[Regis-de-giens]

a bit side of topic, just to discuss :
I was thinking of a system to store energy in the kite to be able to water start more easily and then foil with less power needed than water starts;
based on a spring system fixed on front lines, with a "click system" :
- you make serials loops that would lengthen the spring on the front lines , gradually , loops after loops ( click after click ) (or you can recharge : tension it by hand, ass in the water)
- you prepare your water start and release the click system that will shorten the front lines during a pair of seconds

you need to manage the rear line length, maybe by trimming a lot in anticipation.

I tried to reproduce this by just pulling the front lines directly by hand or the trim during the water start, but not successful to really save some low end for now and since the kite becomes hard to drive (bar becomes unusable time by the time you can release the front lines)

[Tomlutz]

There is however one big advantage of entraped air : the internal pressure provide an additional stiffness that allows more efficient foil profile, like less "twisting" of wingsfor exemple. On the contrary on a SS (so, without internal pressure ) you need to impose more Angle of Attack on wings (to avoid their collapse), and you loose some efficiency (drag) and thus less speed and pull during the travel of the kite, very useful in lightwind.

I don't think the stiffness is the main reason for better performance. I am also paragliding and fly single skin paragliders. However, unlike the kite market there are high AR single skin paragliders (AR of 6), I don't know of any high AR single skin kites (not counting LEI, which in fact are also single skins). The main obstacle is the increased drag due to the fact that there is no skin on the bottom for better aerodynamics. Hence single skins just do not fly fast, otherwise they have quite good performance, in some respect even better performances and even better stability than double skins even though they are not at all stiff. Tom

[joriws]

Crashing in this topic already 3 pages late, sorry about that, as I consider myself as father of flywheel effect theory with kite inertia. Which seems to haunt Peter a lot.

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First to proof that inertia flight exists. (attached quick powerpoint sketch)

Views are pseudo-3d view on left, side view in the middle oriented by kite lines and top view on wind window edge on the right. Note! Inertial energy vector is the opposite to drag vector. Note! Vector lengths/widths are not in propotion, they are just indicating directions.

So drag caused by aerodynamical lift can also be countered by inertial energy, by simple equation W=Fs (https://en.wikipedia.org/wiki/Work_(phy ... and_energy). As long as kite keeps flying forward it produces lift force either by true wind or stored kinetic energy (or both). Flying speed goes down => lift goes exp. down => drag goes down too.

Another proof that *inertial* riding / flight exists is "galactical" or kite 360 or "deadmans turn" or what every you call it, sure galactical is about riders inertia (and gravity to loose hf flight altitude) which can be converted to force (work equation again) to pull the kite upwind of you and ride the rider's spiral, and use inertia to rotate anchor point to reposition wind window as quick as possible. Trick also uses kite inertia and gravity to fly from high near zenith down to water level. And for sure all this applies both ways, so kite creates inertial forces too. Do you remember physics calculations from school if car can make the turn on that speed entering the curve, sure in car case it is the friction to surface. In kite it is the rider and kite lines and kite combo.

I believe Horst Sergio is still struggling making "galactial" on almost inertialess Peak4? He has done it with closed cell kites with inertia.. And closed cell foils with hydrofoils has already done multiple galaticals in a row, like kite buggies. I've done many in a row with ice skates.

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I cannot comprehend why Peter insist single skin or tube, the air behind LE is inertial mass or something else. It is turbulent air, which is *included* in *drag force* of airfoil like tip vortex etc, just like adventure_logs says. LE moving forward creates lower pressure behind LE. This lower pressure is filled with atmosphere weight => forcing air to fill the "hole" aka lower pressure area. This is included in total drag in L/D. Go back to YT videos and see airfoil drag vs cylinder drag etc.

I remember Peter you mentioned that you hang glided (I too). Well on hang gliders we have at least 1-skin, 1.5-skin and 2-skin air foils. Going more glider skins lowers the drag by making more efficient airfoil underside and finally by hiding the wing cross bar inside the wing for least "shape drag". then airfoil rigidness and winglets and raked wing tips are other means among steering triangle airfoil-shapes, wires etc, here I just reminded you the airfoil type drag on skins. This L/D then moves speed polars around the coordinates with wing loading. Well kites have speed polars and wing loadings too.

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Now getting rider to plane in any wind. Another quick figure and again vector lengths are just indicating diretions. Key point is that for rider to succeed to start, he must get out of water first, then horizontal speed accelerated to planing speed which reduces board/hydrofoil drag. This can be characterized by *force impulse*, you need certain amount of force with time force applies to succeed.

*edit* forgot to draw water drag of rider/board/hf, but still horizontal component must overcome this drag to allow rider to accelerate to planing / flying speed which reduces drag and allows riding with less lift force..

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So the hypothesis is that kite inertial can be used to counter kite airfoil drag and keep kite airfoil lift going longer, to get the required force impulse to get going. On more inertial kite this force impulse even outside of wind window can continue. Less inertia, the quicker kinetic energy -based impulse assist stops.

Very simple theory, like a flywheel in car engine to get even single piston 4-stroke engine to run over strokes flushing cylinder and even recompress fuel-air-mixture, storing force in *kinetic energy* engine created for later usage. Same applies kinetic energy flywheel electricity storages etc...