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Inflatable race kites

Posted: Mon Jun 17, 2019 2:07 am
by tegirinenashi
Is anything happening in this area or LEI manufacturers completely gave up on this idea? Cabrinha Apollo was a half-hearted attempt to make LEIs relevant at the racing scene, but it didn't go far for several reasons:
1. The kite is C shaped, with significant m^2 loss in projected area.
2. The AR is still lower than high performance foil kites.
3. Typical LE tube is too thick, which negatively affects L/D ratio.

Now, it is obvious that rigid wing should have superior aerodynamics to sail hinging on some frame, this is why America's Cup contenders are equipped with rigid wing sails. Therefore, executed properly, rigid inflatable kite structure should be superior to paraglider. This is assuming the aforementioned 3 Cab Apollo issues are worked out.

Problems #1 and 2 seems to be easier to solve, than #3. Not so long, on this very forum, Bruno Legaignoux suggested high pressure kite. Bruno's kite has inflatable frame so thin that it would be well suited for high performance applications.

However, the inflatable trailing edge has to go. It is the basics of aerodynamics, that while airfoil can have blunt leading edge, the trailing edge should be razor sharp. Can Bruno's design be salvaged so that it would work without rigid trailing edge? One solution is to mimic foil kites and to have elaborate bridle supporting trailing edge. Unlike trailing edge, rigid leading edge distributes the load evenly, so that the LE bridle can be kept minimal -- essentially the same as the existing tube kites.

Elaborate paraglider style bridle is the source of drag. Apparently, it has less effect than LE tube, but maybe foils sit further in the wind window just because of their higher AR? Anyway in this proposed LE race kite design there is only one bridle with nontrivial drag: the trailing edge one.

There is one more advantage of paraglider style trailing edge bridle: it helps for reverse launch. Did I forget to mention that Apollo sucked badly with water relaunch? This is because high AR rendered invalid the traditional LEI relaunch technique. For ultrahigh AR kites the reverse launch seems to be the only option, and with paraglider style trailing edge bridle I suppose the relaunch difficulty should be the same as for foil kites.

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 5:36 am
by windmaker
Good analysis!

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 8:23 am
by james
America’s cup boats don’t use rigid multi section wings.

The control systems and multiple elements make them too heavy to fly in a kitesurfing application.

The cost would be massive even if it was possible, soft foil kites already mimic much of the behaviour of a rigid wing at a lower weight and cost point of view so why do it?

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 2:38 pm
by joriws
PMU - where is your 1000-line post inflatable race kites are superior?

--

Did Epic kites plan or even do some high aspect race kite prototype? What happened to it, obviously did not come as racer but did it fail boosting too?

viewtopic.php?f=1&t=2373964&hilit=Epic+race+kite

Meanwhile
https://m.facebook.com/story.php?story_ ... =635264166
- check the video on above link

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 2:47 pm
by TomW
I'm no expert, and fly low aspect foil kites. But I've noted that when I trim them for more power, something else is going on than just changing the AoA. If feels like the profile changes too. I just get more power.

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 3:07 pm
by joriws
TomW wrote:
Mon Jun 17, 2019 2:47 pm
I'm no expert, and fly low aspect foil kites. But I've noted that when I trim them for more power, something else is going on than just changing the AoA. If feels like the profile changes too. I just get more power.
In short - yes. AoA, Camber and projected area can be changed on the fly with the foil kite's mixer.


Re: Inflatable race kites

Posted: Mon Jun 17, 2019 4:14 pm
by tilmann
thin high pressure leading edge and low aspect - that´s it.

Re: Inflatable race kites

Posted: Mon Jun 17, 2019 10:12 pm
by tegirinenashi
On some more reflection, just thinning leading edge would allow increasing the pressure. Say, the LE is reduced to half diameter, would it allow 2x pressure increase? Experimenting along these lines is relatively cheap, because one can use existing materials (TPU bladders + dacron). Certainly, front bridle has to provide more support, but consider that normal size LE is typically attached to each power line via 4 branches, so that doubling the support to 8 seems to be no-brainier.

To summarize: flat, high AR, thin LE inflatable kite with doubled front bridle and foil-style back bridle. This is certainly above my DIY skills, so I wonder what is the cost of making one prototype kite?

Re: Inflatable race kites

Posted: Tue Jun 18, 2019 2:55 am
by Matteo V
tegirinenashi wrote:
Mon Jun 17, 2019 2:07 am
....Therefore, executed properly, rigid inflatable kite structure should be superior to paraglider.....
A paraglider's (or a foil kite's) inefficiency is not due to it lacking rigidity. A paraglider's or foil kites main inefficiency is due to the cell shape increasing surface area without increasing lifting area. Thus you have more drag. This excessive drag does not amount to much at low speeds (high speed kiting is still low speed in for aerodynamic consideration), but speeds in excess of 100knots do need to take extra area (without extra lifting area) into account as drag will become detrimental.



While this is very counter intuitive, a foil kite is MORE RIGID than an inflatable when airflow is attached, and the wing is working as a foil section. Due to the fully supported underside of a foil kite, along with the load distribution from vertical fabric on the edge of each cell, foil kites maintain their airfoil shape much better than an inflatable through the entire trim range.

Most kiters think that a pumped up strut on an inflatable translates to rigidity. It does not. Grab a strut pumped up to 9psi and see how small force it takes to bend it just a little. That little bend distorts the entire upper surface of the airfoil section, moves the maximum thickness location around in that airfoil section, and changes the camber line. Considering the entire canopy of an inflatable, this is most easily explained as that canopy gaining more of a cone shape, rather than a pivotal sheeting action as you see with a fully supported foil kite.

Many who understand this concept naturally will wonder why a hybrid design of a fully supported LEI is viable. The main reason for that is that efficiency gains at the low speeds kites operate, do not justify the additional cost/complication. A secondary reason you don't see this concept put into production is that the "cone effect" on inflatables can be exploited to yield some good properties in higher wind speeds. One of those good properties of messing up the airfoil section is that it can actually depower the kite by making the foil section less efficient as it changes the airfoil section. And this is why foil kites tend to feel more twitchy in high winds compared to inflatables. Foil kites are just being more efficient and maintaining a more rigid foil section.

Re: Inflatable race kites

Posted: Tue Jun 18, 2019 2:57 am
by RedSky
Maybe do away with the bridle altogether. Use a spar to hold the TE. Four lines and high pressure LE.