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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 12:23 am
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Joined: Mon Sep 23, 2002 1:00 am
Posts: 1805
Location: Cape Cod, MA USA
The guys setting the kiting speed records in the ditch like Rob Douglas use fairly high aspect kites. The ditch is set up to be a broad reach slightly off the wind, but not anywhere near dead down wind.

http://www.luderitz-speed.com/

Ed

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 12:45 am
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Joined: Tue Oct 26, 2004 11:54 pm
Posts: 608
Location: texas
just looking at the luderitz link. why was rob douglas not there? or any american kiter for that matter?

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 10:55 am
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Joined: Sat Nov 08, 2008 8:20 am
Posts: 123
This is a very interesting question as downwind is the opposite of upwind.

This question is very exciting as you can have the 'theory' and the practice.
Theory says that max speed need high AR kite (high AR kite gives less drag and need less time to pass from zero speed to max speed).
But, for speed records in kitesurf for the last 10years, the angle from wind is around 130-140° in 35-50knots of wind, then to have power for such angle, you have 2 ways:
- Having a high AR kite. Using such kite needs to have a larger kite than a medium AR because the kite go more forward in the wind window, then create less power. Rod douglas use a high AR kite and use the largest kite in any contest compare to others. This is due to his power and the AR of the kite.
- Having a medium AR induces the kite deeper in the wind window: deeper in the wind window = more power, then smaller kite. This is the choice of some riders: Caizergues, Salerno.

Then, the choice for the present days depends on the riders.

But in the next year, I can assume that we will go for high AR.
When you see the performance of VESTAS boat, the choice is clear. In the future, the target is to beat records in less wind. For that, we need more performant kites, and more performant boards with less drag.
Performance = more efficiency.

See you in 5years
5 years ago, kites haven't any bridles, then wait and see!

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 6:33 pm
 Medium Poster

Joined: Mon Nov 17, 2008 9:14 pm
Posts: 135
speed has been my thing for a couple of years now and since Nov 2012 I have been playing with 3 different kites from Ocean Rodeo to try to solve the very question you are asking. I have the Prodigy, Rise and Razor to do back to back testing and record all my sessions on GPS to allow accurate analysis afterwards. The thing is there are that many variables it is hard to definitely say which aspect of kite or features are best for a broad reach, I would say it depends on how broad the course is and I am now sure it has a lot to do with body weight and how much power can be efficiently held down. I have decided that holding down heaps of power is counter productive because you kill speed by edging hard to hold that power down therefore I have recently had easier sessions on a kite that sits further forward in the window and needs to build apparent wind speed rather than be grunty from the outset. I have blogged a few back to back sessions on different kites on the Ocean Rodeo forum if anyone wants to read further. One thing I would say is that as with other kite sports the same kite does not suit every rider and given the chance try as many as you can. I have been very fortunate to be trusted with a few kites to figure what works best for speed, the results may just help develop a race kite as the kites may need to be similar for both disciplines.

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 6:40 pm
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Joined: Wed Jan 08, 2003 1:00 am
Posts: 2635
That's ironic,

Coleman...for some reason I had in mind you were talking about wave riding, hence my drifting comment originally.

Good discussion.

Bob
www.kiteridersllc.com

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 7:54 pm
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Joined: Fri Jul 16, 2010 6:27 am
Posts: 3163
Location: Ford Lake, Michigan
coleman wrote:
just looking at the luderitz link. why was rob douglas not there? or any american kiter for that matter?

rob went to the mistral with caizergues look up "Salt and Speed". Never had the wind they wanted, but Rob got a 53.4 knot run anyway.

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Thu Feb 14, 2013 9:17 pm
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Joined: Wed Aug 03, 2005 6:25 pm
Posts: 918
Location: Vancouver
All other things being equal, a higher L/D ratio kite results in a greater forward force component, and therefore higher speed than a lower one... on any point of sail.

And an optimized higher aspect ratio aerofoil (kite) has a higher L/D ratio than a lower aspect kite.

The concept of a kite flying 'deeper in the window' simply refers to a kite that is relatively inefficient... its propulsive force acting in a less than ideal direction. Respectfully, any notion to the contrary doesn't respect basic aerodynamic and force-vector principles.

Cheers,
James

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Fri Feb 15, 2013 12:34 am
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Joined: Wed Oct 16, 2002 1:00 am
Posts: 6245
Location: Denmark
JS wrote:
All other things being equal, a higher L/D ratio kite results in a greater forward force component, and therefore higher speed than a lower one... on any point of sail.

And an optimized higher aspect ratio aerofoil (kite) has a higher L/D ratio than a lower aspect kite.

The concept of a kite flying 'deeper in the window' simply refers to a kite that is relatively inefficient... its propulsive force acting in a less than ideal direction. Respectfully, any notion to the contrary doesn't respect basic aerodynamic and force-vector principles.

Cheers,
James

Yes.

The statement that a lower AR "kite that sits deep in the window" has more power, is a misconception (myth).

It is understandable that many think it is so - because it has more drag thus pulling a bit more sideways - thus one might think/feel it has more power - but this is not the case.

So a higher AR kite will have higher efficiency and as JS says, all other things being equal, a higher resulting forward pull

But of course the handling and reaction of different AR kites can result in the lower AR kite being easier to handle, because of less power spikes - so there are good reasons why you want a medium AR kite for some things.

The reason why kitesurfers are so fast, is IMO mainly because of being able to handle and control the ride in extremely powered conditions, eventhough they are so inefficient that it is a joke (compared to the Sailrocket f.ex).
And when handling is the primary issue, many other factors than efficiency are important.

But in order to move the speed record "a lot", kitesurfers need to be more efficient IMO, meaning better kites with higher L/D and boards that can be controlled at a higher angle to the wind

Peter

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Fri Feb 15, 2013 12:59 am
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Joined: Tue Oct 26, 2004 11:54 pm
Posts: 608
Location: texas
peter,
yeah but i dont think that sail rocket has maxed out its potential either. i think there is going to have to be a new board design that has far less drag in order for kiters to catch the vespas.

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 Post subject: Re: faster downwind: race kite or mid/low aspectPosted: Fri Feb 15, 2013 2:30 am
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Joined: Mon Sep 23, 2002 1:00 am
Posts: 1805
Location: Cape Cod, MA USA
coleman wrote:
just looking at the luderitz link. why was rob douglas not there? or any american kiter for that matter?

That is strange. I had to click around on the site to find this where he is listed. I was surprised to see that some of the record setting kites were described as delta's, which are generally low to mid aspect.

http://www.luderitz-speed.com/luderitz/index.php?option=com_content&view=article&id=63&Itemid=107

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