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Foiling in zero wind..

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Lokihel
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Re: Foiling in zero wind..

Postby Lokihel » Tue Jun 27, 2017 4:34 pm

Regis-de-giens wrote:
Tue Jun 27, 2017 4:05 pm
Lokihel wrote:
Tue Jun 27, 2017 11:42 am
That car thing is actually moving straight upwind, not the same principle :!:
Thank you.
In fact it is downwind :naughty:
(Ok but there is no wind ... :roll: :lol: )
Anyway to catch the "downwind" thing , you need to imagine you are located on the rolling carpet. relative wind will come from the left toward right , as the car does ... so it is pushed by the wind, at higher speed than wind.
It is exactly the same as kitesurfing on a river ( analog to the rolling carpet) and coming back to initial point (or upstream toward the right) without wind.

However a kite could surely not go direct downwind like this véhicule and should S-jibe with a little angle to the wind (the mechanical principle of transfer of energy on a kite does not allow to overcome the wind)
I see what you mean about upwind/downwing.

I still maintain the way it works is not the same although the result is the same.
It is still a mindfuck though, since when the car speed matches the treadmill speed the apparent wind is zero, but since the wheels are turning the propeller is also turning and therefore pushing air. :peng:

Anyway, what I want to highlight is that with that this model works best when going in the same axis as the wind. You will struggle when you go so fast downwind that you reach zero apparent wind, which is why you need to do a series of gybes. Can someone please tell me how you say "tirer des bords" in English??

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cglazier
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Re: Foiling in zero wind..

Postby cglazier » Tue Jun 27, 2017 5:01 pm

Here is a clearer example of downwind VMG much faster than the wind
If a kiteboarder swoops his kite or loops it while going straight downwind then it is acting somewhat like a propeller and might theoretically go faster than the wind. (Of course if the kite stopped moving the kiteboarder would overrun the kite and be in trouble.)

https://www.youtube.com/watch?v=5CcgmpBGSCI


Regis-de-giens
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Re: Foiling in zero wind..

Postby Regis-de-giens » Tue Jun 27, 2017 5:37 pm

indeed this video is easier as it shows the transient stage from speed 0. The combination of both is just a change in referencial , similarly as the river-no-wind debate !

In pure downwind on this vehicule, the direct link between wheel speed and helix rotation speed allows it to always accelerating the rotation with vehcule speed (and even at higher speed than wind) since the helix always opposes a surface backward against the pushing wind) ;
On a kite , the rotation speed is proportonnal to the apparent wind, so decreasing when rider downwind speed increases. So this cannot work with a kite : you would have need to be able to impose the rotation speed and make it higher and higher while accelerating which is obviously not possible.
... just a matter of poor mechanical link type


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