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What does apparent wind really bring ?

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tomtom
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Re: What does apparent wind really bring ?

Postby tomtom » Wed Mar 04, 2020 1:28 pm

And racers can go so fast downwind because they use BIG kites and they can harness every tiny bit of gradient between the wind and they downwind VMG

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Re: What does apparent wind really bring ?

Postby Herman » Wed Mar 04, 2020 1:59 pm

It is easy to get lost in the semantics of describing the concept, "arguably does not exist as a separate entity" was not intended to mean it does not exist, for example! It was to infer that it is a consequence rather than a driver, Ice sailing provide extreme examples etc. and that is loads of fun!!!!!!! The drivers are the aerodynamic forces, the cat 🐈 in the box is ........

If you are really interested in the subject a team have built a machine that can go dead down wind faster than the wind. The presentation of the rig design is very thorough and was on this forum. The presentation includes analysis of the apparent wind in a variety of ways. Recommended viewing and worth searching for it if interested in apparent wind.

Unfortunately the physics still preclude reaching infinite speeds or perpetual motion!!!!

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Re: What does apparent wind really bring ?

Postby Regis-de-giens » Wed Mar 04, 2020 3:11 pm

Well, thank you all for your active participation and different views; this is exactly where I wanted to bring the discussion ! does the apparent wind generate any usable additional pull for the kiter !

The kite sees more wind but at a less efficient traction angle.

As an introduction, rider speed is not limited to wind speed as every hydrofoiler has noticed . BUT this is not linked to apparent wind ; it is just a matter of the force that you can get from the wind (i.e. area, AoA, ect) vs the drag that you generate ( kite and rider and hydrofoil) ; when the two are balanced, you are at the equilibrium, whatever if your speed is higher or lower than the wind, there is no shreshold (except psychologically).

The DDWFTW machine (direct downwind faster than wind) is not the topic here and has nothing to do with apparent wind ; it is just a kind of mechanical transfert that takes a part of the movement to push the wing surface a bit upwind, so that the wing surface having a backward speed is still in opposition to wind even if the kart goes downwind faster than wind. Anyway, nothing related to the help of apparent wind (it also work without apparent wind) .

Then, as some have reported here, the apparent wind (component of the total wind that the kite sees) generates mainly a force perpendicular to the rider speed, hence it burn legs but do not generate speed ; actually it does ... a little bit when you look at the math :it is a "fight" between the pull increase (proportional to wind speed^2) and projection resultant decrease (1 - tangeant(real wind / apparent wind ) if I remember well my calcs years ago. Hence there is an impact, depending on rider angle vs wind, but it is in the range of only 20% of the effective pull without apparent wind , while your legs (your feeling) are square dependant on apparent wind increase.

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Re: What does apparent wind really bring ?

Postby neilhapgood » Wed Mar 04, 2020 3:26 pm

Great thread, really interesting to someone like me with a somewhat unscientific mind.

I am wondering does the benefit of apparent with vary depending on wind strength? In its simplest form is apparent wind essential to ride in light wind and of no use once you have 10 knots plus, (unless going deep downwind)

Thanks

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Re: What does apparent wind really bring ?

Postby Faxie » Wed Mar 04, 2020 3:29 pm

neilhapgood wrote:
Wed Mar 04, 2020 3:26 pm
Great thread, really interesting to someone like me with a somewhat unscientific mind.

I am wondering does the benefit of apparent with vary depending on wind strength? In its simplest form is apparent wind essential to ride in light wind and of no use once you have 10 knots plus, (unless going deep downwind)

Thanks
The harder it blows, the less relevant apparent wind becomes. But I wouldn't say the turning point is at 10 knots... more like 25-30. It's really just a calculation of vectors between real wind speed and direction, plus board speed and direction.

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Re: What does apparent wind really bring ?

Postby Herman » Wed Mar 04, 2020 4:07 pm

Regis-de-giens wrote:
Wed Mar 04, 2020 3:11 pm

The DDWFTW machine (direct downwind faster than wind) is not the topic here and has nothing to do with apparent wind ; it is just a kind of mechanical transfert that takes a part of the movement to push the wing surface a bit upwind, so that the wing surface having a backward speed is still in opposition to wind even if the kart goes downwind faster than wind. Anyway, nothing related to the help of apparent wind (it also work without apparent wind) .
Either you have seen a different presentation or you really do not understand the fundamentals of the physics involved To put it in simple terms: apparent wind brings f,##k all but believe what works for you in order to visualise how to get the most out of your riding.

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Re: What does apparent wind really bring ?

Postby PullStrings » Wed Mar 04, 2020 4:27 pm

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Regis-de-giens
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Re: What does apparent wind really bring ?

Postby Regis-de-giens » Wed Mar 04, 2020 4:41 pm

Dear Herman,
If you speak of the DDWFTW machine, there is a topic for this.
If you speak of apprarent wind efficiency, please kindly be more accurate and constructive than f,**k for everyone's interest

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Re: What does apparent wind really bring ?

Postby Herman » Wed Mar 04, 2020 5:23 pm

Régis-de-glens

Forgive me for the use of the expurgated vernacular but I found your dismissal of the DDTWFWT as irrelevant to apparent wind too insulting to the original thesis on which it was based. The device deserves to be appreciated for its aerodynamics which are a wonderful exploitation of apparent wind.

Regards Herman

Regis-de-giens
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Re: What does apparent wind really bring ?

Postby Regis-de-giens » Wed Mar 04, 2020 6:02 pm

I fully agree with you on the wonderfull concept , and I am a big big big fan of it which I promote a lot around me and which I discovered really astonished; I like it so much that I thought about it a lot, confused by people trying to define if it is a propulsor or not . It is just that I do not think it works thanks to apparent wind, it just works and not disturbed by apparent wind which is very different and amazing in direct downwind. And off course I may (like any other engineer or curious man) mistaken. No offense on my side at all, I can promise !

Back to the apparent wind impact: I'll try tomorrow to recover my calculations to illustrate.


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