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What does apparent wind really bring ?

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

Postby Regis-de-giens » Wed Mar 04, 2020 2:43 am

Hi,
Some thougths about apparent wind, mainly on hydrofoiling, to discuss with you ... and challenge some frequent assumptions i can read here and here, which I do not fully agree with...

Apparent wind (which as you know is due to rider own speed that adds relative wind upstream the kite), has strange and "whaou" effects on your ride, like tensioning your kite line so much higher that you feel the wind has grown up instantly ! But when you stop in the water.... then be carreful : the real wind is still light, when you need to come back if you lost the planning).

Ok, great feeling that we all love, but what i would like to "challenge" is the commonly used argument that this apparent wind is a major parameter to rider's speed, and that race kite efficiency is based on "using" this apparent wind.

I do not agree fully with this. To be a bit provocative to set my point (but not that much...) i would say that apparent winds does not help at all on your final speed .... :bye: you get my point ?

(Ok, do bot beat me on the teeth... :allbegood: we're here to discuss when not riding... 8)

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Re: What does apparent wind really bring ?

Postby PullStrings » Wed Mar 04, 2020 2:53 am

Apparently it brings more wind power to the kite as you move faster making you go even faster in return and so on and so on

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Re: What does apparent wind really bring ?

Postby Jyoder » Wed Mar 04, 2020 3:36 am

i would say that apparent winds does not help at all on your final speed .... :bye: you get my point ?
I don’t understand your statement. What is final speed? Apparent wind is a product of speed, not a cause. How can it help or not? It just exists.

Are you thinking of the changes in kite airspeed, Angle of Attack, lift, and drag?

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Re: What does apparent wind really bring ?

Postby foilholio » Wed Mar 04, 2020 5:05 am

Well apparent wind changes the wind speed and direction. You can change the apparent wind of the rider or kite by changing the riders speed but you can also change the apparent wind of the kite by flying it around. You can change apparent wind so much as to make it come in the complete opposite direction, by say riding a wave really fast down wind in light wind. And you can also generate apparent wind from no wind by say running with the kite.

Now depending on the direction of the wind to the kite sets it's AOA and then the speed of this wind in combination with the size and shape of the kite sets the power it produces. The principle is the faster a kite goes and the more AoA or camber it has the more power it makes. The complication is the more AoA or camber it has it tends to go slower. The optimum is the maximum speed with a little AoA or camber gives the most power. You can be confused when oversheeting the kite or sheeting it heavily you will get even more power for a short while but you are likely just exchanging kite inertia for kite pull(power).

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Re: What does apparent wind really bring ?

Postby Herman » Wed Mar 04, 2020 11:09 am

Regis-de-giens wrote: Ok, great feeling that we all love, but what i would like to "challenge" is the commonly used argument that this apparent wind is a major parameter to rider's speed, and that race kite efficiency is based on "using" this apparent wind.

I do not agree fully with this. To be a bit provocative to set my point (but not that much...) i would say that apparent winds does not help at all on your final speed .... :bye: you get my point ?
Apparent wind neither helps nor hinders it just "is". It is just an inevitable consequence of the aerofoil having velocity. The resultant forces will depend on how the rider sets the aerofoil in the actual flow at the aerofoil.... simple as!! Or is it just me!! (& Jyoder)

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

Postby Regis-de-giens » Wed Mar 04, 2020 11:38 am

yes everybody is a bit right (IMO); I suggest we simplify and center the discussion by removing the A0A and acmaber things ; indeed the pilot may have to adapt the AoA during the rid when apparent wind appear , but that is not my point.

This topic could sound obvious for some of you, but it is ti share our point of view.

My point was that a lot ( yes a lot) of people think that apparent wind helps to gain speed. The faster you go, the higher wind, the faster you go , ect .... But not (or not significantly) and this last sentence is a wrong view to me ; it is more a "feeling" because kite pulls harder .

I would try to say it with my words.:
- Foilholio is right : assuming a transverse ride 90 degree of the wind, the total wind in the kite increases but also it changes its direction

- unfortunately this direction change is against the rider speed: the new window is more backward than in static (why some people wrongly say the kite is in the power zone: NO : in straight line and kite stable, the kite is always at the edge of the wind window, only the window has turned backward

- consequence is that the angle of traction is more downwind since the window is more downwind ; Whereas the effective traction used to build rider speed is the projection of this pulling force along the rider direction (Force * cosinus of the angle)

- so the angle increases, the cosinus decreases and the effective traction decreases ; so kite pulls harder in the harness (square to total wind) but only a more part of this pull is transformed to speed ...and this part is very close (in value) of the real wind , i.e. without apparent wind.

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Re: What does apparent wind really bring ?

Postby Herman » Wed Mar 04, 2020 12:03 pm

Apparent wind does not help you go faster and, arguably, it does not exist as a separate entity in real life. Apparent wind is just an expression that is used for the relevent vector in the mathematical analysis. It is the very fundamental aspect of the thinking that apparent wind exists as an entity that can make you go faster that is flawed imho.

The actual flow over the aerofoil is what counts and this is the summation of the wind and apparent wind vectors. Once you have added the 2 vectors you can look at AOA and the force vectors generated by the wing.

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Re: What does apparent wind really bring ?

Postby Onda » Wed Mar 04, 2020 1:05 pm

Obviously, no one of you have ever watched a skilled race foiler in low wind, say 8 kts.
These guys easily travel with 20 kts speed almost downwind in such conditions. Much much faster than the wind speed is. It looks surreal at first and totally counter-intuitive. "How can that kite stay in the air?!"
But when you start to contemplate on apparent wind, it becomes obvious.

Apparent wind certainly exists in reality, and it certainly helps you to increase your riding speed, because for the kite it is the "real" wind (also for the rider, by the way). It is simply the geometric sum of the static wind and your speed of movement taking into account the angle of your movement.
To eliminate the "side pull" effect (mentioned above), the rider has to point more and more downwind, the faster he gets. The only limiting factor that prevents to increase speed limitless is the drag of the kite.

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Re: What does apparent wind really bring ?

Postby tomtom » Wed Mar 04, 2020 1:23 pm

Its simple not true about more and more downwind - you very quickly loose pressure this way as soon as your direct downwind VMG start approach real wind speed - which ALWAYS is your only source of energy.
As soon as you loose this power gradient between your kite and wind you are done. No ammount of dragless kite help you.

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Re: What does apparent wind really bring ?

Postby tomtom » Wed Mar 04, 2020 1:27 pm

And actually if you have really drag less kite you can go infinite speed - but only on reach course - because on every other course you downwind VMG will exceed wind speed


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