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The Foil Zone => Wingsurfing, Windfoiling, Wingfoiling, Wing SUP => Topic started by: Phils on October 07, 2020, 04:14:55 AM

Title: wind vs current
Post by: Phils on October 07, 2020, 04:14:55 AM
For some reason, I am confused about this.

For kitesurfing, the conventional thinking has been that when the wind and current are in opposite directions, you get more effective "wind power".  Is that still true with a foil?

If I am foiling upwind with a wing or a kite, won't a current traveling WITH the wind give my foil more lift?
Title: Re: wind vs current
Post by: Solent Foiler on October 07, 2020, 04:58:31 AM
For kitesurfing, the conventional thinking has been that when the wind and current are in opposite directions, you get more effective "wind power".  Is that still true with a foil?


Correct. The foil makes no difference. The ONLY significant impact current has is to change wind speed and direction.

The best way to think about it is to take the wind out of the equation. In zero wind you'll just drift on the current and you'll feel the apparent wind of that drifting. That's the only effect there is. There is no impact of the current on the foil.

Put wind back in and the wind you feel will be the vector sum of the drifting apparent wind and the weather wind. Start foiling, and you add the apparent wind of that movement to the vector sum as well.
Title: Re: wind vs current
Post by: PonoBill on October 07, 2020, 06:23:16 AM
Absolutely correct as far as the wind speed goes. But for foil lift when you turn into the current its speed across the foil wing adds to the speed of the board as a vector sum, so you get more lift for less speed. The Gorge is one big current vs. wind laboratory, especially in the spring when current in the river just rips. You can point upriver and be almost standing still while you fight to keep your foil in the water.
Title: Re: wind vs current
Post by: VB_Foil on October 07, 2020, 06:24:23 AM
I find it is generally much harder to get on foil if the wind and current are going in the same direction.  You end up trying to fight the loss in apparent wind.  Upping the wing/foil size is a good call.  Staying upwind is also very difficult in these situations. 
Title: Re: wind vs current
Post by: Solent Foiler on October 07, 2020, 07:07:45 AM
Absolutely correct as far as the wind speed goes. But for foil lift when you turn into the current its speed across the foil wing adds to the speed of the board as a vector sum, so you get more lift for less speed. The Gorge is one big current vs. wind laboratory, especially in the spring when current in the river just rips. You can point upriver and be almost standing still while you fight to keep your foil in the water.

Hmm - I don't understand how that can be. How does the foil know if it pointing upstream or down except for the impact of the wind on it? The foil doesn't know its speed over ground...

An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind.

Title: Re: wind vs current
Post by: Califoilia on October 07, 2020, 10:23:53 AM
Hmm - I don't understand how that can be. How does the foil know if it pointing upstream or down except for the impact of the wind on it? The foil doesn't know its speed over ground...

An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind.
Same way this plane doesn't know what's causing the air/wind to go over its wings but is still able to "takeoff"....
https://www.youtube.com/watch?v=IPOtDPHjW-Y
...the foil wing doesn't know what's causing the water to go over its wing at whatever speed to liftoff in.
Title: Re: wind vs current
Post by: tarquin on October 07, 2020, 10:44:36 AM
I was lucky and got a ride on one of the coaching ribs to watch the GC 32 foiling cats in the solent years ago. They figured out pretty quickly when the wind was light they could turn into the current to get foiling quicker.
 The first boat to do it won that race by miles. Then the others did the same.
Title: Re: wind vs current
Post by: Solent Foiler on October 07, 2020, 10:55:04 AM
Hmm - I don't understand how that can be. How does the foil know if it pointing upstream or down except for the impact of the wind on it? The foil doesn't know its speed over ground...

An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind.
Same way this plane doesn't know what's causing the air/wind to go over its wings but is still able to "takeoff"....
https://www.youtube.com/watch?v=IPOtDPHjW-Y
...the foil wing doesn't know what's causing the water to go over its wing at whatever speed to liftoff in.

But that proves my point. The only thing the foil sees is flow over it, current or no current. Just because you're heading against the current, the foil doesn't know this so won't generate more lift. The only thing driving the foil is the wind, the speed of the current that the foil is working in is irrelevant save for the impact on the apparent wind. The frame of reference is the water it's in, not the movement of the water over the land.

I repeat above "An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind."
Title: Re: wind vs current
Post by: PonoBill on October 07, 2020, 12:21:27 PM
The foil doesn't know anything, it's just a chunk of carbon. But if you're in a five knot current, floating along with the current there is no flow over the foil. Now you grab a rope to hold you in place and there is five knots over the foil--you'll pop right up--river foilers do that all the time. Instead of a rope we have a wing and enough wind blowing against the current (say 10kts at 50% wing efficiency) to hold us still relative to the current. Once again, there are five knots over the foil and up you go even though your forward speed relative to the land is 0.

If the wind is blowing the same direction as the current and you head downwind, the current has you moving at five kts and the wind is blowing 10kts. The windspeed relative to you is now 5kts and the water flow over your foil is 0 mph. Assuming the same wind wing efficiency (50%) the most water flow you can produce across the foil is 2.5 kts, but every knot you go faster than the current is subtracted from the wind speed, so you won't get anywhere near that amount of flow over the foil wing.

Of course, actually everything is moving at the speed of light, but we'll leave that explanation for next time.

An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind.

Some day I'll figure out why so many people believe that's meaningful--even some pilots. It's silly. To have the same lift in a 50 knot tailwind the plane has to have 150kts ground speed. Conversely in a 50kts headwind to get the same lift it goes 50kts ground speed. Yes. the airspeed is the same. And your point is???
Title: Re: wind vs current
Post by: Califoilia on October 07, 2020, 12:30:35 PM
Yes Pono, better explanation wrt water flowing over the wing when pointed upstream vs downstream. Sometimes seeing is believing...
https://www.youtube.com/watch?v=_0_bnsVUuFM

Edit: That looks really fun btw. ;D
Title: Re: wind vs current
Post by: bigmtn on October 07, 2020, 12:34:01 PM
how often do you see planes take off going downwind?
Title: Re: wind vs current
Post by: Phils on October 07, 2020, 01:00:37 PM
If I am on a river trying to kite a foil upwind, I want the wind and current to be in the SAME direction.

If I am on a river trying to kite a surfboard upwind, I want the wind and current to be in the OPPOSITE direction.

Is that correct?

Bill, to be more precise, we are all moving through Space and Time at the speed of light.
Title: Re: wind vs current
Post by: Solent Foiler on October 07, 2020, 01:08:47 PM
The foil doesn't know anything, it's just a chunk of carbon. But if you're in a five knot current, floating along with the current there is no flow over the foil. Now you grab a rope to hold you in place and there is five knots over the foil--you'll pop right up--river foilers do that all the time. Instead of a rope we have a wing and enough wind blowing against the current (say 10kts at 50% wing efficiency) to hold us still relative to the current. Once again, there are five knots over the foil and up you go even though your forward speed relative to the land is 0.

If the wind is blowing the same direction as the current and you head downwind, the current has you moving at five kts and the wind is blowing 10kts. The windspeed relative to you is now 5kts and the water flow over your foil is 0 mph. Assuming the same wind wing efficiency (50%) the most water flow you can produce across the foil is 2.5 kts, but every knot you go faster than the current is subtracted from the wind speed, so you won't get anywhere near that amount of flow over the foil wing.

Of course, actually everything is moving at the speed of light, but we'll leave that explanation for next time.

An aeroplane flying at 100 knots airspeed has the same lift if its flying with a 50 knot headwind or 50 knot tailwind.

Some day I'll figure out why so many people believe that's meaningful--even some pilots. It's silly. To have the same lift in a 50 knot tailwind the plane has to have 150kts ground speed. Conversely in a 50kts headwind to get the same lift it goes 50kts ground speed. Yes. the airspeed is the same. And your point is???

I'm sorry - you've lost me. Everything you said I agree with, and doesn't contradict anything I've said, I don't think.

What I'm disagreeing with is your statement "But for foil lift when you turn into the current it's speed across the the foil wing adds to the speed of the board as a vector sum..."

What I'm reading that as saying is that an aeroplane flying at the same airspeed needs to adjust the amount of lift depending on whether it's flying into a headwind or not, which isn't true. My analogy isn't silly - it's demonstrating that the current in which the foil is working does not impact the amount of lift it produces - only the amount of drive can do that (assuming the foil keeps its shape).

Water current can only influence drive through the apparent wind it produces.

Two examples:
1) foiling on a lake in 20 knots
2) foiling on a river in 15 knots wind against 5 knots current

Both scenarios a identicle from a physics perspective. The presence of current has no effect as the only difference between the example is the movement of water over land, and the presence of land cannot change the amount of drive presented to the foil.   
Title: Re: wind vs current
Post by: Solent Foiler on October 07, 2020, 01:13:10 PM
how often do you see planes take off going downwind?

Not often!  ;)
Title: Re: wind vs current
Post by: PonoBill on October 07, 2020, 03:44:15 PM
Bill, to be more precise, we are all moving through Space and Time at the speed of light.

Yup. Even more correctly, through spacetime.
Title: Re: wind vs current
Post by: SimonP on October 07, 2020, 03:52:57 PM
Wind against current increases lift regardless whether you are on a foil or a displacement craft. You can point higher upwind and point lower downwind without stalling.
I got stuck in a gybe the other day perfectly balanced for several seconds in a current pointing directly downward but going nowhere. It's a very odd but cool feeling.
Title: Re: wind vs current
Post by: foiled again on October 07, 2020, 08:12:10 PM
"Water current can only influence drive through the apparent wind it produces."
 
This is definitely true. If you only have a foil board and no hand wing and you point it into the current, you are not going to get any lift, you are just going to get sent down stream because there is no hand wing to utilize the apparent wind that is produced. 

If you want to understand this from the layman's perspective, read the holy grail for all airplane pilots called "Stick and Rudder: An Explanation of the Art of Flying" by Wolfgang Langewiesche first published in 1944 and still available in bookstores everywhere.
Title: Re: wind vs current
Post by: LaPerouseBay on October 07, 2020, 09:35:44 PM
Bill, to be more precise, we are all moving through Space and Time at the speed of light.

Yup. Even more correctly, through spacetime.

Hmmm...  That spacetime taradiddle refers to a sum total in 4 dimensions.  I'm aging, so I'll stick to 3.   

Michael puts a number on how we all move thru space, in kph. 

https://youtu.be/IJhgZBn-LHg
Title: Re: wind vs current
Post by: Admin on October 08, 2020, 02:16:12 AM
The Gorge is a perfect place to feel this firsthand because we get Easterlies (wind and water same direction) and Westerlies (wind and water opposite directions). 

On a foil you can get foiling a bit quicker in an opposing current when the current is strong (but it is very subtle, not enough to choose a different wing size).  When the current is light the difference becomes pretty negligible.  It does feel very cool though when you hit a pocket of strong opposing current going downwind at low speed and get suspended. 

The Event site in the springtime is a perfect test spot for this because you have a sandbar protected area inside of no current, then an opposing current in the channel, then a large eddy with matched current.  Aside from crossing the eddy lines which will make your foil jump every time you can feel the difference as you pass from matched flow to counterflow or no flow to counterflow.  Pretty cool.
Title: Re: wind vs current
Post by: foiled again on October 08, 2020, 09:25:33 AM
We have kind of the same thing, as that described by Admin,  in San Francisco Bay by the Golden Gate towards the north side, especially within about an hour each side of slack. We call it "Voodoo Chop". It helps to make the whole experience unique and fun.
Title: Re: wind vs current
Post by: PonoBill on October 09, 2020, 05:54:28 PM
Hmm...  That spacetime taradiddle refers to a sum total in 4 dimensions.  I'm aging, so I'll stick to 3.   

Michael puts a number on how we all move thru space, in kph. 

https://youtu.be/IJhgZBn-LHg

Cool video, thanks for that. I never heard of this guy but I subscribed.

Spacetime is easy to experience even for geezers. Toss a ball--it's a three dimensional object moving through time. The complexities start when you realize that time is different for every object. It varies with relative velocity and position in a gravitational field. It really is just a dimension, not a static thing. In fact at the quantum level it winds up dropping out of virtually every equation. It's reasonably accurate to say it doesn't exist at all, and if it doesn't exist in the waves (or particles if you insist on being classical) then how could it exist on the macro scale which simply consists of a LOT of quantum stuff. It's an emergent phenomenon.

For those interested in having their view of reality permently bent, twisted, stapled and mutilated I recommend Carlo Rovelli's "The order of Time" -- or for that matter anything else Carlo has written.
Title: Re: wind vs current
Post by: LaPerouseBay on October 09, 2020, 09:28:07 PM
Hmm, I usually sidestep the quantum stuff.  Has any of that been proven yet?  Or is it all theoretical?  A quick search and all I found was 'reality is what you make of it.'  K. 

Anyhow, I had recently watched parts of the below linked video.  Also, years ago, Michael's zooming earth...

When you guys were claiming that "we all move at the speed of light" thru spacetime, something didn't seem right.  So, I had to google it.

In the words of Lewis Carroll Epstein:
“You can’t go faster than the speed of light, because you can’t go slower than the speed of light. You are always going the speed of light through spacetime. If you use some of your speed to go through space then there is less speed through time.” 

So, I used Epstein and Professor Green's intuitive explanation for time dilation to conclude that: your claim that we are all moving the speed of light thru spacetime is incorrect.

Because: According to Dr. Greene, motion affects time (and vice versa).   

Since I'm aging, I must be traveling slower than the speed of light, thru spacetime.  It sounds like a zero sum game in 4 dimensions to me.  Change one, the others adjust...

That said, living in 2 dimensions, as a 'flatlander' never made no damn sense to me neither. 

https://youtu.be/XFV2feKDK9E?t=9241
Title: Re: wind vs current
Post by: Phils on October 10, 2020, 12:50:33 AM


Because: According to Dr. Greene, motion affects time (and vice versa).   

Since I'm aging, I must be traveling slower than the speed of light, thru spacetime.  It sounds like a zero sum game in 4 dimensions to me.  Change one, the others adjust...



You are correct in that motion does affect time but that does not invalidate the statement that we exist in spacetime at the speed of light.

Motion affects time in that as you move faster, time slows down for you.  (this is correct mathematically and has been proven with atomic clocks on airplanes)  My conclusion is get a faster foil and live longer.
Title: Re: wind vs current
Post by: PonoBill on October 10, 2020, 07:12:20 AM
Hmm, I usually sidestep the quantum stuff.  Has any of that been proven yet?  Or is it all theoretical?  A quick search and all I found was 'reality is what you make of it.'  K. 

You are typing on a device that could not work unless quantum stuff was real. Enjoying the heat from a sun that would not ignite and continue it's thermonuclear burn unless quantum tunnelling was real.  If it wasn't true then electrons would spin down into the nucleus of the atoms that form you and turn all the protons to neutrons. Atoms would have a tiny volume and you would be part of a small black hole that used to be our solar system. So yup, it's way more than just theoretical. Even though it's been central to science since the turn of the last century no one REALLY understands the why of it--that's at least partly why it's called quantum mechanics. We've got the operating manual, but no explanation of why it works this way. But all the resultant equations work with a level of effectiveness beyond classical physics, which is always inaccurate or totally incorrect at small or large scales. Even really weird stuff like quantum entanglement has ben demonstrated at our scale, with experiments that work shockingly well.

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