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Author Topic: wind vs current  (Read 1434 times)

Phils

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wind vs current
« 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?

Solent Foiler

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Re: wind vs current
« Reply #1 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.
I'm 5'10", 65kg riding:
Gong Flint 5'6 95L
Gong Catch 5'3 34L
Gong Veloce L and XL on 100 monoblock mast
Gong Pulse - 4m, 5m
Gong Superpower 6m

PonoBill

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Re: wind vs current
« Reply #2 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.
« Last Edit: October 07, 2020, 06:27:53 AM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

VB_Foil

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Re: wind vs current
« Reply #3 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. 
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Solent Foiler

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Re: wind vs current
« Reply #4 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.

« Last Edit: October 07, 2020, 07:54:31 AM by Solent Foiler »
I'm 5'10", 65kg riding:
Gong Flint 5'6 95L
Gong Catch 5'3 34L
Gong Veloce L and XL on 100 monoblock mast
Gong Pulse - 4m, 5m
Gong Superpower 6m

SanoSlatchSup

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Re: wind vs current
« Reply #5 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.
Me: 6'1"/185...6'0" Kings Foil Board...6'0" Chelu Foil Board...9'6" Bob Pearson "Laird Noserider"...9'6" Costa Azul Wide Body...14' Lahui Kai "Manta"...8'0" WaveStorm for when proning urges still hit, and 7'3" Chuck Glynn foil board backup.

tarquin

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Re: wind vs current
« Reply #6 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.

Solent Foiler

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Re: wind vs current
« Reply #7 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."
I'm 5'10", 65kg riding:
Gong Flint 5'6 95L
Gong Catch 5'3 34L
Gong Veloce L and XL on 100 monoblock mast
Gong Pulse - 4m, 5m
Gong Superpower 6m

PonoBill

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Re: wind vs current
« Reply #8 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???
« Last Edit: October 07, 2020, 12:27:51 PM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

SanoSlatchSup

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Re: wind vs current
« Reply #9 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
« Last Edit: October 07, 2020, 12:32:43 PM by SanoSlatchSup »
Me: 6'1"/185...6'0" Kings Foil Board...6'0" Chelu Foil Board...9'6" Bob Pearson "Laird Noserider"...9'6" Costa Azul Wide Body...14' Lahui Kai "Manta"...8'0" WaveStorm for when proning urges still hit, and 7'3" Chuck Glynn foil board backup.

bigmtn

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Re: wind vs current
« Reply #10 on: October 07, 2020, 12:34:01 PM »
how often do you see planes take off going downwind?

Phils

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Re: wind vs current
« Reply #11 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.

Solent Foiler

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Re: wind vs current
« Reply #12 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.   
I'm 5'10", 65kg riding:
Gong Flint 5'6 95L
Gong Catch 5'3 34L
Gong Veloce L and XL on 100 monoblock mast
Gong Pulse - 4m, 5m
Gong Superpower 6m

Solent Foiler

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Re: wind vs current
« Reply #13 on: October 07, 2020, 01:13:10 PM »
how often do you see planes take off going downwind?

Not often!  ;)
I'm 5'10", 65kg riding:
Gong Flint 5'6 95L
Gong Catch 5'3 34L
Gong Veloce L and XL on 100 monoblock mast
Gong Pulse - 4m, 5m
Gong Superpower 6m

PonoBill

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Re: wind vs current
« Reply #14 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.
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

 


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