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What force turns a twintip?

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sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Tue Jun 04, 2019 2:53 pm

I think I know now where the steering wheel is, thanks!
It would be nice to know how the steering wheel works 😂 Still a bit confused as to why loading front foot turns the board downwind. May be given constant slippage sideways the board just weathewanes around/by the rear fins. Like an arrow.

In this case the nose of the board and front fins should stay out of the water even when I load the front a bit, which probably means 60% still on the back foot. Watching the Progression beginner videos the front fins really are out of the water constantly when edging. Could it be as simple as that?
Also the back of the board works like one big fin too, so board might just weathewane by the back of the board, even when one of the front fins is in the water. This would also call for more weight on the back foot though, not 50/50?

When we have say 80% on the back foot the center if gravity is at the back close to center of (horizontal part of) pressure. So no weather waning happens - like an arrow with light head and heavy tail.
May be?
In other words my newest great idea (irony here) is we move center of mass over center of pressure to go straight and move center of mass forward to weathewane board downwind. For this to work center of pressure should not move much. This is not obvious given angle of attack changes too. Watching the videos the front if the board is generally totally out of the water at all times really during edging while back is mostly under water.so center of pressure is probably somewhere under back foot, not in the middle of the board?

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Re: What force turns a twintip?

Postby we » Tue Jun 04, 2019 6:11 pm

A kiteboard doesn't carve like a snowboard. Snowboards and skis are concave. When they flex and edge, the snowboard moves parallel to the curved edge. Kite specific snowboards get rid of the concave edge so that you go straight when you edge. In kiteboarding, weighting the tail, nose or edge doesn't turn the board. Weighting the back foot makes the edge in the water small, and weighting the front foot makes the edge long. With a long edge you can go up wind with less pull from the kite.

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Re: What force turns a twintip?

Postby Matteo V » Tue Jun 04, 2019 6:58 pm

sergei Scotland wrote:
Tue Jun 04, 2019 2:53 pm
Still a bit confused as to why loading front foot turns the board downwind.
If you stand on the board and weight your front foot, where does the pull from the harness go? - It has to move more toward the nose because by shifting your body weight forward, you shift your hips forward. Thus when you are moving, the pull will be at the nose and the nose will get pulled downwind.

But this is not how we primarily steer the board. That did work at subplaning speeds in windsurfing, but once planing, the rails were used to "carve" turns. Funny story on my first planing experience - I leaned the sail back and NOTHING HAPPENED. And I was coming in really close to shore. I had to ask how to steer the windsurfer when planing and was told that I needed to step on one rail or the other.

The reason why kiteboarding does not use moving your body forward back IS because we are always on plane. So even though the physics of your concern is valid, you do not really want to try to steer with your body position. Body position is used to dig the rail, you are not using (at planing speeds) the kite's pull to turn you.

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Re: What force turns a twintip?

Postby sergei Scotland » Tue Jun 04, 2019 9:07 pm

Matteo V wrote:
Tue Jun 04, 2019 6:58 pm
sergei Scotland wrote:
Tue Jun 04, 2019 2:53 pm
Still a bit confused as to why loading front foot turns the board downwind.
If you stand on the board and weight your front foot, where does the pull from the harness go? - It has to move more toward the nose because by shifting your body weight forward, you shift your hips forward. Thus when you are moving, the pull will be at the nose and the nose will get pulled downwind.
Cool, this actually adds to the weatherwane effect I imagined so would make turn downwind faster - at subplaning speeds when waterstarting.
The best explanation I have for now on that - even if this is for subplaning speeds only.
The weather wane effect probably also exists - it is the force which keeps the back from sliding downwind while nose slides under the push of the front foot as you explained. After all push by the back foot is as strong (at 50/50), so push by front foot alone is not enough for this reason?

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Re: What force turns a twintip?

Postby sergei Scotland » Tue Jun 04, 2019 9:28 pm

Matteo V wrote:
Tue Jun 04, 2019 6:58 pm
sergei Scotland wrote:
Tue Jun 04, 2019 2:53 pm
Still a bit confused as to why loading front foot turns the board downwind.
But this is not how we primarily steer the board. That did work at subplaning speeds in windsurfing, but once planing, the rails were used to "carve" turns. Funny story on my first planing experience - I leaned the sail back and NOTHING HAPPENED. And I was coming in really close to shore. I had to ask how to steer the windsurfer when planing and was told that I needed to step on one rail or the other.

The reason why kiteboarding does not use moving your body forward back IS because we are always on plane. So even though the physics of your concern is valid, you do not really want to try to steer with your body position. Body position is used to dig the rail, you are not using (at planing speeds) the kite's pull to turn you.
Absolutely wonderful story. I had no idea!
Still can't visualize this with a flat kiteboard without rocker. I mean I can see how it works on a surfboard or windsurf board because those always have rocker, but can't visualize how edging on a flat kiteboard causes it to go upwind (and conversely edging less causes it to go downwind).
I looked and looked at your picture from your original reply and still don't get it. May be I am getting thick 😂 and too old.
I mean I can see how snowboard or a ski carves on hard snow/ice, but can't visualize this with edge causing turn in water. Rocker yes, but many boards have none.

Obviously edging more creates a bigger /taller "keel" out of the back and centre of the board as board sinks deeper. Is it that this increase in submerged surface is bigger in the middle of the board than at the tail (because if the curves edge) and this causes center of lift/water pressure to move forward? So force pushing up and upwind at say 45 degree angle gets bigger around center of the board quicker (with increase of edging angle) than at the tail thus generating rotating moment around center of mass which is closer to back foot than front foot?
Ha-ha that's a long question!
But if this works as described for whatever reason this is wonderful news to me as this is exactly how a skateboard or snowboard works, does it not?
And obviously I have no reason not to beleive you! It makes an easy job of controlling direction.
Is my description if how it might work any good or do you have a better one? I mean board sinking deeper causing quicker increase of total force pushing upwind at the front than at the back (because of the curved edge)?
In other words center of pressure/lift moves forward with increase of edging angle and moves backward with decrease of edging angle on a board with curved edge. Is this what it is?

This also means that a board with a lot of curve in the edge is more aggressive in turning upwind with edging?

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Re: What force turns a twintip?

Postby sergei Scotland » Tue Jun 04, 2019 10:16 pm

And thinking about sinking a bit more 😂.
The same logic works for a board with straight edge too. Taking an extreme case when most of the board is out of the water with small angle of edge (wait mostly on back foot moving fast enough) - center of pressure is way at the back. Now edge a lot . Board sinks a lot but board edge is not parallel to water surface (front is always up so both fins are visible most of the time). But there is a long new wet stripe along the dioganal of the board going all the way from front to back. The center of pressure of this diaganal is in the middle obviously, thus moving total center of pressure forward. In exreame edging case - almost vertical-most of the board is under water so centre of pressure is bang in the center of the board.
Just trying to visualize movement of centre of pressure with edging. Any good?
I understand theory of planing actually describes it differently but the total result of moving centre of pressure probably holds?

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Re: What force turns a twintip?

Postby Matteo V » Wed Jun 05, 2019 3:37 am

sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
May be I am getting thick 😂 and too old.
Naaa, some of these concepts are just counter intuitive. Then sprinkle some blatantly obvious and completely intuitive stuff, and it gets all mixed up. It is tough to keep it all straight. And sergei, to be honest, I may not be explaining the concepts in as simple a way as they actually exist. Not to mention that there is the possibility that I have some concepts wrong, or that some of these concepts are not actually defined - but rather just understood only as a feeling by even those at higher levels of study.



sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
Still can't visualize this with a flat kiteboard without rocker. I mean I can see how it works on a surfboard or windsurf board because those always have rocker, but can't visualize how edging on a flat kiteboard causes it to go upwind (and conversely edging less causes it to go downwind).
I looked and looked at your picture from your original reply and still don't get it. May be I am getting thick 😂 and too old.
I mean I can see how snowboard or a ski carves on hard snow/ice, but can't visualize this with edge causing turn in water. Rocker yes, but many boards have none.
How you engage an edge for a desired outcome is based on it's shape. And always remember that you have two primary ways of engaging that edge - using your body to rotate the board to point in another direction, or using the rail shape by edging. Different shapes will react differently to each, and to both.
sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
but (I) can't visualize this with edge causing turn in water. Rocker yes, but many boards have none.
Snow is pretty hard on groomer right? I mean on groomed surfaces you really do not sink in more than an inch or so. But water is much softer - you sink all the way. But what happens when you start going fast on water? Does it seem to suddenly get a bit more solid like a groomed slope? And that is where kiteboarding differentiates from windsurfing. In kiting there is no super slow movement where you are actually displacing the water in order to support your weight. It is either "sunk up to your neck" or yanked out up on a pretty much instant plane with no displacement mode. Look up some vids of a snowboard on water. If a snowboarder goes fast enough, the water can support them just like the snow can.



sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
Obviously edging more creates a bigger /taller "keel" out of the back and centre of the board as board sinks deeper. Is it that this increase in submerged surface is bigger in the middle of the board than at the tail (because if the curves edge) and this causes center of lift/water pressure to move forward? So force pushing up and upwind at say 45 degree angle gets bigger around center of the board quicker (with increase of edging angle) than at the tail thus generating rotating moment around center of mass which is closer to back foot than front foot?
Ha-ha that's a long question!
But if this works as described for whatever reason this is wonderful news to me as this is exactly how a skateboard or snowboard works, does it not?
I am at a bit of a loss on this one. I will try to come back to it later if you want me to.



sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
Is my description if how it might work any good or do you have a better one? I mean board sinking deeper causing quicker increase of total force pushing upwind at the front than at the back (because of the curved edge)?
More rocker does produce quicker turns upwind, but more rocker is less efficient at getting upwind on a linear course. This is why almost all TT kiteboards, even park and freestyle boards have less rocker than a wakeboard. Wakeboards have more rocker because the tow point (on a boat) is lower than the tow point of a kite. More rocker helps stop a lower tow point from making the rider "pearl" as easily. It also allows the rider to center their weight on the board more with out pearling. But kiters just cannot sacrifice the upwind capabilities by having that much rocker, for the low (and unneeded) payoff.




sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
This also means that a board with a lot of curve in the edge is more aggressive in turning upwind with edging?
Again, yes! But don't confuse turning upwind (changing directions) with going upwind (making upwind progress). If you do confuse those two, then you would question why kiteboards are flatter and wakeboards are more rockered.
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sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Wed Jun 05, 2019 7:59 am

Thanks Matteo. Your insight on edging is very important to me and something I did not see explained so clearly anywhere.
Whatever the physics I guess you nailed it. I did not realised that actual micro control of a planing board is so similar to a skateboard (and as your example shows to a planing surfboard where sail control becomes secondary at planing speeds as you say) and simple change of edging angle works in such a similar way.
Obviously there are differences - on a skateboard any "edging" is steering wheel constantly turned. On a TT obviously one can add more edge, TT will want to go upwind (because of extra sinking and change in the way water hits it, details are obviously tricky, but I guess it just feels like board "curving" upwind). Now (after adding some edge) a kiter can
1)keep turning upwind or
2)can shift weight forward to adjust position of center of mass to be over the new position of the centre of pressure,which stops turning of the board (steering wheel back to "straight").
I am quit convinced that as you said this (change in edging angle) is the primary way a kiter changes direction and keeps balance.
If course I also agree that the other 2 ways you described are important too and rotating the board with your body is probably used more in wave riding for sudden big 180 turns when there is no pull of the kite and no slippage whatsoever.

This actually raises an interesting question-does edging cause a turning moment on a flattish board going straight downwind?
I'd think no if edge is straight (no curving turns on a door?) , but what about a normally curved flat edged board?
Considering we are still on the plane? Do I just engage the other edge to finish the turn and board turns? I guess answer is yes?

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Re: What force turns a twintip?

Postby sergei Scotland » Wed Jun 05, 2019 9:03 am

Matteo V wrote:
Wed Jun 05, 2019 3:37 am
sergei Scotland wrote:
Tue Jun 04, 2019 9:28 pm
Obviously edging more creates a bigger /taller "keel" out of the back and centre of the board as board sinks deeper. Is it that this increase in submerged surface is bigger in the middle of the board than at the tail (because if the curves edge) and this causes center of lift/water pressure to move forward? So force pushing up and upwind at say 45 degree angle gets bigger around center of the board quicker (with increase of edging angle) than at the tail thus generating rotating moment around center of mass which is closer to back foot than front foot?
Ha-ha that's a long question!
But if this works as described for whatever reason this is wonderful news to me as this is exactly how a skateboard or snowboard works, does it not?
I am at a bit of a loss on this one. I will try to come back to it later if you want me to
As a kid I was doing radio controlled gliders. My instructor was an aero engineer.
He suggested we should design a new glider. He explained how to make sure glider is stable going forward - the center of pressure on a side projection of a glider outline must be on top of center of mass or behind. For a glider on top or very slightly behind is ideal so glider goes straight without any input but turns very easily as soon as there is a small turning force. So we made a drawing of the my preffered outline on paper with square grid and counted number of squares to the front and to the back of required center of mass position (which is known for a given wing profile).
It appeared that my preferred outline had too many squares at the back which meant the glider will be weathewaning too much and will be too difficult to turn. So I decreased the size if the vertical tail to even the numbers. The glider was direction-neutral as a result and was reacting very clearly to entering an area of lift with one wing. When say right wing entered area of thermal lift glider would turn away from the area - turn left-by itself indicating lift to me.
All I needed to do was turn right to enter the area. Glider went up!

This example tought me to think about center of pressure as a point where the force generated by push of air or water is applied.
Now we know that any physical body will rotate /turn if a force is applied at any point but center of the mass. I. E. if we have a bar or a board with a rider and apply a force the item will always rotate unless the force applied goes rught through the centre of the mass. For a bar it is straight in the middle obviously.
So to go straight without turning a kiter must adjust the total centre of mass to be exactly over the centre of pressure.
If centre of pressure moves the board starts turning.
Matteo - obviously you know all of this - I am terrible at explaining the obvious!
What I am trying to say though is that there are 2 ways to turn without using sudden body twists (like a glider) . One is move centre of the mass, the other is move center of pressure (forward or backward).
Obviously as you pointed out edging causes board to turn upwind without changing mass distribution. So there must be a change in the position of center of pressure forward. This seems to be happening because of board sinking more with increase of edging angle. The deeper board sits in the water the more forward center if pressure moves. With board fully underwater it will be obviously in the middle. With front out of the water somewhere closer to the back obviously.
By changing edging angle we control sinking and position of center of pressure.
As the force created by pressure of the water is (partially) directed upwind it causes board to turn upwind when centre of pressure is in front of the centre of mass and downwind when center of mass is in front of centre of pressure.
Just my 2 cents.
I guess one does not need to know all of this to ride a kite, but I hope it will help me to stop my board going upwind too much next time I attempt a waterstart and my board starts curving upwind too much by itself. As you said I need either or both - move center of the mass forward and/or decrease of edging angle to move center of pressure backward. Now I know why, at least in my head!
Edit :moving center of mass forward when board is almost flat on the water can probably move centre of pressure forward too by board making more contact with water along the length of the board. Which might just eliminate effect of moving centre of mass forward. Tricky.

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Re: What force turns a twintip?

Postby iriejohn » Wed Jun 05, 2019 9:09 pm

Nobody has mentioned "deflection".


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