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What force turns a twintip?

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Herman
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Re: What force turns a twintip?

Postby Herman » Thu Jun 06, 2019 9:17 am

Hi sergie! When thinking of turning moments applied to static riding you may find it helpful to analyse it around the centre of lateral resistance of the board, ie. the board is not a foil or wing, particularly true for flat twin tips. The force applied to the board is a what goes through the feet, a combination of mg and kite pull. More force on the aft foot will create a greater turning moment into wind. Carving turns involve a different set of forces.
Lateral resistance for tt.: Hold it in water in ridden position, push on nose it bears away, push on tail it hardens up!

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Re: What force turns a twintip?

Postby Matteo V » Thu Jun 06, 2019 1:41 pm

Herman wrote:
Thu Jun 06, 2019 9:17 am
Hi sergie! When thinking of turning moments applied to static riding you may find it helpful to analyse it around the centre of lateral resistance of the board, ie. the board is not a foil or wing, particularly true for flat twin tips. The force applied to the board is a what goes through the feet, a combination of mg and kite pull. More force on the aft foot will create a greater turning moment into wind. Carving turns involve a different set of forces.
Lateral resistance for tt.: Hold it in water in ridden position, push on nose it bears away, push on tail it hardens up!
Excellent way to illustrate the concept! And edging is definitely not foiling (as in horizontal push from a foiled fin).

The only problem with this suggestion is that it is illustrating "displacement mode" instead of "planing mode". When planing, the application of force to a particular part of the board is of minor significance, compared to the power generated from the rail (edge) moving at high speed through the water.

While this way of putting it is butchering the real physics at hand, think of speed suddenly making the water more viscous. Slowing down makes the water less viscous.

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Re: What force turns a twintip?

Postby sergei Scotland » Thu Jun 06, 2019 4:35 pm

Herman wrote:
Thu Jun 06, 2019 9:17 am
Hi sergie! When thinking of turning moments applied to static riding you may find it helpful to analyse it around the centre of lateral resistance of the board, ie. the board is not a foil or wing, particularly true for flat twin tips. The force applied to the board is a what goes through the feet, a combination of mg and kite pull. More force on the aft foot will create a greater turning moment into wind. Carving turns involve a different set of forces.
Lateral resistance for tt.: Hold it in water in ridden position, push on nose it bears away, push on tail it hardens up!
Thanks Herman!
IMHO "more push on the front foot" ="move center of mass forward". For example if a human had a shape of a ball and did not have legs he would roll to the front of the board and board would go downwind. Having 2 legs is just a convenience 😂
In physics they say that body turns when force (or better sum of all forces) applied does not go through the center of mass. IMHO what we do is we move center of mass. If we were lying on the board we would need to crawl front.
I totally agree to your way of explaining it though. Your explanation is equivalent to moving centre of mass forward, just has more details in it as to how to do it, thanks.
And I do not quit agree with Matteo here that it does not apply to planing. Laws of physics do not change.
My latest greatest idea is that while planing moving weight also moves centre of pressure in the same direction (angle of attack is smaller when we load front).
The move of centre of pressure I imagining might just make moving center of mass to be a less efficient method while planing. I mean move center if mass forward -20cm might cause move of the center of pressure forward by say 16cm, so not much change in turning moments happens.
This is if Matteo is right about loading front or back not being main way of turning TT while planing. I do not have enough experience but his windsurfing example is really convincing!

sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Thu Jun 06, 2019 4:49 pm

Matteo V wrote:
Thu Jun 06, 2019 1:41 pm
Herman wrote:
Thu Jun 06, 2019 9:17 am
Hi sergie! When thinking of turning moments applied to static riding you may find it helpful to analyse it around the centre of lateral resistance of the board, ie. the board is not a foil or wing, particularly true for flat twin tips. The force applied to the board is a what goes through the feet, a combination of mg and kite pull. More force on the aft foot will create a greater turning moment into wind. Carving turns involve a different set of forces.
Lateral resistance for tt.: Hold it in water in ridden position, push on nose it bears away, push on tail it hardens up!
Excellent way to illustrate the concept! And edging is definitely not foiling (as in horizontal push from a foiled fin).

The only problem with this suggestion is that it is illustrating "displacement mode" instead of "planing mode". When planing, the application of force to a particular part of the board is of minor significance, compared to the power generated from the rail (edge) moving at high speed through the water.

While this way of putting it is butchering the real physics at hand, think of speed suddenly making the water more viscous. Slowing down makes the water less viscous.
Thank Matteo. As for significance of changing load - see above what I came up with (irony on myself here). Does this sound familiar or plausible?
IMHO you are right.
I had another bright idea as well. It might help to forget the pull of the kite and consider rider on a plane without any external forces applied. Rider got to some decent speed and got planing (may be a motorboat like they do in surfing huge waves).
Now - if i load front or back in this situation I will obviously not turn.
If I edge I will, even if I am riding a flat door and I suspect even if I am riding a rectangular door (lightwind TT with straight edge).
What force turns me?
I think if we can understand this we will understand everything.

Actually people here suggested this approach earlier, I should have paid more attention.!

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Re: What force turns a twintip?

Postby knotwindy » Thu Jun 06, 2019 5:31 pm

sergei Scotland wrote:
Mon Jun 03, 2019 8:56 am
Question for someone who knows physics of TT planing:
there is not an obvious force turning TT downwind. Upwind is easy as kite rotates you upwind all the time through the hook plus any rocker does the same. On the other hand it is not obvious which physical force actually rotates board to go downwind especially during waterstart and transitions but also simple changing directions for example a carving turn downwind into toeside.
Anyone can explain? Ta!
Complicate things much?
If you know what makes a tt go upwind
And you decrease that same force what happens?
It goes less upwind.
What happens if you decrease it further?
It goes straight.
What happens if you decrease it further? :o
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Herman
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Re: What force turns a twintip?

Postby Herman » Thu Jun 06, 2019 6:03 pm

I realise referring to static lateral resistance is a major over simplification as the board will generate pressures as it has an angle of attack to the water due to edging and angle of leeway etc. I only offered this model of thought to help visualise the area of the board that is in contact with the water. The centre of lateral resistance will obviously change as the contact shape between board and water changes due to edging and leeway angles. Riding in steady state on a steady course will result when yaw, pitch and roll turning moments around the point of lateral resistance are zero.
This is passing the time while I recover from a torn hamstring, and my fluid dynamics notes are on the top shelf and so I am trying to keep things simple. Have fun calculating how lateral resistance will change with pitch, yaw or roll variation. If you get a handle on that add some bubbles to the equation for a real challenge.
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Re: What force turns a twintip?

Postby elguapo » Thu Jun 06, 2019 7:09 pm

Herman wrote:
Thu Jun 06, 2019 6:03 pm
I realise referring to static lateral resistance is a major over simplification as the board will generate pressures as it has an angle of attack to the water due to edging and angle of leeway etc. I only offered this model of thought to help visualise the area of the board that is in contact with the water. The centre of lateral resistance will obviously change as the contact shape between board and water changes due to edging and leeway angles. Riding in steady state on a steady course will result when yaw, pitch and roll turning moments around the point of lateral resistance are zero.
This is passing the time while I recover from a torn hamstring, and my fluid dynamics notes are on the top shelf and so I am trying to keep things simple. Have fun calculating how lateral resistance will change with pitch, yaw or roll variation. If you get a handle on that add some bubbles to the equation for a real challenge.
interesting post....
...but what does this have to do with the new kitewings though?

Herman
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Re: What force turns a twintip?

Postby Herman » Thu Jun 06, 2019 7:44 pm

It's about as relevent to kitewings as it is to controlling a TT as you blast through chop looking for a decent ramp to get the thing into a much lower viscosity gaseous fluid where all your problems just melt away!

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Re: What force turns a twintip?

Postby iriejohn » Thu Jun 06, 2019 8:18 pm

A twintip turns by deflection.

sergei Scotland
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Re: What force turns a twintip?

Postby sergei Scotland » Fri Jun 07, 2019 12:12 am

I am trying to figure out how edging causes a turn while planing. May be it is as simple as using lift of the board at an angle to the vertical?
If I were to plane on a rectangular piece of plywood without kite at all the door creates a lifting force obviously which is vertical + some drag directed back obviously.
When I edge: the lift force direction (!) obviously changes in vertical plane. Now it has a horizontal part (vector) directed left if I am going left foot forward and use hillside edging.
This horizontal part of the force alone will cause change in direction - even if it goes through the center of the mass and does not cause board to rotate as such. If I am holding this position my path is not straight anymore and obviously a sector of a circle. This is very similar to a carving ski or snowboard turn and probably feels as one.
Now if I want to make this turn sharper while I am carving like that - I'd need to increase lifting force and part of it directed to the centre of the circle I am riding on, probably by increasing angle of attack or by increasing edging angle.
As Herman pointed out - while statically riding the total moment of rotation for the board is pretty much zero or very small.
This is evident from the fact that board does not rotate. While on a carving turn like this rider must probably be constantly moving center of mass slightly to stay very close to center of lift/pressure, so board does not rotate left or right, controlling which way the board is pointing while being helped by rear fins.
I guess what I am trying to say is that changing edging angle drastically changes the horizontal part (vector) of the lift/pressure force even if angle of attack of the board is constant.
Obviously the same horizontal vector can be changed by changing angle of attack. Less angle of attack (push on front leg) without change in edging obviously reduces horizontal vector along with vertical lift thus increasing radius of the curving turn (I am still on the "no kite or behind a motorboat" case as simpler situation to consider.
I guess I digressed from original question of what turns the board as this is more about what curves the path of the rider. The board orientation might be just following the path like a weathervane, rotated by the rear fins?

Interesting bit of this thought experiment is that it does not require a curved edge or any rocker at all. Any shape works really. Which is kind of consistent with the fact that some kiteboards are almost round (I hear of people kitesurfing on round pieces of wood) and some even have negative edge curve like a snowboard....
Does this make sense?


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