The physics of kiteboarding

[Dave_5280]

https://www.surfertoday.com/kiteboardi ... teboarding

[Faxie]

It started with the mention of the coanda effect. Well, I thought, very good someone finally seems to get it. And then it went downhill very fast unfortunately...

Coanda is a result of Newton's laws and those laws works on both sides of the kite.

2 Good papers that mention coanda:

https://user.uni-frankfurt.de/~weltner/ ... ternet.pdf
http://www.prirodopolis.hr/daily_phy/pd ... 20hork.pdf

A kite 'pulls' itself forward because it's a tethered object (and that's why it cannot be considered the same as a regular wing). It doesn't push itself forward against it's own generated turbulence. Turbulence is a result of flow separation, contributing to drag, and drag pulls the kite backwards. Ever heard of backstalling? Gee I wonder how that happens?

The aoa and window parts aren't completely right either, and so is the wakeboard comparison. A boat doesn't change it's speed, force and direction because of the change in speed, force and direction of the wakeboarder. Even if you're the Hulk and can brake to a standstill, the boat will keep going forward (though will circle around you offcourse). A kite is limited by the window, which is a direct dynamic (apparent) result of the actions of the kiter (or basically, the tether).

[ieism]

This does not explain why I can't nail my foil gybes, so it must be incorrect.

I'm not a scientist, but I didn't get the boat part and turbulence either. I can stick my willy in the air and it will create turbulence, but it sure won't fly.

The parachute / paraglider windwindow I liked. I'm going to steal that when I'm teaching friends to kite.

[andikite]

I don't agree completly with this article. It's said that altering the direction of the fluid flow (due to coanda effect) creates a force in the kite.

The main force of a kite and all wings is the lift created due to the diference of fluid pressure in extrados and extrados as result of the diference in the flow speed in both parts. It's not necessary to change the direction of the air flow. Air flow in extrados is accelerated by the curvature of the profile (also due to coanda effect) and more air speed means lower pressure. In intrados you have less air speed and more pressure. The result is the lift.

Is true that in a kite the intrados has also curvature... So this effect is reduced and it are less efficient that a Foil kite that has a flat surface in intrados (closed aerodynamic profile).

[rnelias]

It started with the mention of the coanda effect. Well, I thought, very good someone finally seems to get it. And then it went downhill very fast unfortunately...

Coanda is a result of Newton's laws and those laws works on both sides of the kite.

2 Good papers that mention coanda:

https://user.uni-frankfurt.de/~weltner/ ... ternet.pdf
http://www.prirodopolis.hr/daily_phy/pd ... 20hork.pdf

A kite 'pulls' itself forward because it's a tethered object (and that's why it cannot be considered the same as a regular wing). It doesn't push itself forward against it's own generated turbulence. Turbulence is a result of flow separation, contributing to drag, and drag pulls the kite backwards. Ever heard of backstalling? Gee I wonder how that happens?

The aoa and window parts aren't completely right either, and so is the wakeboard comparison. A boat doesn't change it's speed, force and direction because of the change in speed, force and direction of the wakeboarder. Even if you're the Hulk and can brake to a standstill, the boat will keep going forward (though will circle around you offcourse). A kite is limited by the window, which is a direct dynamic (apparent) result of the actions of the kiter (or basically, the tether).

I think this is the same case of explaining why planes fly. There's no unique explanation or, in other words, there are two possible and physical explanations that are equally right.

Here's a good reference on this subject.

https://www.scientificamerican.com/arti ... n-the-air/

Bernoulli principle and/or third Newtons law, both are right... (and the Navier-Stokes equations come from Newton's second law too ...) (*)

Moreover, depending on the format of the kite, it'll have a completely different behavior acting more like a true wing or, in the opposite side, as a simple "bag or air". High aspect ratio kites, like racing foil kites, or more modestly, big air 5-struct kites, can lift us vertically with no big effort and these kites behaves more like a true wing. However, we also have to consider kites with more C-shaped curves like the kites used on wakestyle or surf/foil kites. These kites behave more like a "bag of air" pulling us downwind. They don't fly like "pure wings" and produce more power when are flown deeper in the wind window.

The beauty on this subject is that designers can play with the kite's design to extract more of one side and specialize the kite when the market requires.

Kites when combined with hydrofoils are even more impressive as we can understand that there are two wings "fighting" in different physical enviroments. The kite in the air and the hydrofoil in the water while the rider is taking advantage of this physical equilibrium.

[andikite]

It started with the mention of the coanda effect. Well, I thought, very good someone finally seems to get it. And then it went downhill very fast unfortunately...

Coanda is a result of Newton's laws and those laws works on both sides of the kite.

2 Good papers that mention coanda:

https://user.uni-frankfurt.de/~weltner/ ... ternet.pdf
http://www.prirodopolis.hr/daily_phy/pd ... 20hork.pdf

A kite 'pulls' itself forward because it's a tethered object (and that's why it cannot be considered the same as a regular wing). It doesn't push itself forward against it's own generated turbulence. Turbulence is a result of flow separation, contributing to drag, and drag pulls the kite backwards. Ever heard of backstalling? Gee I wonder how that happens?

The aoa and window parts aren't completely right either, and so is the wakeboard comparison. A boat doesn't change it's speed, force and direction because of the change in speed, force and direction of the wakeboarder. Even if you're the Hulk and can brake to a standstill, the boat will keep going forward (though will circle around you offcourse). A kite is limited by the window, which is a direct dynamic (apparent) result of the actions of the kiter (or basically, the tether).

I think this is the same case of explaining why planes fly. There's no unique explanation or, in other words, there are two possible and physical explanations that are equally right.

Here's a good reference on this subject.

https://www.scientificamerican.com/arti ... n-the-air/

Bernoulli principle and/or third Newtons law, both are right... (and the Navier-Stokes equations come from Newton's second law too ...) (*)

Moreover, depending on the format of the kite, it'll have a completely different behavior acting more like a true wing or, in the opposite side, as a simple "bag or air". High aspect ratio kites, like racing foil kites, or more modestly, big air 5-struct kites, can lift us vertically with no big effort and these kites behaves more like a true wing. However, we also have to consider kites with more C-shaped curves like the kites used on wakestyle or surf/foil kites. These kites behave more like a "bag of air" pulling us downwind. They don't fly like "pure wings" and produce more power when are flown deeper in the wind window.

The beauty on this subject is that designers can play with the kite's design to extract more of one side and specialize the kite when the market requires.

Kites when combined with hydrofoils are even more impressive as we can understand that there are two wings "fighting" in different physical enviroments. The kite in the air and the hydrofoil in the water while the rider is taking advantage of this physical equilibrium.

About your last paragraph... What a wonderful sport!!

[bragnouff]

Shame to put all those efforts into writing that paper, and self-sabotage it with the Comic Sans MS font...

[Regis-de-giens]

Bernoulli principle and/or third Newtons law, both are right... (and the Navier-Stokes equations come from Newton's second law too ...) (*)

Moreover, depending on the format of the kite, it'll have a completely different behavior acting more like a true wing or, in the opposite side, as a simple "bag or air". High aspect ratio kites, like racing foil kites, or more modestly, big air 5-struct kites, can lift us vertically with no big effort and these kites behaves more like a true wing. However, we also have to consider kites with more C-shaped curves like the kites used on wakestyle or surf/foil kites. These kites behave more like a "bag of air" pulling us downwind. They don't fly like "pure wings" and produce more power when are flown deeper in the wind window.

The beauty on this subject is that designers can play with the kite's design to extract more of one side and specialize the kite when the market requires.

Kites when combined with hydrofoils are even more impressive as we can understand that there are two wings "fighting" in different physical enviroments. The kite in the air and the hydrofoil in the water while the rider is taking advantage of this physical equilibrium.

Fully agree and I must say that I do not agree with the "restricted point of view of the article" ; of course simply redirecting the airflow is a major contributor, we cannot forget that, and even if you do not consider Bernoulli effect by using a flat profile for example (no difference fo curvature intrados vs extrado : indeed, even a simple rigid thin flat sheet can produce lift if it has an angle of attack, just by redirecting the airflow along the Angle of attack) : air changes its direction, so the counter reaction of this inertia change is a opposite lift on the flat profile) ; some very simple hydrofoils or children kite work like that with flat profiles ... So it is a combination of many physics like said above, with dirreretn impact if you talk about a race kite or a nasawing or a spinnacker...

Also I disagree with the sketches showing the kite effort in the direction of the ride ... resulting force is along the lines only (line only work in tension) ; then this force is partly stopped by the anti-drag plane of the board, and the resulting working force ON THE HARNESS is along rider direction.

PS: This is why apparent wind does not provide any effective pulling force to accelerate the rider (but only uneffective efforts in your legs against the board...) , which tends to make you believe that apparent wind due to rider speed creates additional speed of the rider ; ... but not in reality...

[Matteo V]

...... of course simply redirecting the airflow is a major contributor, we cannot forget that, and even if you do not consider Bernoulli effect by using a flat profile for example (no difference fo curvature intrados vs extrado : indeed, even a simple rigid thin flat sheet can produce lift if it has an angle of attack, just by redirecting the airflow along the Angle of attack) :

Curvature of a foil section allows a time (delta) component for the change of direction of air molecules.

A flat plate allows for no time to redirect the air molecules.

Removing the extra time from the equation increases energy consumption. Thus a flat plate is less efficient, and completely ineffective at some sub mach 1 air flows.

[Regis-de-giens]

maybe it is my language skill that limit my understanding of your statement ; yes a flat profile without curvature offers a fluid redirection thanks to the AoA , I hope you agree: look at war planes : almost or no curvature (while they fly actually, even if not optimum at low speed ) ; another example : if you put your arm and hand out of your car at 100 km/h, you get a good lift if you incline your hand a bit, while no curvature along you hand.

I fully agree that a curvature of the profile will add efficiency (in term of lift/drag ration it will decrease the drag for a similar lift), but a very important part of the lift comme from the AoA increase, this is obvious (which is the reason why you get more lift on a LEI when you sheet in, while kite curvature does not change overall with bar throw );

I know that curvature (camber and profile thicknesses) will impact drag, hen ce efficiency, I agree and I like playing a lot with cambers on foil kites, I just wanted to clarify that lift is not only a matter of Bernoulli or Coanda. Multi parameter lift ; if you look at Bernoulli only (i.e. pressure difference due to change of length between intrados and extrado lines), you get a certain lift but a small one (the one when kite is depowered) which needs to be completed by a AoA of the wing, which happens when you sheet in your bar.

PS: by the way this is the same for the hydrofoil acting on the water; a flat hydrofoil provides a good lift thanks to AoA, and the more speed you target, the less curvature you want on the foil profile.