The physics of kiteboarding

[Matteo V]

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.

OK, this is tough without diagrams but......


First, completely disregard the underside of the wing or foil section. Why? - because flat plate or foil section, it is always redirecting the air molecules and thus always producing lift from that redirection. So there is kind of no difference there.


Now consider the angle of attack of the top surface of the wing or foil section. If this is hard you conceptualize, just think of the direction the molecules on top of the wing leave the trailing edge of the wing.


Then consider camber. A thick highly cambered foil section has a greater angle of attack on top of the wing. A thinner less cambered foil section will have a lower angle of attack. Consider the direction of those molecules when they leave the trailing edge.


Now if the above is clear to you, then consider this. A flat plate, even if you ignore the instantaneous direction change the molecules, is going to change the direction of the air molecules to the same vector on the top and the bottom. Given the head on cross section becomes a low pressure wake behind the trailing edge, the air molecules on the bottom side of will once again try to change direction and move toward that low pressure wake - upwards!

But with a foil section, the air molecules on top of the foil section will leave the trailing edge at a steeper angle than the air molecules on the bottom foil section. This effectively means that those top molecules meet the bottom molecules with more vertical energy. Thus the molecules on the bottom are prevented from moving up again. And the low pressure wake is not filled with molecules from the bottom of the wing, but rather from air on top of the low pressure wake.

All together, this means that a foil section has more potential to move more air molecules downward, than a flat plate, and can do that with less loss of energy/energy input.

[Regis-de-giens]

articulated way to explain which would require long discussions , thank you for the writing effort;
And anyway I agree with your conclusion which , from my understanding, seems coherent with mine : flat plate produce lift without Bernoulli & Coanda effect, and curvature of foil will adds additional lift with lower total drag, hence more efficient (at certain flow rates);
But Coanda/bernouilli alone does not provide the major/full lift, that was my point, so we shall not neglige this in the development of a wing. Like find the "compromised" curvature which works at several AoA (the real and unit definition of AoA = angle between kite chord and wind direction) , and optimized at different AoA depending on kite type and riding style.

[Delta99]

Jeeez, thanks guys. The level of engineering skills in this group - aa-amazing. This was some fun reading.

[Matteo V]

But Coanda/bernouilli alone does not provide the major/full lift, that was my point, so we shall not neglige this in the development of a wing.

Here is the real eye opener - coanada, Bernoulli, Newtonian...... all fail to describe flow over a foil section and the resulting forces > completely.

Like find the "compromised" curvature which works at several AoA (the real and unit definition of AoA = angle between kite chord and wind direction) , and optimized at different AoA depending on kite type and riding style.

That's a sail on a sailboat. Adjustable area, camber rake..... that is the closest to the holy grail so far, though there is some promise for heavy adjustable flexible shapes in the works by some researchers.