Foil Theory

From FoilDesign

Need some contributions here..... Put stuff about AoA, aerodynamics etc here

Forces acting on a kite

This needs to be updated to use math plugin(see KiteForum.com)

F = 0.5 ρ * A * C * v²

• F = forces in Newton
• ρ = density (kg / m³)
• A = surface in m²
• C = constant
• v = speed (in m/s)

F is a vector: it has both magitude and direction. The direction is a combination of both up (Lift) and back (Drag).

Likewise, when you talk about airfoils, the C constant depends on whether you want to know the lift force or the drag force. So there are two constants C(l) and C(d), called the lift and drag coefficients.

C(l) and C(d) also depend on the angle of attack (AoA). This is the angle at which the wind is hitting the foil. AoA depends both on how the kite is set up and where it is in the sky. At some AoA, C(d) becomes greater than C(l). This is called the stall angle. The kite (or wing) falls out of the sky.

The graph of C(l) and C(d) versus AoA is called a drag polar. The best drag polars are developed by experimentation, but we can guess through computer simulation as well. See one guess at a drag polar for the Zeroprestige Skoop profile

So in order to know the Forces on a kite:

1. Pick an AoA. 2. Find C(l) and C(d) for that AoA 3. Compute F(l) and F(d)

For curved inflatable kitesurfing kites, F(l) is horizontal at the sides and you have to integrate around the shape to get the total lift.

All of this is just an approximation, and a bad one at that because there is so much going on in areodynamics (vortices, boundary layer effects, etc.) that is very difficult to model, especially for a physics project.

Check out Zero Prestige for links to more than you ever wanted to know about kite physics.

Profile Thickness and Center of Pressure

The thickest point of the profile is a geometric property, you can find it with two parallel lines, rulers or so.

The center of lift is quite different. It depends from flow around the profile, so it differs depending from the angle of attack and from the different circumstances: regular flow, stall, turning and else. In other words, the center of lift depends from the pressure distribution along the profile - and this is a changing thing (better say a more difficult thing, but it is easy to calculate :) for regular conditions ).

Bye Joe

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Hi,

Yes, this is all true, but there is some correlation between the two, in that they are generally near the same chord point, and moving the thickest point forward will also generally move the CoL forward (under given conditions), and vice versa.

Cheers,

Dave [Aberdeen]