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The science
Aerodynamic force
The wind blows along +x at speed U. The kite of area A feels dynamic pressure q = ½ ρ U2, giving lift L = q A CL (perpendicular to the wind, upward) and drag D = q A CD (along the wind). Below stall the lift slope of a low-aspect-ratio plate is CL = a α with a = 2π / (1 + 2/AR) (Prandtl lifting-line, AR = 2.5 here) and the drag follows the parabolic polar CD = CD0 + CL2 / (π e AR). Beyond the stall angle (15 degrees) the flow separates: CL falls along k sin 2α and drag rises towards the flat-plate value.
Equilibrium
Three forces act on the kite: the aerodynamic force (D, L), the weight (0, -mg) and the tether pull T. In equilibrium the tether pulls with the opposite of their sum, so T = √(D2 + (L - mg)2) and the tether rises from the anchor at the angle β = atan2(L - mg, D). The kite can only fly (beta > 0) once L > mg, i.e. above Umin = √(2 m g / (ρ A CL)). The best glide ratio of the parabolic polar is (L/D)max = ½ √(π e AR / CD0) at CL = √(CD0 π e AR).
Checks
(1) A damped point-mass on a stiff tether is integrated in time until it rests and its angle and tension are compared with the closed form. (2) At Umin the tether angle must be zero. (3) A numerical search for the best L/D is compared with the formula.
Honest simplifications: a point-mass kite (no pitching moment or bridle geometry), constant wind with height, a flat tether (no sag or tether drag, which matters for real kites), quasi-static response to gusts, and empirical post-stall coefficients. Use it for force bookkeeping, not for kite design.
What to try
- Raise the wind from 3 to 12 m/s with the Fighter kite preset: the tether climbs and the tension grows with U squared.
- Increase the angle of attack past 15 degrees and watch lift collapse in the left graph while drag keeps rising.
- Add mass with the Heavy box kite preset and find the minimum wind speed at which the tether first lifts off the ground.
- Turn on gusts and watch the tether angle swing along the middle graph.