Document the reference frame(s) that are used

[ufechner7]

In which reference frame are the following vectors define?

• v_a (apparent wind speed vector)
• the points that are for example used in the Panel struct

[jellepoland]

Reference frame of v_a, and general reference frame:

Y defined spanwise, looking at the kite from the front (so seeing the LE first) the front left is positive.
X is defined chord wise, from LE to TE, positive.
Z is defined as positive upwards. From tip to mid-chord.

Local reference frame of panel
- x_airf (np.array): Unit vector pointing upwards from the chord-line, perpendicular to the panel
- y_airf (np.array): Unit vector pointing parallel to the chord-line, from LE-to-TE
- z_airf (np.array): Unit vector pointing in the airfoil plane, so that is towards left-tip in spanwise direction

[ufechner7]

@jellepoland Nice, but:

• What would be a good name for the general reference frame? My suggestion: VSM
• What would be the origin of this reference frame?

[1-Bart-1]

I use the same general reference frame in my quaternion kite model. The origin in my model is the center of mass, so that zero torques result in linear motion of the origin.

[ufechner7]

Well, nothing wrong with using the centre of mass, but the models imported from surfplan are not using the COM as origin, and surfplan is not even able to calculate the COM.

So we must provide the option to use a different reference point, only derived from the geometry of the kite.

[1-Bart-1]

Any reference point should work, as long as it lies on the xz-plane. Why not let the user provide the reference point? And then default to COM if available, and a point in the middle between the wing tips if no COM is available.

[ufechner7]

See: https://github.com/Albatross-Kite-Transport/VortexStepMethod.jl/blob/main/docs/src/reference_frames.md

[ufechner7]

Done.