Reference Frames

Introduction

Reference frames are needed for following purposes:

for creating a CAD model of the wing (or the wings)
for defining the apparent wind speed vector va_vec
for calculating the lift and drag and side force coefficients
for calculating the resulting forces and moments

A geometric model is always created using the CAD reference frame. It can have any origin (with respect to the kite), but usually either the center of gravity of the body or the bridle point/ Kite Control Unit is used.

Y is defined spanwise, towards the right tip (seen from the front, the left side).
X is defined chord wise, from LE to TE, positive.
Z is defined as positive upwards.

Kite aero body frame (KA)

The body-fixed frame of the kite is the KA frame of KiteUtils.jl, aft-right-up:

X is defined chord wise, from LE to TE, positive.
Y is defined spanwise, towards the right tip (seen from the front, the left side).
Z is defined as the cross product of X and Y, so positive upwards.

The body-axis force components are Fx, Fy and Fz, with the matching coefficients cfx, cfy and cfz.

The origin of the KA frame can be defined by the user by passing the keyword argument kite_body_origin = ... to the BodyAerodynamics constructor.

The turn rates

The turn rates $\mathrm{omega} = [\mathrm{omega_x}, \mathrm{omega_y} ,\mathrm{omega_z}]$ are defined in the KA frame. The unit of the components is $\mathrm{rad}~\mathrm{s^{-1}}$.

Input and output

when running a simulation, the turnrate of the kite must be provided on each time step
the apparent wind speed vector va_vec is defined in the KA frame
the resulting forces are defined in the KA frame
the moments and moment coefficients are defined in the KA frame
CD is along the apparent wind, CL along va × y and CS completes the wind axes, so they match Fx, Fz and Fy only at α = β = 0