Axes and Coefficients

Reference values

All coefficients are based on the reference values set in the Geometry step (Step 2: Geometry):

Symbol	Name	Used for
S	Wing reference area (m²)	All force and moment coefficients
c	Mean aerodynamic chord (m)	Pitching moment Cm; CG and neutral point in % MAC
b	Wing span (m)	Rolling moment Cl and yawing moment Cn

with the dynamic pressure q = ½ ρ V² of the cruise speed in the project’s air.

Moments are taken about the centre of gravity set in the Flight step.

α (angle of attack): The aircraft is pitched by α relative to the wind. Positive α is nose up.
β (sideslip): The aircraft is yawed by β relative to the wind.

Coefficient	Definition
CL	Lift, perpendicular to the wind, / (q S).
CD	Drag, along the wind, / (q S).
CY	Side force / (q S).
Cm	Pitching moment about the CG / (q S c). Positive pitches the nose up.
Cl	Rolling moment / (q S b). Positive rolls right.
Cn	Yawing moment / (q S b). Positive yaws the nose right.

The tables use body axes for the moments.

CLα: Lift-curve slope, per radian.
dCm/dCL: Slope of the pitching moment against lift. Its negative is the static margin.
Static margin: (x_NP − x_CG) / c, in percent of the mean chord. Positive is stable.
CYβ, Clβ, Cnβ: Side force, rolling and yawing moment per radian of sideslip, from the β 5° run at α 2°.
∂C/∂δ: Control effectiveness per degree of deflection, from the ±5° and ±10° runs at α 2°.

Positions in the Aircraft tab (hinge points, CG, motors) and the inertia are in the solver’s axes, in millimetres:

The CG offset is measured from the point the CFD took its moments about.

The Gazebo model uses ROS/Gazebo body axes FLU: +X forward, +Y left, +Z up, in metres.