Blade Coordinate System

Figure 2 illustrates the coordinate system of a blade. The origin of the coordinate system is at the center of rotation in the blade hub. The $z$-axis is coincides with the rotor's axis or rotation. For simplicity, we assume that there is no coning (i.e., the blades are perpendicular to the axis of rotation). Thus, the $x$-axis points through the blade. In fact, the locus of spar midpoints is on the $x$-axis. The $y$-axis points towards the back of the blade.

  

Figure 2: Blade Coordinate System

Figure 3 illustrates a cross-section of the blade. The $y$- and $z$-axes are as described above. The $x$-axis points out of the paper. This figure also illustrates the surface coordinate system for the single-celled beam that the blade is modeled as. The $s$-axis starts at the trailing edge and loops around as shown. For simplicity, the front edge of the blade is not considered, because it would require a multicelled analysis.

  

Figure 3: Blade Cross Section Schematic

There is one subtlety. Due to blade twisting, the local coordinate system of the cross-section is slightly different than the blade coordinate system. In the cross-section coordinate system, the $y$-axis points through the trailing edge, regardless of the angle-of-attack.