Dear Andreas,
The flapwise stiffness and edgewise stiffness are defined about the principal elastic axes of a cross section. The principal elastic axes need not be parallel and perpendicular with the chordline, although they are often quite close to this in practice. The flapwise stiffness and edgewise stiffness are the integrals of the modulus of elasticity times the square of the flapwise and edgewise distances, respectively, from the blade section elastic center to the differential area element over the cross-sectional area of a section. That is, the flapwise stiffness = ∫∫ E(x,y)x^2dxdy and the edgewise stiffness = ∫∫ E(x,y)y^2dxdy, where E(x,y) is the modulus of elasticity and x and y are the flapwise and edgewise distances from the blade section elastic center to the differential area element, respectively. The stiffness and orientation of the principal elastic axes can vary for different cross sections along the length of a blade, depending on the blade geometry (external/airfoil shape and twist) and material constituency (composite laminate lay up, spar, etc.).
I hope that helps.
Best regards,