# FlpStff and EdgStff

Hallo

I do not really understand what the flapwise and edgewise stiffness is. Are these stiffnesses around the principal axis?
Is the 1st principal axis oriented along the chord line and the 2nd perpendicular to it?

I always thought that flap and edgewise are terms concerning the whole blade and not a cross-section.

Please can you explain it for me or give me some hints.
I would also appreciate a tip for good literature or papers concerning this topic.

Thank you very much

Andreas Ferber

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,

Thank you for your rapid reply. It helped me a lot.
In some weeks I have to validate the structural module of a wind turbine code, I wrote. Therefore I wanted to use NREL software and probably the wing described in this document:

“Definition of a 5-MW Reference Wind Turbine for Offshore System Development”

As long as there is no angle given for the difference to the chord line, I assume that the principal axes are along the chord line and perpendicular to it. When I reference to the blade coordinate system used in the FAST user’s guide, then the Flpstff is the stiffness for bending around the y-axis(the 1st principal axis) and the Edgstff the stiffness for bending around the x-axis (the 2nd principal axis) ?

Another question:
Do you know how I can get access to the NREL Unsteady Aerodynamics Experiment wind tunnel database
Dr. Schreck didn’t answer my requests and I do not want to bother him anymore

Thanks a lot again

Andreas

p.s.: I think there is a mistake on page 74/5 in the FAST user’s guide in the definition for the Edgstff

Dear Andreas,

In the NREL 5-MW baseline wind turbine report you refer to (nrel.gov/docs/fy09osti/38060.pdf), the structural twist is specified in Table 2-1 and the aerodynamic twist is specified in Table 3-1. The structural twist gives the orientation of one of the local principal elastic axes (the local x-axis; the local y-axis is perpenendicular to it). The aerodynamic twist gives the orientation of the chordline. While, in general, these twist distributions can differ, you are correct that in this turbine the distributions are the same. In FAST, you are correct that FlpStff is the bending stiffness about the principal elastic y-axis and the EdgStff is the bending stiffness about the principal elastic x-axis.

Scott Shreck is the correct contact at NREL for obtaining the UAE data. We simply are very busy and cannot always respond quickly.

Best regards,