Blade Structure file

Each blade is divided into a number of structural elements that are used in the finite element solver. These elements are defined by two nodes. The Blade structure file  defines the position of each of these nodes and the structural properties of each element. An element will use the structural properties input to the innermost of its two nodes (i.e. the node closest to the blade root).

The following properties can be input:

BlFract (-) distance from the root to the node, normalized against the blade length
StrcTwist (deg) this will determine the direction of the stiffnesses. For a straight blade with no cone, an angle of 0 means the edgewise stiffness will be in the rotor plane. More info here  Blade structure twist . Generally the same as the  Blade shape twist  
BMassDen (kg/m) distributed blade section mass per unit length
FlpStiff (Nm²) flapwise section stiffness, given about the principal structural axis of the cross section oriented by the structural twist angle
EdgStiff (Nm²)
edgewise section stiffness, given about the principal structural axis of the cross section as oriented by the structural twist angle
GJStiff (Nm²) blade torsion stiffness
EAStff (N) blade extensional stiffness
FlpIner (kg m) distributed flapwise section inertia, given about the principal structural axis of the cross section as oriented by the structural twist angle
EdgIner (kg m) distributed edgewise section inertia, given about the principal structural axis of the cross section as oriented by the structural twist angle
ElasticCenterOffsetX (m) offset of the elastic center with respect to the chordline midpoint, along the chord line, positive towards the leading edge
ElasticCenterOffsetY (m) offset of the elastic center with respect to the chordline midpoint, prependicular to the chord line, positive towards the suction side
MassCenterOffsetX (m) offset of the mass center with respect to the chordline midpoint, along the chord line, positive towards the leading edge
MassCenterOffsetY (m) offset of the mass center with respect to the chordline midpoint, perpendicular to the chord line, positive towards the suction side
ShearCenterOffsetX (m) offset of the shear center with respect to the chordline midpoint, along the chord line, positive towards the leading edge
ShearCenterOffsetY (m) offset of the shear center with respect to the chordline midpoint, prependicular to the chord line, positive towards the suction side
GShearAreaFlapwise (N) Shear modulus times shear area in the flapwise direction
GShearAreaEdgewise (N) Shear modulus times shear area in the edgewise direction



Note:  the first node of the blade is always located on the pitch axis. No structural offsets are allowed for that node. If a non-zero value is set for these offsets, you will get the  All offsets have to be zero at the root  error.

Note:  the last node is only used to define the location of the tip (in case there are structural offsets). All other parameters will be ignored



The position of the structural centers (i.e. the Elastic, Mass and Shear centers) will depend on the BlCrvAC and BlSwpAC parameters of the  Blade shape file  and the offset parameters in the Blade structure file .
To understand how these are linked, the following example shows how to obtain the position of the Elastic center  step by setp.

1. Defining the  Pitch axis  



This is equivalent to seeing the blade from the tip towards the root, as illustrated in the figure below. The white vector represents the wind direction



2. Placing the  Chordline midpoint  on the pitch axis. The  Blade aerodynamical station  is perpendicular to the pitch axis.





3 Offsetting the chordline midpoint  BlCrvAC  and BlSwpAC , which define the prebending of the blade.




Note:  in this example, both BlSwpAC and BlCrvAC  are negative, as is commonly the case for conventional prebent blades

4. Twisting the airfoil around the chordline midpoint. Note that in this case it is the  Blade structure twist  which is applied (as opposed to the Blade shape twist , which does not necessarily have the same value).



Note : in this example, the structure twist angle is positive.


5. Pitching the airfoil around the pitch axis



Note:  in this example, the pitch angle is positive.



6. The Elastic center  is offset from the chordline midpoint by
  • ElasticCenterOffsetX  along the chordline, where positive values are towards the leading edge
  • ElasticCenterOffsetY  perpendicular to the chordline, where positive values are towards the suction side


The figure below is a zoom of the previous one.



Note:  in this example, both ElasticCenterOffsetX  and ElasticCenterOffsetY  are positive.