Aerodynamic center offset effects on blade torsion

Wind & Water Structural Analysis
2

Dear @Lin.Yang,

The old AeroCent variable in now obsolete versions of FAST caused much confusion. The following forum topic clarifies it best: NREL 5MW Rotor Geometry.

Here are my answers to your questions:

  1. To get the correct pitching moment and twist, it is important to set both the aerodynamic center relative to the pitch axis and the pitching moment about the aerodynamic center. I say “relative to the pitch” axis rather than some fraction of chord because lifting line models (like used in AeroDyn) don’t really care where the leading or trailing edge are; what is most important is where the aerodynamic loads are applied relative to the pitch axis, which is what AeroDyn blade inputs BlCrvAC and BlSwpAC refer to.
  2. Yes, the pitching moment about the pitch axis comes both from the aerodynamic force crossed with the moment arm (which is offset between the pitch axis and the aerodynamic center) as well as the pitching moment (Cm) term.
  3. The aerodynamic center in AeroDyn is defined as the reference point for the aerodynamic force coefficients and is where the aerodynamic loads are applied, so, the pitching moment should be defined relative to the aerodynamic center, even if that is not 25% chord.

Best regards,

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