Pitch axis - Not on chord line!

The rotor I am modelling has the pitch axis totally separate from the chord line. It is offset so that the pitch axis is located at approximately 30% of the chord length from the leading edge, and is offset downwind by 70 mm. No part of the pitch axis goes through blade.

This is to say that as the blade pitches every single point on the blade will describe an arc about the pitch axis.

Is there a variable that I have overlooked that accounts for this offset. So far I have understood that FAST models the blade so that the pitch axis must go through the chord line at some point between the leading and trailing edges.

Looking forward to your help.


Dear Phillip,

The current versions of FAST (both v7 and v8) assumes that blades are naturally straight along the pitch axis. A blade with a pitch axis that does not pass through the chord has implications on both the aerodynamic loads and structural dynamics. It would probably not be too difficult to modify the source code of FAST so that you can specify the aerodynamic center to be outside the range of 0-1, which FAST currently checks to keep the aerodynamic center between the leading and trailing edges, assuming the pitch axis lies at 25% chord. However, FAST v7 (or the ElastoDyn module of FAST v8) does assume that the structural mass lies along the pitch axis, which does not sound correct for your blade and is likely the bigger of the two problems.

In our current FAST v8 developments, we are working on a new structural dynamics module (BeamDyn) and updates to the aerodynamics module (AeroDyn) that can will enable the modeling of blades that are not naturally straight along the pitch axis (among many other improvements). Once these new modules are available, it should be possible to model the unique features of your wind turbine design.

Best regards,

Hi Bonnie,
thanks for your response. Just to clarify, the blades I am modelling are straight along their span wise and edge wise directions but the pitch axis is offset from the chord line in the global x direction, downwind. In the image below should clarify what I mean. The pitch axis is located at 30% of the chord length but is not on the chord line.

I was hoping I could model this only by modifying the pitching routine PitchCntrl_ACH.f90 however, as you suggested, I will need to modify the FAST source code itself to include a variable that defines the pitch axis distance from the mean chord in the global x direction.

Many thanks

P.S. I know the figure above does not include the aerodynamic pitching moment.