BModes: Blade modes for different pitch angles

Dear Jason,

In BModes and FAST the modes of a blade are defined as flapwise and edgewise modes. So I think the modes are independent from the pitch angle. In this case I’m a bit confused about the input value for the pitch angle in BModes. For the most wind turbines it is not required and can be the pitch angle of the rated conditions or are the mode shapes of BModes depend on the pitch angle?

Thank you very much.

Best regards,

Dear René,

Out-of-plane and in-plane displacements have different effects on the centrifugal loads as a result of rotor rotation, so, the pitch angle will have some influence on the flapwise and edgewise mode shapes. However, my experience with FAST has been that the accuracy of the mode shape (as long as they are close to correct) is not too critical to the results of a loads analysis–the modal frequencies are far more important. So, we typically calculate the mode shapes only at one design point–typically the conditions at rated power (at the rated speed and nominal pitch angle). As wind turbines are getting larger and more flexible, the impact of the mode shapes will likely become more important. You could always do a sensitivity study to see if changes in the mode shape with different operational conditions e.g. rotor speed and pitch angle have an influence on the results.

Best regards,