Rotor Unbalance in FAST


I am developing a FAST wind turbine model to compute fatigue and extreme loads of a wind turbine. IEC61400-1 requires rotor unbalance to be modelled in DLC1.1 to DLC1.5.

Is it possible to introduce unbalance in FAST? How is it done?

If it is not possible, I could make the model with slightly different structural data for the blades. I have tried this approach, but when checking the mass data on the .fsm file, the mass increment on the blade is not what I expected. How is the mass of a blade computed in FAST/MODES?

Best regards,

Iñigo Etxaniz

Dear Inigo,

There are several forum topics that have discussed how to model rotor imbalances in FAST. It is possible to include both structural (e.g., mass) and aerodynamic imbalances. Search for “imbalance” in the forum.

The blade mass and inertia reported in the FAST v7 summary file or ElastoDyn summary file for FAST v8 are computed by integrating the distributed blade mass along the length of the blade. Often a mass imbalance is set by defining different mass scaling factors (input AdjBlMs in the blade file) between the different blades. The blade mass reported in the summary file will be proportional to this input. What are you seeing that is not what you expect?

Best regards,

Thanks Jason,

I will check for imbalance in the forum.

To test how fast is calculating the mass, I made several simulations changing the density of one section of the blade each time. The model of the blade I have has non-uniform “BlFract” positions. I wanted to know how MODES decides the length of each element whos properties are introduced. I checked against two different ways I could make, but results did not match. Neverthelles adjusting AdjBlMs could be a good solution, I think I will go on with this solution.


Dear Inigo,

I haven’t used MODES in a very long time, but if I recall correcctly, MODES divides the blade into 100 equally spaced elements for integration of the structrural properties to form the generalized mass and stiffness matrices. FAST v7 uses the blade elements/nodes defined within the AeroDyn input file to integrate the structual properties.

Regardless, a simple scaling using AdjBlMs is the more common and easy way of introducing a rotor mass imbalance.

Best regards,