# Problems in calculating pitch moment

I would like to ask a question regarding the calculation of pitch moment of a blade section.
Here are my calculations for 5MW blades with 19 sections.

The results of Fast’s calculations are shown below, with 9 sections selected for output.

The pitching moment formula is a very simple algebraic formula,

where the air density is 1.225, the incoming wind speed is 11.4m/s, the chord length is about 3-4m, and the range of Cm is shown in the figure below.

My calculations and the fast output are roughly an order of magnitude different! Is there something wrong with my calculations or formulas? Or is it due to a problem with the settings of my input file for fast?Below is the code I found for the fast calculation.

Best Regards

Dear @Huajian.Xiao,

I’m not sure I understand your question. You seem to be comparing the hand calculation at 19 sections to the AeroDyn calculation at 9 sections. How do the 19 nodes compare to the 9 nodes? Also, why does the AeroDyn output vary in time while your hand calculation is fixed?

Are you using the same angle of attack (to compute C_M) and the same relative wind speed in your hand calculation as is being used by AeroDyn? Please note that in your hand calculation, you should be using the relative wind speed rather than the undisturbed freestream wind speed, U_inf)

Best regards,

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Dear Jason.Jonkman,

Yes, I was comparing 19 sections to 9 sections. Mainly the values were so much different that I did not output all 19 of its sections and did a simple comparison. About my calculation being fixed is due to the fact that I only did the validation work for a single step size.

I would like to consider torsional effects in the dynamic response calculations. Is it valid to simply use the pitch moment and torsional stiffness to calculate the angular deformation? The angular deformation I calculated by this method is relatively small. Is it necessary to consider the geometric relationship between the shear center and the pneumatic center, etc.? Or is there some other functional relationship between pitch moment and angular deformation?

But again, it seems reasonable that the torsional effect of the 5MW blades is not significant in actual operation.

Best regards,

Dear @Huajian.Xiao,

Computing twist only be considering the pitching moment and torsional stiffness misses several important contributions. See the following forum topic for more information: Coupled blade modes in FAST - #40 by Jason.Jonkman.

Best regards,

Dear Jason.Jonkman,

Yes, I agree with your point. Adding torsional degrees of freedom to the overall machine dynamics model requires many considerations and is very difficult.

Now among the methods to consider blade torsion in the calculation of the dynamic response, I understand the following:

• Quasi-static correction methods
• High precision modeling of the blade section alone

Other than that, are there any other ways to take into account the torsional effects of the blade?

Best regards,

Dear @Huajian.Xiao,

I’m not sure what you mean when you refer to “high precision modeling”, but I would say the structural models with torsion could include modal, multibody, beam finite element, super-element, and 3D solids/shells.

Best regards,

Dear Jason.Jonkman,

Is there any data on the moment of inertia of a 5 MW nrel blade in torsion? I found FlpStff, EdgStff, FlpIner , EdgIner, and torsional stiffness (GJStff) through the “Definition of 5-MW Reference Wind Turbine for Offshore System Development”. But I don’t see the Moment of Inertia data in the torsional direction?

Best regards,

Dear @Huajian.Xiao,

The polar inertia (in torsion) is the sum of the flapwise and edgewise inertias at each cross section.

Best regards,