Blade Twist Deflection

Hi Everyone,

Based on my preliminary research, twisting mode of a wind turbine blade is very important to determine the blade structural integrity. Now, I am wondering how I can get the internal twist torque equation when conditions given such as wind speed and structural twist. I am also curious about why FAST doesn’t have an output of twist angle (deflection) of a wind turbine blade, but rather only Adams/FAST can do it. And that’s only for the tip angular deflection.

Does anyone know the answer?


Hello Dongwon,

I don’t know whether the outputs parameter “RootMzb1” or “Spn#Mlzb1” are the torque you are looking at.

And I am also curious about why FAST does not have output of twist angle. Does it can be easily calculated as following if we know the torsional moment say “TwrBsMzt” which is the torsional moment at tower base:

angle of twist = “TwrBsMzt” * L /(G*J), for a cylinder.

Or I was having a wrong understand about the basics.

Hi Lingling,

RootMzb1 (or Spn#Mlzb1) is for bending mode, which gives you a normal stress using sigma = Mc/I. Yes, you’re right, you will get the shear strain by the torsion equation similar to “TwrBsMzt” * L /(G*J). Now, you know you need the torque output TwrBsMzt like you needed moment for the stress. My question is actually about the local torque output. I believe that FAST applies the BEM method and BEM gives us the twist torque equation. If the relationship is simple, then I am wondering why up-to-date FAST doesn’t have angle (or torque) output, but only Adams/FAST can have them.

Hope that anyone can answer! Thanks for looking~

Hi Dongwon,

The elastic torsion (or twisting) of a blade is certainly important for some wind turbine blades (but not all). However, the current version of FAST does not have blade-torsion degrees of freedom (DOFs).

FAST can output the blade twist – e.g. TipRDzb1 for the twist of blade 1 at the tip, Spn1RDzb1 for the twist of blade 1 at blade gage #1, etc. However, all of these twist outputs are zero-valued from because of the absence of blade-torsion DOFs.

That said, FAST can still output the torque (pitching moment) along the blade – e.g., RootMzb1 for the pitching moment at the root of blade 1, Spn1MLzb1 for the pitching moment of blade at blade gage #1, etc. These torques (pitching moments) include contributions from aerodynamic pitching moments and applied aerodynamic forces, gravity forces, and inertia forces crossed with their position vectors. All of these forces, moments, and distances are vectors that orient with the blade as the blade deflects.

The dynamics of a composite spinning blade with bending, torsion, and large deflection is quite compilicated, which is why it has taken a while for NREL to add blade torsion to FAST. However, NREL is working on introducing blade torsion to FAST as described in my Nov 26, 2012 post in the forum topic found here:

Best regards,