Pitch - Flap coupling for FAST or YAWDYN

Hi again everybody,

I promise sometime to stop with all my questioning :slight_smile: … but the opening of the Forum is like an invitation to ask “Everything You Always Wanted to Know About NWTC Simulation codes … But Were Afraid to Ask”.

Anyway: I would like to have a simple “mechanical” coupling between flap and pitch motion in some of the codes (probably YAWDYN or FAST). Not a real “flap-twist” elastic coupling, just a (linear) function correlating a rigid flapping motion wiht a pitching motion, individually for each blade. Each blade should gain an additional DOF representing the rotation about an axis similar to the teeter axis. For a three bladed HAWT… maybe unusual, but possible.

If this could be implemented, it should allow the user to define a generic response curve of flap moment vs. angle (representing some kind of elastic + damping device in the hub).

Or is there a way I could simulate this with the existing codes?

Thanks a lot
Best regards
Claudio Pedrazzi

Hi Claudio,

If I understand correctly, you want to be able to model a blade where the pitch angle is linearly related to the displacement of a rigid flap hinge.

FAST allows you to control the pitch angle through a user-defined routine based on feedback of any output measurement or state in the model. However, FAST does not have a built-in rigid flap hinge DOF. If you could make do by defining a mode shape to approximate a flap hinge, I think you could use FAST as-is do what you want. In the past, we talked about adding a built-in rigid flap hinge DOF. However, there has been little demand and other enhancements have taken priority. I can’t see us adding a built-in rigid flap hinge DOF any time soon.

Although I am no expert in YawDyn, which has a rigid flap hinge DOF, I don’t believe it is possible to control the pitch angle during a simulation.

Another option is SymDyn (available on the NWTC design codes website). SymDyn models the blade with a rigid flap hinge DOF and allows for the pitch angle to be controlled during a simulation. I am no expert in SymDyn either, so I am not sure if you can define the pitch angle to be a function of the flap displacmenent. But a quick skim through the SymDyn User’s Guide should give you a an answer.

A final solution would be to move on to the more advanced, very-flexible, and general-purpose MSC.ADAMS code. If you’ve looked through the FAST User’s Guide you’ll know that FAST contains a built-in preprocessor for generating ADAMS models of conventional wind turbine configurations. However, with the ADAMS code, it is relatively simple to create new configurations and add new DOFs to your model. The drawback is that ADAMS is a commercial code requiring expensive licensing fees and takes some effort to learn.

Please let us know which path you decide to take so that I can advise other users who have similar questions.

Best regards,

Hi Jason,
sorry for the long wait in answering. Yes, you understand me correctly, although the coupling that I wish could have to be non-linear: something like: pitch=f(flap-angle, …), where flap-angle is a degree of freedom for every blade.

I have examined the different alternatives that you suggested, and it was already an help to know, anyway, that FAST is not going to implement this dof. Yawdyn does not seem to provide a way to change the pitch during a simulation.

Actually I do know ADAMS (I have been using it for another job several years), but the company where I work has no ADAMS licence, and I feel that it would be no use at this stage: I completely agree with what Mr. Buhl writes in the “Usage Advice” page of the codes: the level of complexity of ADAMS is useful only in the “design-refinement” stage, when you have the plenty of data it requires.

So, I have been “skimming” the Symdyn user guide, because Symdyn seems to me the only viable alternative, apart from writing something from scratch. The main problem for me is that I do not have a Matlab/Simulink licence either :slight_smile:. I could be willing to ask the company to acquire one, if I can be sure to obtain what I am looking for… so I would be very grateful if someone of you could confirm the following:

Is it possible in SymDyn to let theta(i) be a function of beta(i) AND of some input coming from the control system?
Possibly because of my ignorance in Matlab/Simulink, this is not completely clear to me from the user guide.

Thanks a lot
Best regards
Claudio Pedrazzi

Hi Claudio,

Yes, in SymDyn it is possible to make pitch angle a function of any DOF available during simulation, including blade flap. The complexity of the function you choose is only limited by the features of Simulink, which isn’t much of a limitation.

However, you should realise that in SymDyn the flapping axis is pitched, i.e the order of rotations from the hub frame is pitch then flap (not flap then pitch). SymDyn adopts this convention because a real blade has both in-plane and out-of-plane components when it bends/flaps. If this is not the behaviour you desire, then parts of the SymDyn code would need to be modified. This is not a trivial task but it has been done before by at least one other researcher that has investigated flap-hinge dynamics.


Dr Karl Stol
Department of Mechanical Engineering
University of Auckland

Hi Karl,

it is a pleasure and an honour to be able to interact directly with one of the authors of the program!

The coupling I wish to look into is really a kinematic joint (imagine the two hinges being almost superimposed, like in a gimbal or cardanic joint).

From your answer I believe that SymDyn would do my case. I just have to give the same radial distance to the flapping hinge as the pitch hinge.

If I can, now this brings me to a (hopefully last) question: I will also need the capability to “resist” the flapping motion with some kind of highly non-linear torsional spring-damper: I should be able to write, for each blade independently:

RootTorque(i) = Function of (beta(i), d/dt beta(i))

The function could be segment-wise linear, but it certainly has to be discontinuous in its first derivative, like a spring that changes its K depending on the elongation. Of course the program would than have to supply the aerodinamic portion of restoring moment, depending on the instantaneous pitch and flap of the i-th blade.

Do you think it would be possible without important modifications of the code SymDyn?

Thanks a lot
Best Regards

Claudio Pedrazzi
Senior Engineer
Technical Dept.

Hi Claudio,

You can thank the diligent staff at NWTC for letting me know you had posted a SymDyn question. I’m happy to respond.

The nonlinear feedback that you’re suggesting is definitely possible with SymDyn in Simulink without code modification. SymDyn provides the states (beta, beta_dot) and Simulink allows the nonlinear function. Structural dynamics and aerodynamics are represented as separate Simulink subsystems. Therefore, it’s possible for you to add your restoring flap torque to the aerodynamic torque already calculated (within the block diagram).


Hi Karl,
thanks a lot for your answer (and thanks to NREL who told you there was a message on SymDyn!).

I would like to ask you if, to your knowledge, SymDyn can run on any “open source” scientific software (I am thinking to Scilab - Scicos scicos.org/ , they are, to my knowledge, free clones of Mathlab + Simulink).

I guess the answer is “no”, but I had to ask :slight_smile:!

Best regards and Happy Christmas!


I don’t have any experience with open-source alternatives to MATLAB/Simulink. Sorry, I can’t help you here.