Hello everyone,
I am unsuccessfully trying to understand how FAST takes coning into account to calculate the deflection of the blade.
Our blade has been coned 4º forward and FAST results show a greater deflection of the tip (compared to 0º).
Is it possible to have the equations that calculate this or at least have an explanation of the concept?
Until now, I have followed Jason Jonkman’s Master Thesis and I have managed to calculate the tip deflections quite accurately when coned 0º, but I’m stuck with coning!!
Thank you very much,
Ohiana Goikoetxea
Dear Ohiana,
Perhaps I’m not understanding your question, but upwind coning will lead to increased blade deflection. In a downwind-coned blade, the centrifugal loads tend to cancel some of the aerodynamicly induced deflection. But in an upwind-coned blade, the aerodynamic loads and centrifugal loads both act to produce blade deflection.
Please not that In FAST, blade deflection is defined relative to the undeflected blade (pitch) axis. Perhaps you were thinking that deflection was defined relative to the unconed rotor disk?
Best regards,
Dear Jason,
First of all, thank you for your quick answer.
I am Ohiana’s co-worker. She posted my request, as I was not registered yet.
Let me explain my problem a little better :
I have understood why the upwind coning leads to greater deflection. Centrifugal loads act to bring the blade into the plane of rotation. If cone is upwind, those loads are in the same direction as aerodynamic ones, and if coning is downwind centrifugal loads are in the other direction.
As far I have been representing centrifugal loads as explained in you Master Thesis, equation 3.77, p.50. (nrel.gov/docs/fy04osti/34755.pdf.).
Centrifugal loads add an extra stiffness to the blade thus reducing deflection.
But this equation is not valid anymore if the blades are coned, is it?
What is the equation used by FAST to link coning with tip deflection?
Hoping that I have made myself clear,
Best regards
Guillermo Leturio
Dear Guillermo,
The introduction of coning would add a COS(PreCone) factor multiplying Omega^2 in Eq. 3.77 from that report.
That said, Eq. 3.77 is not actually implemented in FAST, in contrast to what is implied in that report. Instead, FAST implements all inertia-related terms through direct implementation of the kinemetics, which are based on the specified turbine geometry and all system degrees of freedom. But these equations are far beyond what can be written in a forum post. They are documented only in the “Unofficial FAST Theory Manual,” mentioned in this forum post: Coupled blade modes in FAST.
I hope that helps.
Best regards,