FAST Tail Furling Convention Clarification


I’ve been modeling a tail-furling turbine in FAST and having initially excluded the furling aspects using fixed yaw positions got results which look pretty reasonable so the basic model seems good.

I have now moved onto implementing the tail furling but am not getting sensible results for the tail behavior and I’d like to check if my understanding of the tail furl axis parameters are correct.

  1. TFrTilt is 90deg when tail furl axis is vertical and the arbitrary point is uppermost
  2. TFrlSkew is positive as drawn in figure 17 of v6.0 manual
  3. 0deg position of tail is inline with skewed position for input and output parameters. Positive values indicate clockwise movement and vice versus
  4. TFrlDSSP, TFrlUSSP, TFrlDSDP & TFrlUSDP are located relative to tail zero degree position
  5. TFrlDSSP and TFrlDSDP generally relate to tail position at it’s neutral position (i.e. position at 0m/s wind)

I’d like to make sure before I go into more details on my issue (I may not need to!).


Dear Mike,

Here are my answers to your questions:

  1. Yes, when TFrlTilt = 90deg, the tail furl axis will be vertical (or at least, parallel to the tower centerline at the tower top).
  2. Yes, TFrlSkew is drawn in the positive sense. Please note, however, that when TFrlTilt = 90deg, the value of TFrlSkew doesn’t matter. This is because TFrlSkew determines the projection of the tail-furl axis in the nominally horizontal plane and when the tail-furl axis is vertical, there is no horizontal component.
  3. I’m not sure what you are asking here. The input parameters related to the tail-furl geometry are defined with respect to a tail-furl angle of zero, regardless of the initial condition of the tail-furl angle. See the last paragraph of the “Tail-Furl” section of the “Model Description” chapter of the FAST User’s Guide for more information. When TFrlTilt = 90deg, positive tail-furl rotations occur counter-clockwise when viewed from above.
  4. Yes, the tail-furl stop spring and damper angles are defined relative to a tail-furl angle of zero.
  5. I’m not sure what you are asking here. The tail-furl stop spring and damper angles determine the tail-furl angles beyond which spring and damping moments apply.

By the way, I suggest you set the tail-furl Coulomb-friction damping, TFrlCDmp, to zero. The present implementation may lead to numerical problems with the solution.

I hope that helps.

Best regards,