Sign Conventions

Hello all,
I am a new user of FAST and hope that I don’t ask foolish questions but here are my first queries: -

Note I have used the search function on this forum and have found some useful info on this topic but not conclusive to me for my queries. eg [url]]

To put my position into perspective I am creating a model of our windturbine which is very similar to the SWRT as documented in the SWRT Final Report by Corbus and Meadors. The main differences are that ours is a 6 kW variant and it furls the other way, eg nacelle rotates clockwise when furling and viewed from above. The other difference, which I understand cannot be modeled in the FAST yet, is that we also have a horizontal plane as well as a vertical fin at the tail. I have taken Test15.fst and have started modifying the variables to match our turbine.

However I need clarification on the following things so far: -

  1. Referring to fig. 17 of the user manual, if TFrlTilt were say -80 deg would this effectively lean the top of the furl axis towards the reader given that the angle indicated on the figure is positive OR is it the TFrlSkew variable that governs the direction of ‘lean’? Eg if we set TFrlTilt = 80 but then set TFrlSkew to -15, would this then lean the top of the furl axis towards the reader. By this I mean that TFrlTilt effectively only has useful a range between 0 and 90 (although -90 and 90 is specified in the manual) and that if you allocate a value to TFrlTilt you must then assign a value, even if it is ‘0’, to TFrlSkew to define which direction the top of the axis is leaning.

  2. This question in a way is very similar to 1) but is regarding the tail fin. Referencing fig 19, assuming we had a rectangular vertical fin with the long edge perpendicular to the ground and TfinSkew = 0, TFinTilt simply is the angle of the long edge (trailing edge) with respect to the ground. Eg TFinTilt = 10 deg leans the top of the tail fin upwind in the ZX plane. Likewise by setting TFinTilt = 0 and setting TFinSkew = 10 deg this would then rotate the trailing edge away from the reader in the vertical plane, eg causing the rudder on an aircraft to rotate anti clockwise. I feel that TFinBank is self explanatory from the figure.

  3. Lastly my problem lies in that we have a horizontal stabiliser that effectively aids the lift of the tail as it has a crude airfoil, is set at 14 degrees angle of attack WRT the horizontal plane, and therefore produces lift in the vertical sense to help lift the tail when furling. This is not my design and feel that a correctly calculated TFinBank angle should do just that, however, analyse it I must, and short of dipping into the hard coding of FAST I welcome any suggestions. So far I have considered simply setting TFinBank to + 45 deg and giving a value to TFinSkew as per figure 17 which will tend to lift the tail earlier than a fin which is vertical with no skew angle.

Many thanks in advance from sunny England. No really…it is sunny here today!

In addition to this I want to add something about the SWRT FAST model and its correlation with the actual model.

In the SWRT report in figure 7, page 16 , the rotor shaft clearly is shown on the right hand side of the yaw shaft if looking downwind. According to figure 18 in the FAST user guide, Yaw2Shft would be defined as a negative value eg rotor axis it is displaced to the right WRT the yaw shaft . In the SWRT_Furl.dat the value for Yaw2Shft is positive.

From running the Test15.fst it would appear that before furling the nacelle sits at angle of approximately - 11 deg, eg clockwise rotation from wind direction. Then, when furling, this angle decreases, it rotates in a positive sense, eg counter clockwise when viewed from above. And this appears to be correct behavior for a Burgey turbine with the rotor shaft mounded to the RHS of the yaw shaft when looking downwind.

I base my positive angle sign convention on the right hand rule; make right hand into a fist and point thumb upwards. Thumb indicates direction of axis and fingers indicate positive rotation about that axis.

Does anyone have any clarification on this as it would appear to me that the FAST model seems to reflect the correct furl behavior but the lateral offsets in the model are the inverse of the sign convention.

Yours hopefully.

Phillip from a not so sunny England today. Rain forecast soon. Rain coat at the ready!

Dear Philip,

I’m not really sure I understand your first question (a Figure would help for clarification). Hopefully it is clarifying if I say that projecting the tail-furl axis onto the horizontal plane can be used to define TFrlSkew and TFrlTilt is the angle in a vertical plane from this horizontal line to the tail-furl axis.

I agree with the explanation in your second question.

Modeling a horizontal stabilizer would ideally involve a change to the source code, but in the absence of that, choosing an appropriate TFinBank sounds like the next-best solution.

Regarding the SWRT (Bergey 10-kW turbine), the rotor spins counter-clockwise when looking from upwind to downwind, but FAST can only model rotors that spin clockwise. So, many of the signs of the various inputs and outputs (such as Yaw2Shft) need to be “swapped” to mimic the appropriate effects. See the following forum topic for more information:

Hopefully the sun will show itself again. In cloudy places, I’ve heard the term “blue clouds” to describe those rare sunny blue skies :laughing:.

Best regards,

thank you kindly for your response. Regarding your last point first, that answers a lot of my problems that FAST only models clockwise rotation when looking downwind. What is more irritating is that I do remember reading that somewhere but of course have conveniently forgotten that!

Regarding point 1. Yes I understand that TfrlSkew is the angle projected onto the horizontal plane by the furl axis measured from the fore-aft datum, positive in the counter-clockwise direction when viewed from above. Also that TFrlTilt is as you describe it. My question was that even though TFrlTilt can accept a value between -180 and 180 deg, are its only practical angles, in this particular arrangement, between 0 (horizontal) and 90 (vertical) and it is TFrlSkew which defines which direction the top of the axis leans when looking downwind. Eg Assuming that TfrlTilt = 80 deg as shown in figure 17, would a positive TFrlSkew lean the top of the tail furl axis to the left of the machine when looking downwind, negative TFrlSkew would lean the top of the tail furl axis to the right? Likewise if TFrlSkew = 0 then the furl axis would not lean either left or right when looking downwind.

I shall simply ignore the horizontal plane for the moment until I am more familiar with FAST. I may experiment with TFinBank to see if it improves the furl response.

Indeed, cloud blues is one way of putting it. I like to take the Scottish attitude, which is, there is no such thing a bad weather, only incorrect clothing! That said, today we are again blessed with clear blue skies! What is what I love about living on an island facing the gulf stream coming off the Atlantic. You just never know what the weather will be like, hour to hour.

Many thanks.


Dear Phillip,

Actually, the valid angles for TFrlTilt are between -90 and 90 degrees (inclusive). Otherwise, your understanding is correct.

The tail-furl angle (TailFurl) is defined following the right-hand rule about the tail-furl axis when your thumb points to the “top” of the hinge, as you refer to it. We could have limited TFrlTilt to be between 0 and 90 degrees, but allowing TFrlTilt to be negative means that the user can define the “top” to be lower than the “bottom”, which effectively allows the user to control the sign of TailFurl. For example, a tail-furl axis defined by TFinSkew = 30, TFinTilt = 60 is identical to a tail-furl axis defined by TFinSkew = -150, TFinTilt = -60, but positive TailFurl in the first case is identical to negative TailFurl in the second.

I hope that helps.

Best regards,

thank you. Your last post was highly informative and has cleared things up for me a great deal. I have been running the model with the yaw and furl DOF disabled in order to refine the model before complicating things. Now I have that nuget of information from you, I will crack on with the furling behavior.

Yours…bathed in glorious English sunshine…while it lasts!