# Precone angle in WTPERF

Hi everybody,
I have two more questions

1. the manual of WTPERF says that the precone angle has to be always positive. In a sample input file, coming possibly from an earlier release, I read the comment
Precone angle, positive downwind
Am I correct in assuming that, for example, a +5 in this field represents blades coned toward the wind (that is the normal way to help avoid blade-tower collision for upwind turbines)?
2. I am a bit puzzled because I run a few tests, with precone angles ranging from 0 to 20 degrees, and I found an increasing CP. Is that correct? I always thougt that every deviation from 0 deg, both for tilt angle and precone angle, meant a loss of performance. Is this due to the fact that I have to reduce the radius of the rotor, because it is intended normal to wind?
Thanks a lot
Best regards from Italy
Claudio Pedrazzi

Thinking his post didn’t get through, Claudio sent me email on this subject. I thought it would be useful to post the response I emailed to him.

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Dear Claudio,

Denver is shut down due to a blizzard, so I am responding to your email from home. The snow is over 60 cm deep out there!

I don’t know what’s wrong with the forum. I’ll check on it when I return to work.

It’s been so long since I thought about precone in WT_Perf, I decided to read the manual. It says: “The PreCone in degrees should be a positive value regardless of whether the turbine is downwind or upwind.” Generally, the difference between being preconed upwind or downwind is how the aerodynamic bending moment either adds or subtracts from the rotationally induced bending moment; it’s not an aerodynamics issue. As WT_Perf is an aerodynamics-only program, there is no rotational component to the root bending moment, so it makes no difference what the sign it.

I haven’t had a chance to read the code to make sure, but my guess is that the precone appears in the equations only in the form of COS(PreCone), so the sign should not matter.

As for the impact of changing precone on the power coefficient, increasing the precone will reduce the power–not necessarily the power coefficient. Increasing the precone reduces the swept area, so the power available from the wind goes down as the converted power does. As Cp is the ratio of converted power to available power, I would think that the effect on Cp would be small. I suppose precone may change the angle of attack some and that could change the performance. I’d have to run your case to be sure. Can you verifiy that the converted power goes down when the precone goes up.

Another thing to keep in mind is that WT_Perf uses an iteration scheme that attempts convergence of the solution to the BEM equations. Your choice of tolerance can have a significant effect on the answers. I was working on a newer version of WT_Perf last summer that uses a more-robust solution method that gives better answers. Unfortunately, it runs much slower than the currently-released version and I was pulled off the project before I could find a way to improve the performance. I hope to get back to finishing that in a few months.

Happy Holidays!

Marshall

Dear Marshall,
thanks for your answer. The original post did not get through, because of some special character (possible the “degree” sign). I reposted it successfully, so we share our discussion with the forum, what I believe is a very good tool.

Now, you are right, of course: the power is going down! Here is a little table of my experiment (I deleted the uninteresting parts):

Precone___Power____Cp
0________62.509____0.4554
5________62.034____0.4571
10_______60.617____0.4624
15_______58.278____0.4711
30_______46.227____0.5185

Obviously the two last entries were “just to see” where is the trend!

It is a “small” wind turbine of 50 kW/25 diameter, for very low winds. I understand that the effect is small, certainly under the “numerical noise” of a simulation, but I was puzzled by the increase. And still have difficulties in physical understanding of this behaviour. Could it be that, due to the tilt angle (5 degrees) the upper part of the rotor disc is working with a better angle of attack? But then, this should apply only for about 5 degrees of precone.

Anyway, it is a small effect and not blocking at all for my usage of WTPERF. I am looking forward to the new version of it, even if slower!

Best regards,
and a Merry Christmas and Happy New Years to you and to all the NREL team!
Claudio

yes, Happy belated New Year to all.