# Strange values of Ct

Hello,

My local Ct values in the FAST output gives some values which are 3 to 6 time less than what I get by calculation thanks to the Cl and Cd (which are good).
The Cn is ok too, so I’m wondering if don’t have to multiply the Ct by something to get the real one, like r/R for instance…? (I tried this one, it didn’t work too)

Dear Franck,

I’m not aware of any problem with the outputs of Ct from FAST. Which version of FAST and which version of AeroDyn are you using? Which output are you channel are you examining?

Best regards,

Dear Jason,

Thank you for your answer. I’m using the version v8.09.00a-bjj of the 30-September-2014 for FAST and the v14 of Aerodyn.
I take the mean value of the output CTang** for the last time steps for each printed local radius from the file “.AD.out”.

It is very strange because I take the CNorm one column before and the values are right…

Best regards,

Dear Franck,

I downloaded FAST v8.09 and ran a test to compare Cl and Cd against Cn and Ct from AeroDyn v14 and my hand calculation matched the AeroDyn output exactly. Are you sure you used the correct transformation? The transformation I used is as follows:

Cn = ClCOS(alpha) + CdSIN(alpha)
Ct = ClSIN(alpha) - CdCOS(alpha)

(Note the minus sign on Ct by convention.)

Best regards,

Hello Jason,

I’m using the same formulas than you externally for the calculation except that I don’t put alpha in the cosinus and sinus but alpha+twist+pitch. But it is more correct, no? The calculations gives me good results for Cn and Ct compared to my vortex panel code, but they are still different compared to the output of FAST.
My output Ct is lower than my calculated one.
Are there no 3d effects on the Ct due to the induced Cd at all? Compared to the vortex panel code the Cp is the same, the Ct and Cn fit with the calculated ones of FAST, but not the Cd because of these 3d effect which are not considered in FAST apparently… So I’m expecting some difference on the Ct consequently, but with the calculated values it is not the case…

Dear Franck,

The Cn and Ct I was referring to in my prior post are normal and tangential to the chord, so no, the twist+pitch are not included in the transformation. I believe you are seeking the coefficients normal and tangential to the rotor plane, but AeroDyn v14 does not output those directly. (AeroDyn v15 within FAST v8.12 can output the coefficients normal and tangential to the rotor plane, which are referred to as Cx and Cy – see Appendix E of the draft AeroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/AeroDyn_Manual.pdf.)

I’m not sure I understand your question about 3D effects on Ct due to induced Cd.

Best regards,

Dear Jason,

I understand better why my Cn and Ct values in output where wrong now, I thought they were normal and tangential to the rotor plane…
For the 3d effects it is no more a big deal, I suppose that FAST doesn’t consider it at all because when I deactivate them in my panel vortex code I get similar aerodynamic coefficients.