# RtAeroPwr and RtAeroCp - AeroDyn15

Dear all,

I have a doubt about the outputs of AeroDyn15.
How is RtAeroCp calculated?

I ran a simulation in order to obtain the Cp-Lambda curve for the 5MW wind turbine. Having set RtAeroPwr as output of AeroDyn, I tried to obtain the values corresponding to the RtAeroCp through the classical formula of power, that is:

Cp = RtAeroPwr / ( 1/2 * rho * V^3 * pi * R^2 )

I found that the results obtained with the formula match with the values of RtAeroCp ONLY IF I put as radius 61.5, that is the blade length, instead of the actual radius, that is 63.

Shouldn’t be calculated with the actual rotor radius?

Here I’ll put the comparison between the two different cases.

Best regards,

Dear Matteo,

RtAeroCp in AeroDyn v15 is calculated as follows: RtAeroCp = RtAeroPwr/(0.5AirDensRtAero*RtVAvgxh^3)

Are AeroDyn v15 outputs RtAero and RtVAvgxh as you expect they should be? Output RtAero is computed using the maximum rotor radius (including the hub radius), measured as the distance normal to the shaft. Output RtVAvgxh is computed as the disk-averaged relative wind speed (wind speed minus structural speed) projected along the shaft.

I’m not aware of a bug pertaining to output RtAeroCp. Are you running this case using the AeroDyn driver? Have you set the hub radius (HubRad) correct in the driver input file?

Best regards,

Dear Jason,