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,