Dear Mahmoud,
We’ve had a discussion on this forum regarding how FAST calculates RotCp. Please see the forum topic found here: Betz law and RotCp. Hopefully this clarifies things a bit.
The power coefficient predicted by WT_Perf is calculated purely from steady state aerodynamics. FAST should produce similar results to WT_Perf under these conditions (but not identical results because the aerodynamic algorithms are slightly different).
I hope that helps.
Best regards,