Zeja,
Precone angle (permanently designed into the rotor hub) is arrived at using the calculated centrifugal force for a blade and the design lift the blade is intended to generate. The Arc Sin of thrust / C.F. produces P.C. angle, generally around 2.5 degrees. So for one thrust condition (100% design thrust) and one RPM (100% Nr, and assuming the C.F. is calculated correctly) the coning angle = Pre Cone Angle. This angle relieves the steady stress in the main rotor hub generated by Beam Moment or Mb and thus the peak stress levels which are the sum of the steady and max positive oscillatory beam moments.
