Schlag suggests that the calculation of the linear head loss in a conical pipe can be done in the following way:
The head loss due to friction or linear head loss in conical pipe can be calculated according to Schlag (1927) by replacing the truncated cone with a straight imaginary line section of equal length L and constant diameter d such that:
d=([(d1 - d0)(d1^4)(d0^4)]/(d1^4 - d0^4))^0.2