Calculating F1 in Johancsik torque and drag model

[Ekaterina Wiktorski]

Hi! Is anyone experienced With T&D calculations using above mentioned model:

?
I have a trouble to understand what exactly F1 is and how to calculate it. I know it is an axial force, so it is drag? And once you calculate drag for one element, you use it as F1 to calculate drag in 2ns element, is it correct?

 

[BvU]

Page 22 in vdPoll thesis An investigation of the stress-strain behaviour of a GRE cylindrical structure used for a drilling-with-casing application and its influence on torsional vibrations