Tension profile

The cable tension increases, going down stream (towards the exit), in each and every bend. If it were not for the aerodynamic forces, the cable tension would never decrease going down stream. This would result in rapidly increasing tension. This is the reason why it is typically impossible to pull a cable through a long duct by means of a pre-installed hawser. The tension in the hawser, where it passes through bends, would be so large as to damage the duct or break the hawser itself. The reason why aerodynamic cable installation results in much greater achievable installation lengths than does cable pulling is because of the resultant distributed body force. It is possible for the cable tension to decrease from one bend to the next bend going downstream, towards the exit, because of the aerodynamic propulsion.

When cable is blown through a duct, hawsers are typically not used. Consequently the travelling end of the cable is free, and the tension there is zero (except when traversing bends, in which case it can be negative, i.e. in compression).