Jet head propagation and speed for runs CygX1_T7 (red), CygX1 (blue) and CygX1_T9. Run CygX1 slows down to the turbulent phase earlier than CygX1_T7, but display the same average speed during the turbulent phase, as shown by the almost parallel propagation curves. Run CygX1_T9 displays a different behaviour, decelerating to a plateau in the instability growth phase and with a lower average speed than the other two runs, which appears to decelerate after t = 12 000 s.