Originally Posted by
henra
Could it be due to the relatively instantaneous change of the airflow from static hover with its specific airflows through the disc to 'normal flight' airflow which happens when going through ETL? The Rotorblades 'suddenly' getting clean air from the front. This could potentially lead to some 'overshooting' of the blade track, somewhat similar to a gust hitting the disk.
It certainly could, but I would expect inflow roll to have at least as big of change, since the front of the rotor system is first and the rear is last to realize the airflow changes. However, the roll seems minor compared to the amount of forward cyclic needed to keep the nose where it was.