In most helicopters, the horizontal tail surface is operating to a greater or lesser degree in the downwash from the main rotor in powered flight. Surely this must mean that as soon as the collective is lowered, the airflow over the surface is modified and causes a nose down pitch before the descent is established. Unfortunately, in practice it is impossible to disentangle the various effects. However, it is noticeable that larger helicopters still exhibit the nose down pitch even though their inertia means that they don't alter the flight path into a descent quite so quickly as for instance an R22.