CBL - thanks for the comments. I realise that Momentum is a vector quantity by virtue of its velocity component and I should have mentioned that I was comparing magnitudes rather than the whole vector quantity. I was meerly trying to demonstate that when using the ground as a reference system, things get more complicated and more difficult to visualise.
Overpitched - Ask yourself this - What is wind? It is simply the movement of air relative to the ground. No ground = no wind. The helicopter does not know whether there is ground there or not but it still flies. So how can wind affect the aerodynamics of a helicopter.