I don't really understand this negative Airspeed business, if you hovering in a 10 knot headwind, you have a IAS of 10 knots, in order to maintain that you need some power, as you turn around you will gradually start moving relative to the ground, but a given power setting will always (all other things being equal) maintain that 10 Knot IAS, what has negative airspeed got to do with anything?