Someone could rewrite the aerodynamics of helicopters from this thread alone. Specific wind conditions can either help or hinder power. It only needs to vary a few degrees or knots to change from an aid to the rotor efficiency to an airframe drag or vice versa. Tailwind or crosswind, but never headwind (Unless you're in a 47 & it's gusting 55kts). I'd say 9/10 of the time you want a bit of extra torque to turn downwind above 8kts(Excluding mediums and above). If there was no effect at all helicopters would fly as fast backwards as they do forwards. I've seen a lot of people try but they don't!