Nick, I'm not attacking you, just responding to all your posts at once.
You said, "It takes no more power to take off down wind than it does to hover in still air. "
I don't agree with this. When you take off with a tail wind, you must accelerate your helicopter first to zero airspeed, then beyond. Hovering in still air requires less power than accelerating and climbing from the still air hover.
If you meant to compare power required to take off with a tail wind to the power required to take off from a still air hover, I wouldn't argue. I think if there is a difference it would be small.
You also said, "any wind is good wind if you don't take off."
I always thought that in wind speeds slower than translational lift speed, the power required to hover with any relative wind would be greater than that for still air.
Finally, you said, "...make a careful downwind landing and note the power...needed to complete a careful approach. Compare downwind to that needed for still air, there is no difference."
The power required to approach along the same path, with the same speed gradient will be greater if you have a tailwind.
Practically, you can do the approach with the same power requirement down to zero airspeed by shallowing your approach angle with a tailwind.
What really amazes me in this discussion is the strength of the tail winds that have been mentioned. We have a published limitation of 15 kts for downwind takeoffs and landings. Even without that limitation there would still have to be considerable mitigating circumstance before I'd consider any downwind approach/landing in 15kt or stronger winds.
Matthew.