David,

In your original example you turned a helicopter through 180 degrees whilst maintaining 0 groundspeed. Effectively, as far as the airframe is concerned, you are now flying backwards. The increased power requirements aren't due to the turn or the air over the rotor disc but the fact that the fuselage is now 'downwind'. The same additional power demand is required if you lift into the hover downwind and this area of manoeuvering does require caution for a number of reasons, including restricted power. As I mentioned before, the rotor disc will still experience the same airspeed (and translational lift) when downwind but this is offset by extra power needed to hold the tail against weathercocking and issues caused by the horizontal stabiliser and possible tail rotor interference. If you were to maintain 20 kts forwards airspeed (i.e. forward flight) whilst turning then there is no such limitation on handling and you can be relatively 'care-free'. Also, if you allow the helicopter to drift with the wind you can pedal turn without any problem as the airspeed will be low. The one area that can catch people out when doing this very close to the ground is the visual illusion issue caused by varying groundspeed which will cause inadvertant airspeed changes. This is now back to the 'downwind turn' myth.

p.s. Retreating blade stall is a high speed issue and can be disregarded in hover manoeuvres.