Hi Overpitched,
If you are flying in an unaccelerated mass of air (unaccelerated means it is not changing velocity), it doesn't matter what the speed of the air is. The helicopter (or bird, or balloon) will not know or be affected by the speed of the airmass. Airspeed is airspeed - if you fly 50 KT airspeed surrounded by a 50 KT southbound airmass, the helicopter will not handle, respond, or perform any differently than if the airmass was not moving. The only way you would know if the airmass was moving would be by looking at a ground reference (or a GPS). Turning downwind has no meaning under these circumstances - there is no "downwind" in a constantly-moving mass of air unless you choose to reference it to the ground.
Ride in a hot-air balloon and you'll decide that maybe the air isn't moving at all, in fact the Earth is turning and the air is still. IT DOESN"T MATTER, until you are trying to fly consistent GROUNDSPEED instead of airspeed.
Inertia only rears its head if the airmass accelerates (changes speed or direction), i.e., wind shear, thermals, wind gusts, downdrafts, etc. That WILL affect performance no matter what maneuver you may be performing at the time. If you are turning downwind and the wind suddenly increases, the inertia of the helicopter initially resists the acceleration, so there is a brief decrease in airspeed (the lighter the aircraft, the briefer the change). This is a result of a CHANGE in the air's speed - if the air's speed remains constant, their is no such inertial effect.
OK, now on to the theory of quantum strings....