Overpithed:
The way I see it is if you are doing 60 kias with a 30 knot headwind you are traveling across the ground (or through space) at 30 knots.
If you turn 180 deg (or the wind does) without a change in attitude or power you will still be doing 30 knots across the ground( or through space) but will now have a 30 knot tailwind ie ias = 0
Here's a simple test that any real helicopter pilot can do: Go up on a day when there's a brisk wind - say, 2,000 feet and fly directly into that wind. Slow to 40 knots IAS, get it stabilized and in trim. Now bank the ship and start a constant-attitude turn - your choice of direction. Do
not mess with the pitch attitude of the ship. Keep it going around and around, as many times as you can stand. You'll notice a couple of things:
1) Your indicated airspeed will never vary. As you turn away from the wind, your IAS will not decrease by the amount of the wind, or vice-versa;
2) If you look out at the horizon, you won't be able to tell when you're turning down- or upwind. Oh, if it's really smooth out and if you're really perceptive, you
might notice a tiny little increase in performance when coming around into the wind, and a corresponding tiny little decrease in performance when turning downwind, but the gains and sags are in the order of only a couple of feet of altitude. As you keep going around your overall altitude will remain the same;
3) If you look at the ground, you'll see that the ship will be describing a curly-cue as it translates across the ground in the direction of the wind.
As an experiment, I have done this in aircraft ranging from Cessna 150's to Bell 206's to BO105's. (Of course, in the Cessna the speed was just above stall speed, not 40 knots for you nitpickers out there.) The results are always the same. If you have not done this, please do not make any more theoretical postings about what you think "should" or "will" happen. Only those who have actually gone up and quantified it should post in this thread - not guys who've screwed-up a downwind turn close to the ground and have now erronesously concluded that such turns are dangerous.