The wind direction really has little effect on the rotor performance in a hover, any wind is good wind if you don't take off. The rotor really doesn't know which direction the wind comes from, and tail rotor power increase is nearly always less than the performance gain from the wind speed.
In a downwind takeoff, three factors complicate the matter (let's use 15 knot winds as an example):
1) The aircraft must accelerate from minus 15 knots velocity through zero velocity. At the zero velocity point (where the aircraft is scooting along at 15 knots groundspeed) the power will be at the maximum for the procedure. (If you were foolish enough to have been at max power when in the steady downwind hover, at this point you will settle and ground contact is a real possibility.) As you then continue to accelerate, the power will decrease (or the climb will begin) and you are off.
2) The takeoff distance (when compared to still air) will be the sum of that needed to get to zero plus that needed for a normal take off. With a 5 degree nose down as "normal" you will take you about 110 feet just to get to zero knots airspeed. Additionally, the acceleration will seem very flat relative to the ground and obstructions, since you are always 15 knots faster, so your climb angle will be considerably shallower than a still air takeoff.
3) If you have a power failure anywhere along the takeoff, you will not be able to stop the aircraft prior to touchdown, because that would require that you decelerate to zero, then continue decelerating to a rearward 15 knots. Count on needing to run it on at about the wind speed.
That was all for a dead downwind takeoff. For a cross wind, the issue is much cleaner. Let's assume the same 15 knot wind.
Since the rotor doesn't know where the wind is coming from, the performance is better from the start, it takes much less power to hover, so your acceleration for takeoff is greater, if you chose to use the extra power. The distance to takeoff is less than that of a zero wind takeoff, so you are ahead there, too. The only two complications are:
1) You gave up the chance to shorten your takeoff by about that 110 feet if you had taken the headwind.
2) If you have a power failure, you will not have the advantage of that extra 15 knots of airspeed on touchdown, plus you must compensate for the cross wind on touchdown, thus complicating the touchdown a bit.
All in all, the rules might be:
1) Take a headwind component as often as possible, always if it allows good takeoff paths and barrier clearance.
2) If dictated by the scenario, take a cross wind with the assurance that it is better than still air.
3) As a last resort, take the down wind if you have good power margin above the "steady" hover and if the takeoff path allows extra distance for barrier clearance and allows a running touchdown if the engine quits.
I have been trying to boil down a rule of thumb, but its a complex situation. If you have a 15 knot wind, and a helo that climbs at 500 FPM at 45 knots:
At 20 knots, the upwind allows a 40% climb gradient (40 up at 100 across), still air allows 7% and downwind allows 4%. In other words, for a slow speed climbout, you have 10 times the climbout gradient into the wind as down wind! Rule of thumb might be, if there are barriers, don't go downwind.
[ 03 September 2001: Message edited by: Nick Lappos ]