Downwind turns equal disaster??

[overpitched]

In nil wind if I was flying thru the sky(space) heading south at 100 knots in a 206 at 1400 kg my momentum would be M X V = 140,000 kt/kg

Add a 50 kt tailwind I'm now flying thru the sky at 150 knots in a 206 at 1400 kg my momentum is 210,000kt/kg

The practical application from the pilots point of view is that in both cases he is doing 100kias. From a physics point of view the momentum of the aircraft would increase turning downwind & decrease when turning upwind and that can only be caused in this case by accelleration. Which means unballanced forces.

If the wind was not a force that acts on an aircraft to either assist it or hinder it then navigation would be a hell of a lot easier.

Nick.. The problem I'm having seeing it from your point of view is you seem to be treating the aircraft as if it has no motion relative to the air.
If you pick a fixed spot in the air say a hot air balloon floating with the wind. wind is 50 knots from the north so the balloon is heading south at 50 knots. Agreed ?

I'm in a 206 heading north but at 100 kias. I decide to fly around the balloon and head back south. To maintain a constant speed relative to the balloon all I have to do is fly in at 100kias and back out at 100kias simple

But what I have done with regard to the laws of motion is fly in at 50 knots accelerate and fly out at 150 knots. As I haven't changed power or attitude the force that provides the acceleration can only have come from the wind.