This is for those who believe that despite a constant wind the aircraft has to accelerate through the air when it turns downwind:
Imagine a 747 flying along at 500 knots. Now imagine a model aeroplane flying around that passenger cabin at 10 knots. Are you trying to tell me that when the model is facing the back of the plane and turns towards the front it suddenly has a 500 knot tailwind? And are you trying to tell me that there is a difference aerodynamically between the model flying towards the nose of the aircraft and turning towards the tail compared to flying towards the tail and turning towards the nose?
So if you agree there is no difference then imagine the 747 flying in a parcel of air that moving along at a constant 100 knots. It is the same effect. The aircraft is blissfully unaware of the steady wind, the airspeed stays constant whether it turns into wind or downwind.