The discussion so far has focused on the forces and accelerations that the glider "feels". There seems to be agreement that a glider would feel the same forces and therefore experience the same accelerations in both the no-wind and the 100 kt headwind turn. It's pretty clear that the glider gains a LOT more kinetic energy in the case where it is turning downwind when it starts with a 100 kt headwind. My question is where all that energy comes from. One explanation would be that the glider would need to lose quite a bit more altitude when turning downwind from a 100 kt headwind in order to convert potential into kinetic energy. Unfortunately, that explanation doesn't fit with the suggestion that the wind should have no effect on the "vertical performance" of the glider. So... where DOES all that kinetic energy come from?