ORIGINAL: Andrew McGregor
There's a component of total energy (sum of potential and kinetic) that is due to the glider's position in the moving airmass. So in a no-wind situation the glider has much less total energy than in the high-wind case. So as it turns downwind, it exchanges some of that total energy from potential (pointed upwind in a moving airmass) to kinetic (pointed downwind in a moving airmass). The change in total energy is the same as in no wind, it's just there was a lot more energy than you thought to start with because of the wind.
If I am walking at 3 mph toward a parked car and collide with it, the force of impact is calculable, based on the relative velocity between me and the car. Now if the car is moving away from me at 2 mph, and I collide with it while walking again at 3 mph, the force of impact will be calculated base on a relative velocity of 1 mph. My total energy hasn't changed, only my velocity relative to a car. Relative to another car moving toward me at the same time, my relative velocity is simultaneously different. My total energy of motion relative to the medium I am acting on in order to move hasn't changed (still moving 3 mph relative to the earth), but my kinetic energy relative to various cars is different for each car at any given time. Same with a plane. The plane is acting on the air in order to move, so motion of the earth beneath is not relevant unless in contact with it (or I want to plot a deduced reckoning course).
I think we probably agree, but we might just be trying to state the same thing in different ways. I think we all agree that a steady wind doesn't affect an airborne airplane's airspeed or vertical speed. And, of course if you hit the ground at a higher groundspeed, due to flying downwind, more damage will be done. That statement seems to sum up shoe's and my arguments.