If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope. If it turns downwind it needs to get some kinetic energy from somewhere, quickly
Not true.
An aircraft does not feel wind at all. Wind is totally irelevant to the aircraft, and the turn downwind makes no difference whatsoever.
The problem is the shear (change of wind). If the glider turns downwind while descending through a shear (20kt tail increasing to 40kt tail), now the glider has lost 20kt of airspeed, and will need to lose height to regain speed (trading potential for kinetic).
The above scenario can happen, but is unusual. The big problem for a glider is final approach into 40kt of wind, and then flying into the lee of a hill or trees, so that the wind suddenly reduces to 10kt. Now the glider has lost 30kt of airspeed, and needs to readjust by closing the airbrakes.
Just remember. Wind has no effect on aircraft flight, but a rapidly changing wind (shear) does.
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