It's possible for the descending glider to maintain a constant forward component of velocity. Explain

It's -neccesary- for a descending.....

Gliders are gravity powered.

Gravity gives propulsion. Pitch controls speed. Dive more to speed up. Dive less to slow down. Just like rolling a ball down various inclines.

multiflyer

Gravity doesn't power anything. The energy to run a weight driven clock comes from whoever winds the clock. The energy that powers downhill skiers comes from whatever powers the ski lift. Gliders are powered by tow planes, winches, arm muscles, and the heat of the sun which causes thermals.

OK, point well taken. However, without gravity, there would be no conversion of potential energy into thrust. If there were no gravity, your weight, skier and glider would just sit where you put them.

Hydropower is solar power in that the sun powered the lifting of the water into the clouds. However if the water did not want to run down hill, the turbines would not turn. All very interesting, and I see your point.