Drag has an upward component on every descending object.
Jump out of your glider at 10,000ft and you will accelerate at 9.8 m/s/s until drag acting opposite to gravity increases ehoung to stop the acceleration (about 200mph I think).
Now pop your chute.
The extra drag from the chute (acting upwards) reduces your descent speed and keeps you in the air longer than would have been the case without one.
But drag also has a component along the flight path. In the falling case the flight path is vertical and all the drag acts along the flight path. However, in the glide some of the drag acts upwards while the bigger part acts along the flight path.
You are correct to say that gravity is the force that acting on a glider in a downward sense. However, the glider's ability to resist that force is directly dependent on drag.
What I find amusing about this discussion is how the whole idiology can be changed. Point and power people would probably press the point that extending a speedbrake (not spoiler) would increase drag and thus slow the aircraft. You I think see the result of using a speed brake as causing height to be lost quicker.
Are you not thus saying that the increase in drag by extending the speed brake has caused the aircraft to descend more quickly and thus varying the drag using speedbrake has a direct effect on the amount of time that the aircraft can remain airbourne?
Regards,
DFC