The point about water ballast is actually not entirely accurate - it's not to stay up longer, it's to increase speed. Gliders have a certain angle of attack at which they fly most efficiently (optimal glide angle, i.e. trading the least height for every meter forward). If you add water ballast, the speed corresponding to that angle of attack is increased. The downside is that your downward speed also increases - so when the weather gets difficult or thermals get weak, pilots dump the water.
> The downside is that your downward speed also increases
You're saying the down component of the velocity vector also increases with the velocity itself. Not that heavier objects fall faster, which we know to be untrue from Galileo's famous experiment. I think you understand that, but I thought I'd clarify it.
No you're wrong. Galileo's experiment doesn't apply here. Heavier objects DO fall faster at terminal velocity. A glider in sustained flight is an example of that.
Galileo just showed that objects accelerate at the same rate due to gravity. But they do NOT fall at the same rate unless in a vacuum.
