I kind of think of it as "free" in the way a ball falling from a height doesn't require any fuel to locomote. Yes, you put energy into the throw. But if you c

I kind of think of it as "free" in the way a ball falling from a height doesn't require any fuel to locomote. Yes, you put energy into the throw. But if you count the guy hauling a hang-glider up a hill (or in other words it's potential energy at the top), I wouldn't think of hang-gliders as energy efficient at all. So either way I'm having trouble reconciling the comparison. Either they're infinitely efficient (zero input energy) or not comparably efficient. Maybe they're counting the twitches / muscle movements and such of steering the apparatus while in flight?