If someone asks some questions on paper-planes and related items, would it be on-topic for this site?
I'd say yes. They are airplanes (well, gliders to be specific).
Besides, if you have questions about how the basic mechanics of flight work, a paper airplane is a good way to learn them. Not only that, but mechanics of flight is certainly within the scope of this site so, by extension, questions about paper airplanes should be fine.
We even have a tag for it. So, yeah. Ask away :).