Considering that origami is an art, and paper airplanes are basically the same, except that you make airplanes with the paper and throw them, is paper airplane making within the scope of crafts.se?
Yes, absolutely, the same as any other paper or modeling craft.
Just as you said, it's a folding paper art. It's also about making models. We've established origami as on topic, and paper crafting. I expect model making will be on topic. Paper airplanes fit into any one of these, so there's no reason to exclude them.
However, the questions should be about the craft itself, and not trivia such as "What's the largest paper airplane ever made?"
And not about making specific models, such as "How do I make a B-52 bomber?", same as with origami where "How can I make a paper giraffe?" wouldn't be allowed.
But "What can I add to my paper airplane to make the point sturdier without adding much weight?" or "Reliably tape parts to paper airplane without creating imbalance" should have a place here.
Yes, absolutely. As the person who's been asking most of the origami questions, I'd actually think of it as an origami sub-category, say, like kusudama, unit origami, rezuru, tesselations, microgami, currency/business card/gum wrapper, or free-form origami, that has its own special issues/techniques. I would, however, recommend tagging it as paper-airplane to be able to pull out the paper aircraft specific questions from the rest of the origami "noise".
The main thing is that the questions should probably be about the physical creation (folding, materials) of the airplane, while questions solely about aerodynamics of paper aircraft might be better served on aviation.SE or physics.SE. Issues in designing said aircraft might be a toss-up.