Oh, I'd say ten? Flying machines already existed in my world, but they weren't widespread as they are here. I first started off with reverse-engineering more simple things, then moving on from there.

Now, designing my own flying contraptions? I began working on my greatest creation a few years ago: a flying fortress! But that was an on-and-off thing for me. In that time, I developed a sort of personal lift device for short-term use.

And I'd love to recreate it all here, but unfortunately, there's always the issue of energy source...