I have flown with a classical guitar. I could not always take it carry on. The best thing is to invest in a good sturdy travel case. Of course, the case may cost as much as the instrument itself, especially if you travel with your knockabout instrument. The main things to look for are a good fit that prevents the instrument from moving inside the case but does not put any extra pressure anywhere, a rigid shell that will not be compressed by a ton of luggage on top, and secure hasps and hinges that can't pop open accidentally. Most airlines try to pack instruments in protected spots, but you never know. And it will likely get dropped, kicked, or otherwise knocked about. Probably not on purpose, but just because it happens. But if you have a good case, and loosen strings to reduce the stress on the soundboard, it should be fine. My guitar survived unharmed not only the airline, but being tossed about from boat to boat in the Bearing Sea in winter.