I saw https://www.youtube.com/watch?v=tohImHa4f5U (Hoarder Sam, "I'm building a drone for the DARPA lift challenge") the other day and it was a pretty good discussion of the "shape of the envelope" of the problem (and what kind of lift ratios actually exist in modern air vehicles), and particularly how they've set up the constraints to eliminate a bunch of "easy" approaches.

It also reminded me that for the first round of the self-driving grand challenge, none of the vehicles even completed the course :-) They really are trying to encourage "out of the box", or at least "not in the obvious box", designs...