Just build them big and put large motors on them !
http://www.youtube.com/watch?v=7VNPcMwwsE8#ws (http://www.youtube.com/watch?v=7VNPcMwwsE8#ws)
http://www.youtube.com/watch?v=OQA1Y5AcwMo#ws (http://www.youtube.com/watch?v=OQA1Y5AcwMo#ws)
The guy who opens the door in the second video is like "OMG! Lay low! Lay low! The bots are still around..." ;)
https://www.youtube.com/watch?v=L75ESD9PBOw (http://www.youtube.com/watch?v=L75ESD9PBOw#ws)
The Volocopter:
http://www.e-volo.com/ (http://www.e-volo.com/)