From the Kickstarter: "Why are your props upside down? The inverted props give an efficiency gain proportional to the amount of obstacles in the air column. We'

From the Kickstarter: "Why are your props upside down? The inverted props give an efficiency gain proportional to the amount of obstacles in the air column. We've found it varied between 10-20% depending on the frame and protection ring design. The inverted props also let us put the props right in the middle of the center of gravity for awesome control response. Even better, with the props on the bottom they keep spinning when you run up against the ceiling making flying indoors a breeze."