The small prop diameter is because of the wake immersed propulsion concept. The idea is to have a propeller radius which only affect already disturbed air in order to minimize drag. The prop is also constant pitch if I understood this correctly. They are combining several proven concepts into one airframe, nobody has done this before. Synergy is a very well chosen name for the design. I like it, in fact I love it... they are on to something I just hope they can find the sweet spot in the design work.