kybernetikos said it well: "spending a lot of novelty points" I would add: in a highly-regulated domain that segregates risk. FAA wants designs that won't kil

kybernetikos said it well: "spending a lot of novelty points" I would add: in a highly-regulated domain that segregates risk. FAA wants designs that won't kill people (including bystanders) when systems fail. Battery: using lithium with silicon anodes: unproven at scale? For landing, the Illium offers only a 60-second reserve after a expected 20-second hover. But can take time to land, particularly in wind: 20-80 seconds is too short. And since it is landing on rooftop helipads, without more reserve you could kill people just by blocking the helipad. Perhaps they could take-off via hover and land conventionally, but that would require stronger gear placed differently. Design: This relies completely on fans for directional stability? FAA even for experimentals, helicopters, etc. requires controllability on power failure, and e.g., 30 minutes of reserve power, more at night. It's doubling risk to integrate power and control, and engines into the wing. If some fans fail, you're adding controllability to power loss; it's unclear other fans could depower dynamically as required. And what if a fan goes catastrophic -- breaking the wing or nearby fans or control lines? Commercial airplanes can fly even when their engines blow up because the engines are largely segregated from flying and control surfaces (unlike military jets). I could imagine a more conventional hybrid stepping-stone to this ultimate concept. If you put fixed horizontal ducted fans at the front and rear of the fuselage, you get the benefit of lower disc loading for the bulk of vertical hover thrust. With colocated batteries, this would reduce power transit. On failure of both, fall back to conventional landing. On failure of one, balance out with tandem-wing alternate. As an aside: for homebuilt tandem-wing airplane, search for Rutan Quickie or Q200