As things stand, aside from Joby (six) the best-funded eVTOL projects err on the side of extra when it comes to rotors, with eHang, Volocopter and Lilium sporting sixteen, eighteen and as many as thirty-two.
And you can probably get away with as few as six for human carriage if you are only dependent on these for vertical manoeuvre either side of extended flights as fixed-wing aircraft, as Joby and Lilium's prototypes are ~ the bulk of the world's fixed-wing aircraft having just two engines.
When it comes to pure multi-copters however it would appear that more is better, though I think personally that if eight is enough, then a dozen (as seen here) would be more than enough.
The above would be configured as an independent quadcopter surmounted by an independent octocopter, with the former tasked purely with lift whilst all is well but with the capability to steer the aircraft should this prove beyond that of the octocopter for any reason.
It therefore offers comprehensive redundancy beside modularity, in that each of the drones is interchangeable along with the size and scope of the payload compartment.
(It is also a significant variation in view of the fact that EASA in their ~ lack of ~ wisdom dictate that even in the event of a forced landing, eVTOLs should retain directional capability: I say that is fine so long as they insist that airliners with a double engine failure and similar rate of descent should be able to maintain a forward speed of zero in much the same way.)
I register the idea in patent and design form if only by way of technical disclosure, because once the octo is flying regularly someone sooner or later is going to suggest twelve propellers instead.
It was Edison who said that you could guarantee that having developed any particular product that there would be any number of people emerging from out of the woodwork to say they had thought of it first.
My thanks to cousin Mat for this marvellous render in SketchUp... turns out that architectural training was not entirely wasted.
