I saw that 18 rotor one a while back...
The key to a good multi-copter with fixed pitch props will be it has to be electric motors and braking ESC's... we got that tech today...
Using piston engines on fixed pitch props...
They are too slow to change power up and down to be very controllable...
You might get one to hover... but show me spins 180 degrees then climbs out like an Inspire 1...
HA!... no way...
The problem now is payload weight (people) verses battery weight and capacity...
It's reported that the 18 rotor above flies great... as long as nobody is on board...
The problem is the same as what effects drones today... Can not store enough energy in a lightweight battery to make it work...
First company to build an economical micro turbine direct driving a generator with a battery to level out amp draw will make this all doable...(large electric multirotor that can carry a person 1 hour)
Either that or a 100x leap in battery power density...
Just look at the power system on the Solar Impulse 2... and remember it's a 40 to 1 glider...
https://en.wikipedia.org/wiki/Solar_Impulse...
Making something that can carry 1000 lbs payload (4 pass + luggage)... take off like a helicopter and go 500 miles on one charge... HA!!... can't be done anytime soon...