Opposing rotors is a proven helicopter design, https://en.wikipedia.org/wiki/Coaxial_rotors https://en.wikipedia.org/wiki/Coaxial_rotors By placing the rotor u

Opposing rotors is a proven helicopter design, https://en.wikipedia.org/wiki/Coaxial_rotors https://en.wikipedia.org/wiki/Coaxial_rotors By placing the rotor under the vehicle you remove the mechanical complexity of the rotor but you don't have any way to land... Unless your landing gear is as wide as the rotor and extending around it, or some kind of arm catching the whole vehicle from the side.