the multi so-called rotor is inherently less efficient
because it accelerates downwards a relatively small amount of air by moving it fast
relative to an equivalent 'proper size' helicopter rotor - this made for acting on a larger amount of air so moving it slower
the lift here comes from momentum (velocity) change not kinetic energy (velocity squared) change
KE is wasted energy
this is the reason why ....
rotary wings are less efficient than fixed wings
rotary wings in hover are less efficient than rotary wings in translational flight (eg moving forward)
rotary wings in hover-out-of-ground-effect HOGE are less efficient than in HIGE
greater efficiency ie minimum KE cost for a given 'thrust' occurs when accelerating fluid that is moving forward
ie the propulsor working on boundary layer fluid - remember a body moving in fluid drags along the adjacent fluid layer
ships with such internal 'jet propulsors' use higher rpm lower torque engine types as ....
BL allows less fluid but more acceleration (for a given efficiency ie momentum vs KE outcome)
in aircraft the (liquid-cooled) electric motor is better suited than the ICE to positioning for BL
and BL propulsion can reduce airframe drag