Two possibilities:
Way 1 - Multisection-Voxel: Build up the Mi-24 like the Nighthawk and use DMZ's HVA-Editor to make the rotor(s) spin; it's pretty self-explaining.
Way 2 - seperate turret: Seperate the rotor from the body and make it the copter's turret; then use TurretSpins= in the code.