"Sorry guys, I made a mistake (my English is not so good). Euan Ramsy is right. I used above instead below. Yes, I would like to say that it is more stable when the CoG is on the bottom....or when the rotor is above of the CoG and vice versa. Thanks."
"Rotor above the frame creates a parasite drag when the downwash hit the arms, thus the efficiency of the air flow will decrease / less lift. In another hand, rotors below will be more efficient, but you will create others problems (lack of motor…"