A quadcopter or drone is a multirotor helicopter that is lifted and propelled by four rotors fixed at the four arms. Quadcopter uses two pairs of propeller that are at fix pitch i.e two clockwise and two anti-clockwise so as to cancel out the angular momentum. Each rotor or a motor is at a different speed so as to give the proper control to the drone or a quadcopter. Controlling the speed of each rotor gives you the desired thrust at each arm and to fly in a desired direction.