A propeller or Rotor in a rotary-wing aircraft is equivalent to the wing of a fixed-wing aircraft. The propeller (rotor) converts the rotational output power of the Brushless DC motor into the movement of air using the principle of Newton's 3rd Law of Motion. Basically, the energy of rotation is converted into the acceleration of air below the disk plane of the propeller, this Thrust is the force that lifts the multi-rotor aircraft, and pulls the fixed-wing aircraft forwards. A propeller is generally associated with a fixed-wing aircraft, whereas a rotor is generally associated with a rotary-wing aircraft. A multi-rotor has more in common with a helicopter, which is of course a member of the rotorcraft family. The use of propellers in a multi-rotor is therefore a bastardisation of their intended role. Propellers are usually denoted in inches and designated as the diameter multiplied by the pitch.