The ubiquitous Outrunner Brushless DC (BLDC) motor has the rotor on the outside, incorporated into the casing. The BLDC motor is essentially a standard DC motor, without the old carbon brushes, which caused friction, sparks and ultimately failure due to wear. The Electronic Speed Controller (ESC) essentially takes the DC voltage input and outputs a three-phase AC trapezoidal (or sinusoidal) voltage to each of the three windings within the BLDC motor. In a sensorless BLDC motor, the back EMF is the feedback loop that provides the ESC with information about the position and speed of the rotor. BLDC motor manufacturers rarely quote their internal design parameters, such as the number of windings, core material, and permanent magnet data. The U5 motor from T-Motor is a typical BLDC motor used in medium sized multi-rotors. The motor speed (RPM) was measured using a handheld laser RPM meter.