Tachometers (also known as revolution-counter sensors) are highly recommended for rotor-wing frames because they allow the autopilot to detect stall or another rotor failure (for rotor-flight it is the rotation of blades that guarantees lift not airspeed or ground speed).
This section lists the tachometer sensors supported by PX4 (linked to more detailed documentation). More detailed setup and configuration information is provided in the topics linked below (and sidebar).
