Definition
A small rotor mounted on the tail boom of a single-rotor helicopter that produces sideways thrust to counteract the torque generated by the main rotor and to provide directional (yaw) control. Commonly called the tail rotor.
Plain English
The little rotor on the tail of a helicopter. It pushes sideways to stop the helicopter from spinning the opposite way to its main blades, and lets the pilot point the nose left or right.
Context Anchor
Seen in helicopter systems, especially when discussing tail rotors and directional control.
Derivation
Auxiliary comes from the Latin auxilium, meaning help or aid. The auxiliary rotor is a helping rotor — it does not provide lift like the main rotor; it exists to support it by cancelling its torque and giving the pilot yaw control.
Why Pilots Care
Without the auxiliary rotor the helicopter body would rotate uncontrollably opposite the main rotor; precise control of its thrust is required for stable hovering and coordinated turns.
Intuition Check
Auxiliary does not mean optional or unimportant here. It means additional to the main rotor and needed for control.
Example Sentence 1
When the pilot pressed the left pedal, the auxiliary rotor changed pitch and the nose swung to the left.
Example Sentence 2
In a right turn the pilot reduced collective pitch on the auxiliary rotor to swing the nose around.