Definition
A helicopter rotor configuration in which two main rotors are mounted on the same vertical shaft, one above the other, and turn in opposite directions. The counter-rotation cancels the torque each rotor would otherwise produce, eliminating the need for a tail rotor.
Plain English
Two main rotors stacked on the same mast, spinning in opposite directions. Because they cancel each other's twisting force, the helicopter doesn't need a tail rotor to keep it pointing straight.
Context Anchor
Seen in helicopter design, especially when comparing different ways helicopters control turning and balance the force from the main rotor.
Derivation
Coaxial comes from the Latin co- (together) and axis (axle). It means 'sharing the same axis.' In this case, both rotors share one central shaft.
Why Pilots Care
This arrangement cancels torque reaction, eliminating the need for a tail rotor and simplifying the aircraft.
Intuition Check
Coaxial does not mean two rotors mounted side by side. It means the rotors share the same centerline and are stacked one above the other.
Example Sentence 1
The Kamov Ka-52 uses coaxial rotors, so it has no tail rotor.
Example Sentence 2
Maintenance crews inspected the coaxial rotors before the next training flight.