From RC Helicopter Wiki
A helicopter will almost always have two rotors, arranged in a variety of ways:
- Two main rotors, stacked on a common axis but rotating in opposite directions, canceling out each other's torque reaction.
- One large main rotor on top, and a smaller but faster turning tail rotor mounted remotely to counter the torque reaction.
- Two large main rotors, mounted on separate axes, and rotating in opposite directions.
Other configurations exist.
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