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What happens if you take off one of the main rotors of a two-rotored helicopter (excluding the tail rotor from the two main ones)?
This is a great question. Without the second rotor, a two-rotor helicopter would spin around like a top and be uncontrollable in flight.
The reason for this is the law of conservation of angular momentum. The simplified version of this law says that if part of a system starts spinning one way, a different part needs to spin the opposite way. When the main rotor of a single rotor helicopter starts spinning, the main body of the helicopter wants to start spinning the other way. The tail rotor provides a force to counteract this spin.
Tandem rotor helicopters (i.e. two-rotor helicopters) actually don't have a tail rotor. The rotors spin in opposite directions to counteract each other so that the body doesn't spin at all. If one of the rotors was removed, the same thing would happen as on a single rotor helicopter. The helicopter body would spin in the opposite direction of the remaining rotor.