The main reason is for stability and control. Having two rotors allows for counter-rotation, which helps cancel out the torque produced by the main rotor, making the helicopter easier to handle and more stable in flight.
Yes, there have been single-rotor helicopters built, but they are not as common as dual-rotor ones. Some examples include the Robinson R22 and the Bell 47.
Single-rotor helicopters have a few disadvantages compared to dual-rotor ones. They tend to be less stable and require more complex control systems to compensate for the torque produced by the main rotor. They also have a higher risk of rolling over during takeoff and landing.
Single-rotor helicopters can be more maneuverable and have a higher top speed compared to dual-rotor ones. They also tend to be lighter and more fuel-efficient, making them ideal for certain missions such as aerial photography or emergency medical services.
The decision to use one or two rotors on a military helicopter depends on the specific needs and requirements of the mission. Single-rotor helicopters are generally better for speed and maneuverability, while dual-rotor ones are more stable and can carry heavier loads. Some military helicopters, like the CH-47 Chinook, have tandem rotors to combine the benefits of both designs.