A coaxial rotor eliminates the need for a tail rotor and creates a safer, more stable machine.
In order to understand how a design with two coaxial rotors far outperforms other helicopters, we have to examine the physics at play.
The physics of helicopters
For single-rotor helicopters, lift-off is generated through the main rotor rotating. This rotation generates torque about the main helicopter, which causes the main fuselage to spin around in the opposite direction. Early engineers designed the tail rotor to counter this torque and keep helicopters stable. Tail rotors are generally much smaller rotors mounted on a perpendicular axis to the main rotor. By controlling the speed of the tail rotor, the pilot can stabilize the craft as well as control the direction of the helicopter.
Slowing the tail rotor would cause the helicopter body to rotate in the opposite direction of the main rotor due to excess torque in that direction. Speeding up the tail rotor would do the opposite. Along with direction, helicopter pilots can control the yaw of the craft by adjusting the angle of the tail rotor. By pitching the tail rotor slightly up or down, the pilot creates a moment arm through the helicopter, which in turn adjusts the yaw of the craft.
Why a coaxial design is so much better
Now that we understand the basic mechanics of single-rotor helicopters, we can begin to see why coaxial rotors might present some advantages. By placing two rotors on a single axis and rotating them in opposite directions, a net-zero torque around the main body of the helicopter is created, keeping it very stable. Through both mechanical means and electronic means, each rotor is perfectly timed and controlled to cancel out the net torque of the other rotor in real-time. This allows the coaxial craft to achieve rather significant hovering capabilities when compared to their single-rotor brethren.
When you think of helicopters, you think of vertical takeoff and the ability to hover. Remove those aspects, and the helicopter functions identically to a plane. As a side note, vertical takeoff isn't exclusive to rotorcraft. However, planes that harness the ability without rotors – mainly the harrier jet – accomplish the task with much less efficiency and stability.
A helicopter's ability to hover and be stable is synonymous with its quality of being a helicopter. In coaxial designs, the improved ability to hover and maintain stable flight ultimately makes for better helicopters. Better helicopters mean that they are easier to control and much safer for the occupants. Theoretically, if one rotor broke in a coaxial system, the craft could still be landed safely.
Lastly, the application of coaxial rotors means that there is no inherent need for the craft to have a gyroscope to provide stability. The rotational effects of both rotors provide for a near-perfect gyroscope, improving the stability of the craft once more.
So why don't we see more coaxial helicopters? They aren't without their faults.
Issues with coaxial helicopters
The first main fault is that the timing of the two rotor blades needs to be near perfect. Speed and directional changes need to be achieved together. Even the slightest fault in calibration essentially makes the aircraft unstable and impossible to fly. A fault in calibration is worse than you probably think for the craft's ability to fly. If the timing is off enough, coaxial helicopters won't produce enough lift to even leave the ground and end up just spinning on the tarmac.
On top of the need for accuracy in the tuning of the rotors, these rotors tend not to be as responsive as a single rotorcraft. When you make an aircraft more stable, you generally make precise movements harder to achieve – it's a constant tradeoff in aerospace engineering. While coaxial helicopters are safe and efficient, they are not well suited for applications where pilots need fine maneuverability. They are, however, perfect for applications where precise hovering is needed.
The coaxial rotor design is one of the most prominent helicopter designs to date. While it has its inefficiencies; it won't be going away anytime soon. The stability of the design is popular within the hobbyist community and even many military and rescue helicopters to date. If you were designing a helicopter, which design would you choose?