RadioHelicopter.com

Torque is the tendency of a force to rotate about an axis. Radio helicopters also have a large rotating mechanism which is called a rotor in this case.

Reactive Torque

The big rotor in a helicopter is creating a lot of reactive torque in the opposite direction. Therefore the helicopter will want to spin in the opposite direction of the rotor. This also means  any change in engine power to the rotor or collective pitch will produce corresponding changes in torque.

There are three ways to deal with this reactive torque:

1. By using a tail rotor which is the most common solution in today’s helicopters. In this situation the force from the tail rotor must match the main rotor to cancel out the reactive torque and preventing the helicopter from spinning.  This movement is called the YAW.
   
2. An alternative way to control yaw is to use a coaxial rotor, because they will spin in opposite directions.

   With electric coaxial rc helicopters this is easy to do with two separate motors when they are spinning at equal speeds. However when trying to control a heli with two rotors and having to change torque between both blades to get movement does require some practice.

3. The least common solution is called NOTAR (No Tail Rotor) and instead uses air pressure to counter act the thrust created by the main rotor. This air pressure is piped down the over sized tail boom and vented out to the sides.

In conventional helicopters there is usually one main rotor consisting of two or more blades.

However to improve stability some electric radio controlled helicopters are configured for a coaxial setup or twin rotors. The benefit of having two rotors mounted is so they can spin in opposite directions and counter act the torque produced by the spinning rotor. It is also important to note that in a coaxial configuration there is no tail rotor. So you might ask yourself how does a coaxial helicopter change directions with no tail rotor. Well as long as the two rotors are spinning at the same speed the helicopter will hover. As soon as one increases speed torque will be generated again and you will be able to change your direction.

In most coaxial helis the top blades are mounted on the main shaft and the lower blades are mounted on a hollow shaft that runs inside of the main shaft. Twin servos (side by side electric motors) then control one of the two shafts independently.

This design is also used in life sized helicopters as can be seen here of a Russian Air Force Kamov Ka-50 attack helicopter.

Example of an electric coaxial radio controlled  helicopter:

Blade CX2