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.