RC electric helicopters with a single rotor (i.e. fixed and collective pitch, rather than co-axial) have a rear rotor to generate sideways thrust. To make the fuselage of electric helicopters remain straight, the rear rotor thrust must equal the torque from the main rotor. Adjusting the rear end thrust causes yaw, or movement of the tail, right or left.
The rear rotor is either driven by a separate motor (motor driven) or via a belt from the main motor (belt driven). The latter method is more complex, and therefore more expensive, but is generally preferred. Let’s see why.
Electric helicopters can have fixed or variable pitch tail rotors. With variable pitch tail rotors the pitch, or thrust, of the tail rotor is varied by commands from the receiver to the tail servos. Power to the rotor comes via a belt from the main motor.
This means the power to both tail and main rotor is constantly balanced, and yaw is controlled by servo command. It is a lot more complicated than this, but that is basically what happens.
With RC electric helicopters utilising a tail motor, the rotor is (generally) of fixed pitch design. There is no complicated gearing mechanism, pitch slider or servo. Thrust is varied by the speed of the motor alone.
Because the tail rotors on RC helicopters are smaller than the main rotors, they must have a much higher RPM for the tail thrust to balance the main blade torque. Even when negative yaw is applied (the tail thrust set lower than the main torque) the RPM is still very high.
However, the size and weight restrictions of electric helicopters mean any tail motor used must be much smaller and lighter than the main motor. If it is too heavy, it will upset the helicopter’s centre of gravity.
Since the motor is underpowered for the work it has to do, it is prone to burning out quickly. Also, lack of pitch control means that, just as with fixed pitch main blades, the craft is less responsive. There is always a lag before it responds to pilot commands, making it hard to fly during bad weather conditions.
Modern refinements, such as brushless tail motors, have improved tail-motor driven Electric Helicopters. However, advanced pilots generally reckon belt-driven as the best RC Electric Helicopters you can get.