Radio Controlled (RC) helicopters are model aircrafts which are distinct from RC airplanes because of the differences in construction, aerodynamics and flight training. Several basic designs of RC helicopters exist, some more maneuverable than others (such as helicopters with collective pitch). The more maneuverable designs are often harder to fly, but benefit from greater aerobatic capabilities.
Flight controls allow pilots to control the collective and throttle (usually linked together), the cyclic controls (pitch and roll), and the tail rotor (yaw). Controlling these in unison enables the helicopter to perform most of the maneuvers an aeroplane can do, and many that aeroplanes cannot, such as hovering and backwards flight; in this manner, they are quite similar in operation to full-sized helicopters.
The various helicopter controls are effected by means of small servo motors. A piezoelectric gyroscope is typically used on the tail rotor (yaw) control to counter wind- and torque-reaction-induced tail movement. This "gyro" does not apply a mechanical force, but rather, electronically adjusts the control signal to the tail rotor servo.
The engines used are typically methanol-powered two-stroke motors, but gasoline, jet turbine and increasingly electric brushless motors combined with a high-performance lithium polymer battery are also used.
What's it worth? Take a look at this Rc Helicopter price guide: sold listings for a value indication.