A radio system is the set of electronics on any radio controlled model which allows that model to be operated via wireless remote control. Before the advent of radio control, powered land models primarily consisted of "tether cars" with nitro or gasoline engines and no radio systems. They went around in circles, held in place by a tether. Model aircraft were either nitro- or gas-powered models operated in a circular motion via a control line or rubber band-powered free flight models. Tether cars, control line aircraft and free flight aircraft are still enjoyed by hobbyists today, although radio control has greatly displaced these types of models.
The radio system consists of a transmitter, a receiver, a battery and several servos.
The transmitter is the hand-held unit which takes control input from the user and converts it to radio signals which the model's onboard electronics interpret as motion via the servos or electric motor control via an ESC. Transmitters are available in a "pistol-grip" style or a stick style. The pistol-grip style, as the name implies, is held like a handgun and its use is limited to surface models. Throttle is controlled via a trigger operated with the user's index finger (again, like a handgun) and steering is via a steering wheel immediately above the pistol grip. Most of these radios are designed with right-handed users in mind as the grip is held in the left hand, the throttle operated with the left index finger and the model steered with the right hand. There are pistol-grip radios on the market that can be reversed for use by left-handed users, although many "lefties" have no problem adapting to a right-handed radio.
All aircraft radios and some surface radios utilize a dual stick configuration. These are held in much the same way one might hold a book. Larger transmitters of this type generally have provisions for attaching a neck strap, thereby reducing the fatigue of holding it in this manner for any length of time. The sticks, or gimbals, are manipulated by the user's thumbs. On a simple two-channel air or surface unit, forward vertical motion of the left stick operates the throttle and backward motion the brake (if so equipped) while horizontal motion of the right stick affects steering. A three-channel radio adds elevator or elevon control with vertical motion of the right stick. In much the same way as found in full-sized aircraft, pushing the stick away from oneself pitches the aircraft down; pulling it back toward oneself will cause the aircraft to climb. Some inexpensive two- and three-channel radios often substitute a thumb-operated slider in lieu of the left stick. Fixed-wing radios with four or more channels add rudder control with horizontal motion of the left stick.
Helicopter radios are similar in operation. Advancing the left stick, or throttle, on a helicopter also changes the collective, or attack angle of the blades, causing the helicopter to lift into the air. Instead of rudder control, the left stick's left-right motion controls a helicopter's tail rotor. Left rudder points the helicopter's nose to the left by swinging the tail to the right. The right stick manipulates the pitch of the main rotor blades. Pushing the stick upwards tilts the main rotor forward, thereby drawing the helicopter forward. Lateral motion tilts the main to the right or left while pulling the stick towards oneself tilts the rotors backwards.
Transmitters are generally powered by eight AA batteries or a 9.6-volt rechargeable pack.
A receiver interprets the signals from the transmitter and sends them out to the servos. The receiver must match the transmitter to work properly. They must both be of the same type and band and have matching crystals (for standard radios), channel settings (for synthesized radios), or be bound to each other (for DSM radios). They operate on a standard 4.8 or 6 volts. Some receivers have a battery eliminator circuit; this allows you to connect the full 7.2 or 8.4 volts from the main battery of an electric model into the receiver. This was origianlly used for older and cheaper models that had mechanical speed controls as a weight-saving measure as well as a sort of failsafe. Electronic speed controls used in most applications contain a BEC. High-current applications such as tractor pulling require that the BEC be overridden and a seperate power source used. Some receivers have a BEC circuit that only gives the onboard radio equipment 4.8 volts (equal to a 4 cell NiCd/NiMH pack). The normal voltage in use currently is 6 volts, equal to a four-cell alkaline holder or five-cell NiCd or NiMh pack. Since any of a receiver's channels will accept power input, a full 6 volts can be sent to these receivers by simply connecting the battery to an unused channel, or by using a "Y" harness to connect it to a channel in use by a servo. This will provide more servo torque and speed. Also, some failsafes do not work properly with 4.8 volts.