Radio Systems

Aircraft radio systems consist of a transmitter (or controller), a receiver, one or more servos, and batteries. Servos are the part of the airborne radio system that convey mechanical movement. Each moving part will need a servo to make that part move. Radio systems are available with a wide variety of features. However, they all share similar quality and basic functions.

When you first look at a radio system, you'll see how many channels it has and then what channel it's on. The word channel is used two different ways: firstly, an airplane radio is very often a 4-channel radio. This means that it controls four functions on the plane (ailerons, elevator, rudder, throttle); secondly, it refers to the actual radio frequency the radio transmits. There are several frequencies legal to use for R/C airplanes and each one has been assigned a channel number.

When deciding on a radio, it is important to decide on how many channel functions you want. Most airplanes use four channels. However, some aerobatic, scale, and sport planes use five or six channels (adding flaps and retractable landing gear). So, you may want to think about the future and what plane you want next, and buy a radio that will control all anticipated functions.

Frequencies

The FCC has set aside 50 frequencies in the 72 MHz band (channels 11-60) dedicated to aircraft use only. No license is needed to operate these radios. However, if you have an amateur (ham) radio operator's license you may be able to use a radio in the 50 MHz band. Also, there are six frequencies set aside in the 27 MHz band that are legal for any kind of model use (surface or air). Just remember, whatever channel your radio is on, check the field where you fly for any other radios on the same channel and do not use your radio when theirs are on!

Many fields have a frequency control system. Before you use your radio, be sure you understand the system and are using it correctly.

AM vs FM / PCM vs PPM

This refers to the signal type (or modulation). A radio wave of any frequency can have different signal types.

PPM (pulse proportional modulation) is usually an analog system. This is very precise (but not digitized). This equipment is more economical than PCM.
PCM (pulse code modulation) signals are digitized and provide the most accurate signal. A dedicated computer in the transmitter and receiver actually use binary code (pulses) in the signal.
AM signals are always PPM. AM is more subject to interference than FM and does not allow for the use of a "Buddy" cord.
FM signals are either PPM or PCM. Usually, only pilots flying competitively in precision aerobatics or scale use PCM.

Control Features

The following features can be found on many economically priced 4 and 6 channel units:

Servo Reversing: Due to servo rotation and control linkages, control inputs can end up reversed (i.e. moving the transmitter stick to the right, viewed from behind, the rudder moves to the left). To remedy this, the transmitter has servo reversing switches. Just flip this switch and everything moves the right way.
Adjustable Travel Volume (ATV): ATV limits the amount of total servo movement. This eliminates excessive control travel and stress on the servo.
Dual Rates: These switches, usually found on the TX face, allow two different total travels for ailerons and elevator and can be switched in flight. For some aerobatic maneuvers, planes need increased control movement, but for normal flight they don't, thus dual rate is used.
End Point Adjustment (EPA): EPA is an advanced feature that allows each direction of movement to have a different travel. For example, if your plane banks too quickly to the left, turn down the left aileron EPA dial. This will not change the amount of bank control to the right.
Exponential: Some planes (especially racers) need more control travel at low speeds and less at high speeds where control response can be overly sensitive. Exponential changes the relationship between transmitter stick motion (linear) and the control servo response (non-linear). This feature can be used to "soften" plane response near the center of stick motion without lessening the overall amount of control at full stick deflection.
Mixing: Many planes benefit from mixing two functions together. In fact, for planes like flying wings or V-Tails, mixing is mandatory. For example, a flying wing's elevators are also its ailerons (these are called elevons). When the radio has mixing, one servo will be installed for each elevon. Mixed together, these servos will respond correctly to both the elevator and aileron inputs.

Computer Radios vs. Standard Radios

A computer radio allows more adjustments and channels to be mixed in a more precise way. Computer radios also have more trims and setup adjustments making the radio installation and flight-trimming process of the plane easier and more precise. Also, the settings can all be saved to memory and settings for more than one plane can be saved. In more advanced computer radios, the mixing is programmable, actually allowing custom combinations of channels and movements (and even control inputs). Competition pilots find this kind of adjustability a must. Many beginners simply find computer radios too complicated.

Standard radios have basic mixing but it is not as precise nor as adjustable as a computer radio. Also, the setting cannot be saved for more than one plane. However, these radios are simpler to use and are less expensive.