Most aircraft radio systems [there is also a completely different class of radios used for cars and boats known as 'Pistol Grips'] come with four or more channels of control capability. Each system comes with just about everything you need to install the radio into a built model. Typically, novice or beginner sailplanes utilize only 2 channels of control, but you are still better off purchasing a radio with at least 4 channels of control. Most 4 channel radios typically are complete with rechargeable nickel-cadmium battery packs that are reliable and can give many years of service. Also, there are still brands of radios that are advertised as 2 channel units, have 2 control levers, but the lever on the left for is for elevator (up and down) control only and one on the right for rudder (right and left). This is not the preferable configuration for most pilots and it can be difficult to find a fellow pilot who will (or can) fly with this setup. [I can do it, I learned the hard way...]
The radio consists of two basic components: a transmitter and receiver. The transmitter is what you hold in your hand and the receiver is what you put in the plane. The transmitter sends signals out on a particular frequency and the receiver receives these signals and translates those signals into movement of motors on board the plane. Those motors are known as servos and move as the control levers on the transmitter are moved. You connect the servos to different control surfaces on the plane such as the rudder or elevator. So, when you move the 'rudder stick' on the transmitter, the rudder moves on the airplane, etc.
Most radio systems have at a range of at least a half mile or so (in the air). This means that as long as you can see your plane, you still should be able to control it.
Some radios have the additional capability of connecting one radio transmitter to another to help the beginning pilot learn. This capability is sometimes known as a trainer or 'buddy box'. A cable connects two transmitters of the same brand and by using a switch on one, allows the other control. If the plane 'gets into trouble', the person helping teach the beginning pilot can release the switch and he or she regains control of the plane without incident. The only problem with this technique is that you need two transmitters of the same type as different brands of radios cannot usually be used together in this mode.
Radio control units that are available today come in many varieties and brands that can be broken down into three types:
| Modulation Method | Signal Quality | Weight & Size of receiver | Cost |
| AM | Good | Small & Light | Lowest |
| FM | Better | Medium Size | Medium |
| PCM | Best | Larger and Heavier | Highest |
For sailplanes, typically only AM and FM radios are used as PCM circuitry tends to be heavier and really doesn't add tremendous benefit. PCM circuitry (and FM) is used and sometimes required on aircraft that have electric motors or gas engines. This is due to the added radio frequency noise (also known as RF noise) that tends to be generated by those setups. Sailplanes aren't infallible though, as some control linkages for controlling even rudders or elevators can generate RF noise, especially if the linkages have metal to metal connections. The bottom line is that you can usually get away with a FM radio control system or even AM radio system, if you have a simpler setup with minimal metal-to-metal linkages (unlike a helicopter which can have many). I only have one PCM radio system and my personal experience with it hasn't been that great. I've been completely satisfied with FM modulation.
AM stands for Amplitude Modulation and was used exclusively until the last 15 years or so. Like AM radios that are in the home, the 'information' that is used to control the plane is transmitted by varying the amplitude of the radio signal.
FM (or PPM) stands for Frequency Modulation and is less prone to interference than AM. The control information is transmitted by varying the frequency of the radio signal.
PCM stands for Pulse Code Modulation and actually is still FM. The difference between PCM and FM is that the control information is sent in the form of a digital data pulse [word] rather than just an analog signal pulse. I suppose you could compare this to your cassette audio tape and an audio CD. FM would be the tape and PCM the CD. Both are completely adequate depending on the application.
Here's a brief [??] history of what radio and radio frequency usage was like in the U.S. and Canada. Originally, and for several decades, there was only one frequency band available for both aircraft and surface (cars, boats, etc.) models. This band was situated in the 27 MHz band and only allowed 7 radio controls to be used simultaneously. The seven available frequency channel numbers [not to be confused with channels of control, like a 4 channel or 2 channel radio] were identified by a single colored flag, each color identifying a different frequency. The 27 MHz band quickly became unpopular in the late 1960s and 70s due to the growth of CB radio usage which transmitted on the same frequency range as the radio units. In order to prevent two people from using the same frequency, you could visually 'see' what frequency was in use by the color of the flag attached to the radio antenna. Interference from the CB radios was common, sometimes resulting in the lost of an aircraft.
After that, the FCC allocated frequency ranges in the 72 and 75 MHz range to be used for modeling and at the same time the HAM operator frequency band (50/53 Mhz) became available for modeling use as long as the operator has a valid HAM radio operators license. The 72 MHz band was to be used for aircraft and the 75 MHz band for surface use. In the 72 MHz band there was only 6 available frequencies which were identified by two color flags, one of them white, again each flag indicated what frequency the radio system was using. Like the 27 MHz band above, to prevent two or more people from using the same frequency could be prevented by not turning on a radio with the same flag colors, until the radio in use was turned off. In the 72 Mhz band, each frequency channel was separated by a 80 khz margin. For example, the first frequency was known as "brown and white" and was situated at 72.080 MHz, the next was "blue and white" at 72.160 MHz, etc. The receivers that could "listen" to these frequencies with this 80 kHz spacing are known as Hetrodyne receivers or "wide-band".
In 1988, the frequency spacing of the radio frequency channels was reduced to 20 kHz which turned the original 6 frequency channels into 50 channels. At this point in time, the frequency flag mechanism was abandoned and each frequency channel was assigned a number, starting at 11 (72.010 MHz) and running to channel 60 (72.990 MHz). What's important to note is that the original 6 frequencies fall in between the newer, assigned channel numbers, and are sometimes referred to as "half-channels". At first, only the even channel numbers were available. In 1991, the odd channel numbers were made available. Some flying fields have restrictions today, in that they only allow even numbers. This allows the older, pre-1991 radio transmitter/receiver combinations to still function without interfering with the newer radios. In Canada, there are even fields that only allow every 4 channels, the older 80 kHz spacing, so that even the "ancient" radios can operate. [in actuality, some of these fields actually use a 5 channel spacing due to the fact that the original channel numbers in Canada weren't always evenly spaced, between channels 32 & 46, and 54 & 58]
Radios that can handle the 1991 standard of 20 kHz spacing between channels are known as 1991 radios or "narrow band" radios; the receivers are known as Super Hetrodyne receivers. The newer radio receivers are also equipped with dual conversion circuitry that greatly improves signal reception and reduces interference. Some of the cheaper AM radios from Futaba, JR, and some others (especially surface radios) still use a single conversion circuit with some proprietary add-on circuitry to act like dual conversion by blocking out interference. Even with perfect, dual conversion circuitry and a 1991 transmitter at 20 kHz spacing, a "wide-band" transmitter, on your frequency can interfere completely with the signal controlling your plane causing it to crash. This is why most flying fields or clubs require that everyone must use a post 1991, narrow band transmitter for control.
Don't bother buying or trying to use an older, 1991 or older radio systems to control your aircraft, it's just too risky and dangerous for others flying with you. If you've an old one that is pre-1991 but was created after 1988, you probably can get it upgraded to 1991 status for minimal cost. If you've got some older servos, you can use these without problems. Even some of the servos that are 20 years old can and will still work with most modern radio systems. Newer servos are, typically, much better, however. Newer servos have reduced their weight by as much as 3/4 and have twice or more torque and speed comparable to any servo created in the 1970's or 80's.
I have personally used mostly Futaba radios over the years and I have found them to be trouble free. I have also tried Airtronics, which I feel is a quality brand as well. Years ago, I used brands known as EK Logitrol and Kraft, of which both are no longer in business. As it turns out, most servos these days work with most radio systems. The major difference in radio systems are the connectors and wiring used to connect to the receiver in the plane. Most of my planes have a mix of different branded servos and receivers from many different companies, but be careful, especially when connecting different servos and receiver manufacturers together as some of the wiring might be incompatible.
Click the Servo Wiring page to read about servos, servo wiring, and different servo connectors, or the The Gigantic Servo Chart page for a listing of all the servos and specs I could find.
Do you want to read what others have written about radio controls and other technical details about radio controls and how they work, click here or read about radio interference by visiting the Radio Interference page.
Do you want to see a chart of all the frequencies and channel numbers in the USA?, visit the Frequencies page.
If you want to order some great books on programming computer radio control systems, visit Don Edberg's Dynamic Modelling site.
