Radio Controlled models are obviously controlled by radio signals. The radio consists of several components. The pilot holds the Transmitter which is used to send signals to the Receiver. The receiver is in the aircraft and controls the Servos. The servos are also in the aircraft and are what actually move the controls such as rudder, throttle, aileron, and elevator. All of these are powered by batteries. These are the main components, each of which I will go into more detail about.
Transmitter
The transmitter is a handheld box with switches and levers that allow the pilot to control the various functions of an aircraft. These may include throttle, rudder, aileron, elevator, flaps, spoilers, landing gear, brakes, bomb drop, etc. Each function requires the use of a "channel", therefore, the more channels your transmitter has the more fuctions you can control. The most channels I've seen on a radio is 10, with 4 being the most common. Each transmitter also has a frequency that it operates on. There are 50 frequencies available for aircraft use (more if you include the HAM band), and more than one person can fly at a time as long as they are on different frequencies. A transmitter has a signal strength of under 1 watt. This is enough to fly a model to the limits of most people's vision.
Receiver
The receiver goes in the aircraft and is therefore quite small and light. It receives the signals from the pilots transmitter while at the same time ignoring any other signals in the area, whether they be from other pilots transmitters, pagers, television, radio, etc. The receiver has connection points for the servos as well as for a battery power source. In order for a receiver to work with a particular transmitter it has to be on the same frequency and have at least as many channels as the transmitter. It also has to be of the same modulation type.
Modulation
When you buy a radio you have a choice of three different modulation types, AM, FM, or PCM. AM, or Amplitude Modulation, is the least expensive and the most susceptible to interference from other sources. It's the same as when listening to an AM station. You might pick up static from power lines or distant lightning strikes. Having said this, my first radio was AM and it worked fine for me. FM, or Frequency Modulation, is the next type and is better suited to an environment with possible interference problems. FM radios usually cost a little more than a comparable AM model. PCM, or Pulse Code Modulation is the last type. It may, or may not, offer an advantage over FM systems. Let me explain why.With PCM every signal that is sent by the transmitter is preceeded by an "on" command and followed by an "off" command. The receiver in the aircraft ignores any and all radio signals until it receives a valid "on" signal from the transmitter. Then the receiver accepts input on it's frequency until it receives a valid "off" command from the transmitter. If there is too much radio interference the receiver is unable to recognize a valid "on" from the transmitter and therefore ignores all signals, including the interference. The question then becomes, "what does the receiver do if it's ignoring all input?" Well, it can be programmed to hold all servos in their last position, or move the servos to a postion that was preprogrammed by the pilot. If you are in the middle of straight and level flight and the receiver goes into "hold" for a moment you will likely never know anything was wrong. However, if you are in a dive and this happens, you may not be able to pull out right when you need to. If you choose the preprogrammed postion option you would usually have the servos return to neutral and the throttle go to idle. This way if a crash does occur at least you won't be moving at top speed.
So is PCM better than FM? Many pilots think so, but others would rather have their plane get "glitched" and have partial control rather than have the PCM hold function kick in and have no control at all. Another reason for FM is that if you are experiencing small glitches you will know there's a problem and can take steps to solve it before it gets worse. With PCM you might never know you're being glitched until the problem becomes too severe to recover from. I own AM, FM, and PCM systems and have never had a problem with any of them. I will probably go with FM for any future radios as I think it offers the best value for my dollar.
Servos
The servos are little motor driven devices which convert the radio signal to a physical movement. They have output shafts to which an arm or wheel can be attached. When the receiver tells the servo to move the output shaft rotates which causes the arm to swing or the wheel to turn. We can then connect the arm or wheel to a pushrod or cable which in turn is connected to the function we need to opereate, such as the throttle. Servos come in different sizes and strenghs. A very strong servo might be used for flaps or landing gear whereas a simple, lightweight servo might be used for throttle.
Batteries
The batteries most used for powering radio controlled aircraft are Nickel Cadmium or NiCad batteries. They come in many different sizes and capacities and can be recharged over and over again. They are usually found in 4 cell (4.8 volt) and 5 cell (6 volt) arrangements and can have capacities anywhere from 50 to 1500 mAh and more. The most common size for airplanes is a 500 mAh, 4.8 volt battery. Helicopters usually have a 1000 mAh battery because of the extra demands of 5 servos and a gyro. mAh means milli-Ampre hour and indicates that the battery should be able to supply 500 milli-ampres (.5 amps) of current for 1 hour. In a nutshell, the higher the mAh rating, the longer you can fly before recharging.