Radio control gear basics explained

[This page refers to standard FM equipment - info on the new DSM rc gear can be read here.]

Understanding how radio control gear operates and what it consists of isn't too difficult, and hopefully this page will help you out if you're curious about how model airplanes can be made to do what they do best -- fly!

A typical basic rc setup will consist of a transmitter, a receiver, one or more servos and battery pack, a fairly typical 4 channel radio set is shown below...

The number of channels determines the number of servos required - generally speaking a separate servo is needed for each channel, although modern day electronic speed controls and mixer units do bend this rule a bit.
One channel means that just one function of the model is controllable, for example rudder movement or electric motor on/off.
Two channels could be rudder and motor while 3 channels could be rudder, motor and elevator.
A typical 4 channel rc airplane will have rudder, elevator, throttle and ailerons, the four primary airplane controls.

There are no rules as to how many channels a model needs, it is purely dependant on the model itself - more complex models may require 7 or 8 channels to operate the four primary controls plus retractable landing gear, flaps, even landing lights or parachute deployment!

Transmitter (abbreviated to 'Tx')

The transmitter is the main box that you hold and use to control your model.
There are several different types of transmitter, the common ones are shown below, and are (from left to right) standard 4+ channel, single stick 3 channel with slide motor control (popular with RTF electric rc airplanes), standard 2 channel with sticks and pistol grip 2 channel (commonly used with surface vehicles):

For the purpose of this page we'll talk about a standard multi channel (4+) rc airplane transmitter.
It consists of 2 sticks, trim levers and if the set has 5 or more channels there will be switches on the top of the transmitter body, so as to be within easy 'fingertip reach'. These switches are normally set up for retractable landing gear and flaps, but can be used for any purpose.
There is also a telescopic antenna and some kind of battery level indicator, either an analogue meter or a digital display.
These main features are shown below:

On a 4+ channel tx both sticks move up/down and left/right to enable two functions to be controlled with each stick. These are the 4 basic channels, and the switches on the top of the body make up the other channels - 5th, 6th etc.

The trim levers are essential for fine tuning the flight characteristics of the airplane, and to iron out any unwanted tendencies that the model may have in the air.

When an input is made by the pilot, by moving a stick or flicking a switch on the transmitter, a radio signal is sent out via the transmitter's antenna and picked up by the antenna of the receiver, which is located within the model.
That signal passes from the receiver directly to the servos, and the end result is a proportional movement of the airplane's control surface, throttle or whatever.

By proportional, we mean that the movement of the control surface (or throttle) is a direct representation of how much movement was applied to the stick of the transmitter - a small stick movement will mean little movement of the control surface, while throwing the stick to its maximum position will mean full deflection of the control surface.

Receiver (abbrev. 'Rx')

In exactly the same way as a normal radio or TV receives the signal from the broadcasting station (for example, when you watch CNN), a radio control receiver receives the signal that is sent out by the transmitter when you move the stick or flick a switch.

The receiver is located inside the model and is directly connected to the servos by small cables.
A wire antenna extends from within the receiver to outside of the model, typically this is 3 to 4 feet long and should never be cut or looped up to reduce its length.
By doing so, its ability to receive the signal from the transmitter is drastically reduced which can result in disastrous consequences by way of the model flying out of radio range too early, and you losing control. NEVER CUT THE ANTENNA!

All radio control systems require a minimum length of antenna to operate successfully.

Servos

As previously mentioned, the number of servos varies according to the number of channels that the radio control gear and the model requires.

A servo consists of a small but powerful electric motor and a set of gears, housed within a plastic casing, which are directly linked to the servo horn located on the top of the servo body (the round disc in the photo).

This horn is the 'hand' of the servo and links to the control surface of the airplane by way of either a rigid (eg wooden) or flexible (eg plastic 'snake') linking rod.

When the receiver picks up a signal from the transmitter, that signal is then sent to the appropriate servo which responds with a direct movement of the horn via the motor and gears.
This in turn moves the linking rod which moves the control surface in question.

Different sized servos are available depending on the usage required and how powerful they have to be, or whether they're going into a large or mini rc airplane etc.

Digital servos are becoming increasingly more common, and offer better all-round performance over their analogue cousins.
On the outside, they look the same but that's about where the similarity ends.

Digital servos give instant control, from stop to full deflection, in a micro-second and do away with the 'lag' sometimes experienced with analogue servos. Also, the torque is much higher at any given point of the servo movement so they have much greater 'holding power'.
For these reasons, digital servos are perfect for demanding rc flying such as 3D helicopter flying where immediate model response is needed by the pilot.

Crystals

Crystals determine which frequency the radio control set will operate on.
RC airplanes, for example, have a set number of frequencies that fall into the 72mhz band (for the US).

Both the receiver and the transmitter need their own crystal to operate correctly, both on identical frequencies.

It's always a good idea to carry at least one spare set of crystals with you so you can change channel at anytime, if someone else is flying on the channel that you normally use.
When buying extra crystals, try and choose a range of widely spread channel numbers.

You can read more about the rc airplane frequencies here.

Batteries

While certain radio control sets require normal 'dry' (alkaline) batteries, it's a better idea to use rechargeable nicad batteries wherever possible.
Although the initial cost is more, this cost is soon recouped as rechargeable batteries have an incredibly long life of around 1000 charges or more.

Nickel cadmium (NiCd) batteries are the most commonly used in radio control gear although the newer Nickel Metal Hydride (NiMH) batteries with an even higher capacity and performance are now being favored.

The battery packs for the transmitter and receiver should be charged before each flying session to ensure that the radio control gear has optimum performance.

The radio control gear battery level is of paramount importance when it comes to rc flying - if even just one of the batteries is low then you will not have control over your airplane for very long!

If you can't use nicads and you have to use dry batteries, it's a good idea to purchase a hand-held voltage meter so that you can always tell the battery condition. Most transmitters have some kind of battery meter, but cheaper ones can be unreliable.

Battery use and replacement should always be made in accordance with your airplane manufacturer's instructions.

You can see a selection of radio control gear for rc airplanes and surface vehicles here.

Related pages

Radio control functions - further info on frequencies and functions of rc gear.

Transmitter modes - the difference between the four modes used in rc flying.

RC airplane frequencies - info on the designated frequencies and a table of them.

Spread spectrum - the latest shake-up for radio systems.

RC airplane controls - how the radio gear acts on the model.

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