Radio control gear basics explained

- how your RC system works

[This page refers to standard FM equipment -
you can read about spread spectrum rc systems here.]

Understanding how radio control gear (rc systems) 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!

The fundamental components of a typical rc system are the transmitter, receiver and servos. Battery packs, or individual cells, are needed to power all three components. However, a receiver battery pack isn't always required; many modern RTF electric rc airplanes use the motor battery pack to power the receiver and servos.

A fairly typical 4 channel radio control system, as purchased, is shown below...

Incidentally, the word channel refers to each separate controllable function of the model. An rc airplane that is just one channel means that only one function can be controlled by the operator, 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.

As a general rule, the number of channels determines the number of servos required and, generally speaking, a separate servo is needed for each channel. However, modern day electronic speed controls (ESCs) do bend this rule when it comes to engine control. Also, large-scale rc airplanes may need more than one servo to do the same job eg one servo per aileron instead of one servo to operate both ailerons.

There are no hard and fast rules as to how many channels an rc model can have, it is purely dependant on the model itself. More complex rc airplanes may require 7 or 8 channels to operate the four primary controls plus retractable landing gear, flaps, landing lights, parachute deployment, camera operation....

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 available, the common ones are shown below and are, from left to right, standard 4+ channel, single stick 3 channel with slide motor control, 2-stick 2 channel 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 or more) rc airplane transmitter...

It consists of 2 control sticks, trim levers and, if the set has 5 or more channels, there will be switches and dials on the face and top of the transmitter body, so as to be within easy fingertip reach. These switches and dials are usually set up for retractable landing gear and flaps, but can be used for any purpose.

There is also a collapsible antenna on top of the tx and some kind of battery level indicator, either an analogue meter, colored lights or a digital display. Modern computer transmitters display much more info than just the battery level though!
The main features of a 6 channel tx are shown below...

On a 4+ channel transmitter 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 any input is made by the pilot, be it moving a stick, flicking a switch or rotating a dial, a radio signal is sent out via the transmitter's antenna and picked up by the antenna of the receiver, located inside 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 at 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')

The receiver is located inside the model and is directly connected to the servos by small cables. A thin single wire antenna extends from within the receiver to outside of the model, typically this is 2 or 3 feet long.

In the same way as a normal radio or TV receives the signal from the broadcasting station, so a radio control rx receives the signal that is sent out by the transmitter when you move a stick or flick a switch. These signals are then passed through to the servos, or electronic speed control, which respond appropriately.

The antenna of the receiver should never be cut or looped up to reduce its length. By doing so, the ability of the rx to receive the signal from the tx is drastically reduced, and this usually has disastrous consequences. The model will very quickly fly out of radio range, and you'll lose complete control. Once that happens, a crash or a lost airplane is inevitable!

Servos

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

A servo consists of a tiny circuit board, a small but powerful electric motor and a set of either nylon or metal gears, all housed within a plastic casing. The gears are directly linked to the servo horn, or arm, located on the top of the servo body (the round disc in the photo). This horn is the 'hand' of the servo and connects directly to the control surface of the airplane by way of either a rigid (eg wooden) or flexible (eg plastic 'snake') servo linkage, or rod.

When the rx picks up a signal from the tx, that signal is then sent to the appropriate servo which responds with a direct movement of the horn, via the motor and gears. This horn movement either pushes or pulls the linkage, thus moving 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 small rc airplane etc. These days, micro servos are widely available and weigh only a couple of grams.

Digital servos are also 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 they 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 and exact model response is needed by the pilot.

Crystals

Tx and RX crystals Crystals determine which frequency channel the radio control set will operate on. In North America, for example, rc aircraft have a designated set number of channels that fall into the 72MHz frequency band, ranging from 72.010MHz to 72.990MHz. There are 50 different channels in all, spread at 20Hz intervals.

Both the receiver and the transmitter need their own crystal to operate correctly, both on identical frequency channels. Each crystal simply plugs in to its respective component, and can be swapped for another channel crystal set at any time.

It's always a good idea to carry at least one spare set of crystals with you so you can change channel 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 - this gives you better options for finding an area of the frequency band that's not being used.

You can read more about rc airplane frequencies here.

Batteries

While certain radio control systems require normal 'dry' (alkaline) batteries, it's a better idea to use rechargeable 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.

Nickel Cadmium (NiCd) cells were traditionally the most commonly used type cell in radio control gear, but Nickel Metal Hydride (NiMH) cells with a much higher capacity and performance are now the norm.

A typical multi-channel rc system requires 8 battery cells for the transmitter plus 4 for the receiver. Where possible, it's always best to use sealed packs for the tx and rx - the individual cells are soldered together and this greatly reduces the risk of losing a connection, which would invariably result in your model going out of control and crashing.
The batteries 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 cells is low then you will not have control over your model for very long!

Browse different types of radio control gear.