General radio control functions explained
Radio control functions and features vary depending on the complexity of the rc system, but the majority of rc systems do all share the same basic functions.
The absolute fundamental purpose of your rc system is, of course, to control the directional movement of your model, and there is a big difference between 2 channel systems and 4+ channel systems in this respect (Incidentally, the word 'channel' refers to each separate function of the rc system).
2 channel rc systems are mainly used for surface vehicles (ie cars & boats) while systems of 3 channels or more are used for aircraft. 2 stick (2 ch.) transmitters traditionally used with surface vehicles are giving way to the 'pistol grip' style transmitter, but they are still available and are used for simple rc aircraft such as gliders.
The following picture illustrates the big difference in stick movement between a 2 channel and 4 channel transmitter.
Stick movements of a traditional 2 channel (shown left) & 4 channel (shown right) transmitter
On a 2 stick, 2 channel transmitter in use with an rc glider, for example, the right hand stick moves from left to right, with a natural center ie the stick is sprung. To correspond naturally with the control of the glider, this stick moves the rudder; moving the stick left will move the rudder to the left and hence turn the glider to the left, whilst moving the stick to the right will turn the glider to the right.
The left hand stick, also sprung, moves up and down and corresponds naturally with the elevator, although in this case up does not mean up; moving the stick upwards (ie away from you) will cause the elevator to drop which will result in a dive, whilst moving the stick down (ie towards you) will raise the elevator and cause the glider to climb.
The radio control functions of a 4 channel set (shown above, right) become more complicated because each stick moves up and down and left and right...
The right hand stick would, for example (*see below), control elevator (up/down) and aileron (left/right) whilst the left hand stick would control throttle (up/down) and rudder (left/right).
Learning to control two actions with one stick may sound complicated but is quickly mastered and with practice quickly becomes second nature.
*NB: Various modes are used with multi-channel rc systems, determining which sticks operate which controls...
- Mode 1: rudder and elevator are controlled by the left stick, throttle and ailerons by the right
- Mode 2: ailerons and elevator are controlled by the right stick, rudder and throttle by the left
- Mode 3: ailerons and elevator are controlled by the left stick, rudder and throttle by the right
- Mode 4: throttle and ailerons are controlled by the left stick, rudder and elevator by the right
See RC transmitter modes in detail
But which transmitter mode is right for you? Well, if you learn to fly rc at a club then the choice will more than likely be made for you, by your instructor and the airplane you are learning on.
The mode is often different between countries; Mode 2 has become the standard for flying in North America, while many British flyers prefer Mode 1 transmitters. Modes 3 and 4 are rarely used, but some flyers do prefer these set-ups to Modes 1 or 2.
There are advantages to either one, and no mode is wrong or right; having the ailerons and elevator on the same stick (Mode 2) puts both primary controls in one place and correspond to a real airplane's joystick. But having the two primary controls on separate sticks (Mode 1) means there's less chance of you inadvertently changing one input while moving the other.
Whichever mode you end up flying with, the chances are that you'll stick with that one for all your flying days. Once you've learned one mode, it's very difficult to start flying on another!
Browse rc systems, if you're looking to buy.
Additional radio control functions
Additional functions, all referred to as channels, are found on either toggle switches or dials, both of which can be located on the face or top of the transmitter within easy fingertip reach (the photo shows the top left corner of a transmitter with many extra channels).
Common radio control functions and features include...
- Flaps.
- Retractable landing gear.
- Control surface mixing.
- Channel mixing.
- Programmable settings and memory.
- Training features.
Flaps are usually controlled by rotating a small dial so that varying amounts of flap can be applied as necessary. Retractable landing gear, or 'retracts', on the other hand, is likely to be controlled by the flick of a toggle switch.
Control surface mixing is only possible on a computer-based transmitter, which are very common these days. A single pair of control surfaces is made to act as two separate pairs would. The most common type of control surface mixing is to combine elevator and aileron deflection into 'elevons'. With such mixing, the transmitter is operated normally but the single pair of control surfaces combine the two different deflections in accordance with the tx stick movements and so move together, as elevators do, and individually as ailerons do.
Other similar control surface mixing includes flaperons (mixing aileron & flap deflection) and spoilerons (mixing aileron & spoiler deflection).
Channel mixing occurs when two separate channels are made to operate in conjunction with one another. For example, you could program an aileron and rudder mix whereby the rudder automatically moves a set amount when the ailerons are deflected.
A memory function of an rc system means that multiple models can be used with just the one transmitter. Once the particular settings for a model are programmed in to the tx (eg trim settings), they can be stored along with saved settings for other models.
This function is very convenient, it prevents you having to start trimming from scratch every time you fly a different aircraft with the same transmitter.
A training feature is the ability to connect one transmitter to a second transmitter, or 'Buddy Box', via a cable. This allows the student pilot to have full control of the airplane, yet allows the instructor to resume immediate control at the flick of a switch, if the student gets in to difficulties.
This way of learning is one of the best methods and should be seriously considered if you join a club - most model airplane clubs will have at least one buddy box system, to enable flight training under direct supervision of an approved model flying instructor.
(Radio control simulators are also a very popular way of learning to fly rc. See this page for more information on these).
Further radio control functions commonly found on a transmitter can be servo reverse and dual rates...
Servo reverse allows the direction of servo movement to be fully reversed, ideal for times when a servo can only be installed in the model in a way which isn't conducive to the required direction of movement.
Dual rates enable the control surface deflection to be limited while still retaining full stick movement, so as to limit the sensitivity of the airplane in the air. With dual rates off the control surface has 100% deflection, but with dual rates on the deflection can be programmed down to, say, a 20% deflection limit.
Dual rates can be found on some RTF rc airplanes these days, and may be described as 'Beginner Mode' (less deflection) and 'Advanced Mode' (full deflection).
Browse rc systems, if you're looking to buy.
The trimming function
All rc transmitters, with the exception of the cheapest toy ones, have a function that allows you to 'trim' your model while it is being flown or driven.
Small trim levers, also called trim tabs or simply trims, are located adjacent to each stick, one for each direction in which the stick moves. The trim levers have exactly the same effect as the main sticks, only to a much finer degree.
Essentially the trims fine tune the servo deflection or motor power of the model. With rc aircraft, the aim of trimming is to get your model flying as straight and level as possible with the main transmitter sticks in their central positions, and with no input from you.
It's quite normal to have to adjust the trims throughout each flying session, as outside factors affect the aircraft's flight characteristics. But once the trims have been set, they are left alone until the natural flight path of the aircraft changes and they have to be adjusted again.
Radio channels
When talking about channels, there are 2 different meanings in the rc world; the number of channels an rc system has and the frequency channel that the system operates on.
Because all radio control systems send out radio signals, just as a normal radio or TV broadcasting station does, various frequency channels are needed so that more than one rc set can be operated at the same time. So, certain channels have been designated for radio control models by law and must be adhered to for safety reasons. Furthermore, rc model aircraft have their own set of channels while rc surface vehicles have theirs.
For more detailed information on this very important aspect of radio control, see the rc airplane frequencies page.
A further point to remember when referring to radio frequencies used for models is the modulation. The word modulation refers to the whole process of the radio signal being sent out by the transmitter, received by the receiver and passed on to the servos.
The 2 standard types of modulation are AM (Amplitude Modulation) and FM (Frequency Modulation), the latter being sub-divided into PPM (Pulse Position Modulation) and PCM (Pulse Code Modulation).
In a nutshell, PCM is the modern digital successor to PPM and does a far better job of guarding against unwanted interference. FM is the most commonly used by nearly all rc aircraft pilots*, while AM is often used on 2 channel surface vehicles and rc toys.
*NB: while FM rc systems are still the norm, the new 2.4GHz spread spectrum systems are quickly becoming more common. Such systems use a completely different frequency band and technology and are without doubt the future of radio control. DSM/DSM2 is a term you'll likely encounter and stands for Digital Spectrum Modulation, the technology used in the Spektrum rc system range. Read more about spread spectrum technology here.
Whichever rc system you have, understanding the radio control functions and features on it is very important, to get the most out of it. As with all things in life, it's not necessary to buy the all-singing, all-dancing, most expensive set you can lay your hands on, but do be satisfied that your system will be able to control your aircraft in all the ways that you need!
Browse rc systems, if you're looking to buy.
Related pages
Radio control gear - what's included in a basic set and how it works.
Transmitter modes - a more detailed look at transmitter modes.
RC frequencies - a table of the designated frequencies and their channel numbers.
Spread spectrum - the latest shake-up for rc systems.
RC airplane controls - what they are and how they operate.
