Getting to grips with a computer transmitter?

JR DSX-12 2.4GHz

The very mention of computer radios is often enough to send the uninitiated running for the shadows. If, until now, you’ve shied away from these marvels of modern technology then settle into a comfy chair and I’ll ease you through an introduction, explaining some of the intimidating features and, hopefully, simplifying the programming of your computer transmitter.

A lot of people are initially put off by computer radios, but with so many examples available it’s actually quite easy to find the right one to suit your needs, even if those needs arent anywhere near that of a competitor in the Tournament of Champions or the Jet World Masters. Most low- to mid-range computer transmitters are more than capable of coping with the needs of a club flyer (and then some).

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The Futaba Field Force 6 and 7 (6EX and 7C series), for example, are ideal entry-level computer radios, with menus that are easy to navigate. As well as providing the basics, these systems are also extremely capable of in-depth program mixing (7C in particular) if so desired.

At the other end of the spectrum the current ‘top of the range’ Futaba 14MZ product offers unprecedented functionality, delivered using colour touch screen technology in one of the most user-friendly packages seen so far (though the price tag may not be quite what the average club flyer is prepared to pay!). Ignoring these top end systems for the purposes of this article, let’s have a general look at what computer radios have to offer.


Model memory. A pioneering feature that gave computer radios a significant advantage over non-computer radios was the advent of ‘model memory’. This enables the modeller to save the settings of different aircraft using the internal memory of the Tx. These settings can be recalled at the touch of a button. Early designs were limited to storing only a few models, but the accelerating development of ‘flash memory’ in the last few years now offers almost infinite storage for the settings of every model in your expanding hangar!  

Pulse mode.
Most computer transmitters are able to transmit in two distinct pulse modes. Depending on the type of Rx being used theres a choice of either PCM (Pulse Code Modulation) or PPM (Pulse Position Modulation, also known as FM). PPM was initially chosen for use with R/C aeromodelling because the electronics required to decode the signal are extremely simple, leading to small, lightweight receiver / decoder units. However PCM, although more complex, is now the preferred mode as it offers greater flexibility and reliability.

The last couple of years have seen the introduction of the new PCM 2048 G3 system, which promises double the resolution of PCM 1024 and results in real-time response. However, from the experiences of fellow club members who use the system, I’m told youd be hard pressed to notice the difference!  

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On-screen display. The majority of computer radios have adopted a ‘standard’ menu system, shown on an LCD screen. The most common things youd expect to see on screen are the Tx battery voltage, timers (counting up and down), trim positions and the selected model name/number.  

Digital trims.
Ever had the frustration of arriving at the flying field to find that all your Tx trim sliders have moved? This and the annoying task of re-trimming every time you change model disappears with a computer Tx, which has the key benefit of digital trim functions. Essentially these are touch-sensitive trim tabs that can be tapped or held in the direction required. The user is often able to program how the trims react to changes of force on the trim tab switches. For instance, when pressing lightly on the trim you get one speed of operation, and when you increase the pressure, the repetition speed and rate may also increase. With most transmitters, the repetition speed and rate of step can be adjusted to meet the requirements for specific models or, indeed, personal preference. In the absence of being able to sense a physical point of reference on the transmitter whilst youre flying, an audible beep helps to reinforce how much input is being entered. The positions of the trims are often also displayed as a ticker marker on the LCD screen.

In order of preference, heres my personal checklist for setting up a new model on my computer Tx –  

Servo reverse. This is the first point of call when setting up any new model on a computer radio. Once youve installed the radio and all the servos are located in their desired channel sockets it’s important to make sure that they’re working in the desired directions. This ‘no frills’ function simply allows the user to reverse the operation of each individual servo as necessary.

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Sub trim. As mentioned above, the majority of computer radios now come equipped with digitally operated trims, which are a godsend. To get the most out of these, however, its imperative to first ‘zero’ the model with the Sub Trim function. With a non-computer Tx youd traditionally set up the aircraft as neutral as mechanically possible at the horns and linkages and then make any minor adjustments using the trim sliders. The drawbacks here were that one click of the trim was sometimes too much, and every time you changed model you had to remember the positions of the trims! The minute increments of digital sub trim allow the user to accurately neutralise all the controls and start from a ‘clean slate’ with the main trims switches. The settings are, of course, saved in the model memory of that particular aircraft, so theres no more remembering trim settings!  
Adjustable travel volume. Also called ‘End Point Adjust’, this function allows the maximum travel of a servo to be set either side from its neutral position. It can also be used to employ differential throws and allow correction for off-centre linkages. Once the travel for each channel is set, it will be the basis for all other secondary functions to follow, such as Dual Rates and Mixing. The travel rate can typically be set anywhere from 1% to 140% in each direction on all proportional channels.  

Dual rate and exponential.
These two functions deal with setting the operation rate and operation curve for any three channels (typically), by movement of an appointed or default switch. The need to be able to switch easily between high- and low-rated movements whilst in flight was quickly realised during the development of the earliest proportional radio transmitters, and as that need is fundamentally unchanged, the basic function also remains the same. The real development with Dual Rates in modern computer radios is the ability to define a percentage of movement per channel (often per side of neutral, also), with the settings being saved in the model memory of each individual aircraft.

Dual Rate is a function thats often featured as an extension to Exponential. In my opinion this is an invaluable tool for any model flyer, allowing stick sensitivity to be programmed via a simple movement curve. On Futaba transmitters negative values aid in producing a ‘dead’ band around stick centre, whilst positive values have the reverse effect. On most transmitters the exponential curves can be programmed individually, either side of the neutral stick position.

is usually activated along with the programmed Dual Rate settings via the relative switch position. On some transmitters its also possible to link on / off control in accordance with the position of a primary stick, such as throttle. A possible situation for this may be the need for more or less control with only slight stick movements at low throttle settings, such as during the landing approach.

On a personal note, for sport, 3D or general fun-fly models, I prefer to set all control surfaces to maximum deflection and employ a -40% exponential curve, or thereabouts. This allows full control at any time but with such a significant dead band I can still fly as if ‘rated’ by limiting the stick movement. This, incidentally, has enabled me to get out of trouble on many occasions without having to worry about the flick of a switch!  

Differential. If your model has a tendency to yaw whilst executing an aileron roll, then you need to employ aileron differential. In the good ol’ days this problem was often tackled by mechanically adjusting the control links and horns of the (usually) single servo.

Most models are fitted with two aileron servos these days and the easiest way to solve the problem outlined, is by using the Differential function of a computer Tx, which allows the user to independently adjust the positive and negative deflection angles of each aileron via the two servos. Most screens will show two percentage values each for AIL1 and AIL2, left and right; each value can be highlighted and adjusted +/- to suit your requirements. In some cases its also possible for the user to trim the amount of down aileron deflection in flight using a fascia-mounted rotary dial.

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Elevon. A far simpler alternative than mechanically mixed elevons, this useful function effortlessly combines two surfaces and two servos to control both pitch and roll. A prime example of its use is in setting up the controls for delta or flying wing aircraft, which require combined aileron and elevator functions. In most cases Rx channels 1 and 2 are the default for the two elevons, and differential operation is also possible.  
Elevator to flap. This function is commonly used with 3D / fun-fly or aerobatic aircraft, mixing flaps with elevator so that when you pull up, the flaps go down (and vice-versa) at a user-defined angle of deflection. This is particularly helpful during tight manoeuvres, where its desirable to eliminate ‘screwing out’ or snap stalling. If flaperon (see below) is activated, the elevators are usually mixed with the ailerons; if not, mixing is with flaps only.  

Flaperons. As Ive already mentioned it’s more common, these days, to have a servo for each aileron, primarily due to the extra functionality thus offered, but also due to the relatively low cost of servos. One of the most common functions available to users of computer radios utilising this wing set-up, is flaperon, which allows the ailerons to be programmed so they operate together in the same direction, offering a flap response as well as aileron control.

Usually used for take-off, landing or just slower flight, both ailerons can be raised and lowered simultaneously at the flick of a switch, or dialled in using a rotary knob. Whilst the Flaperon function is active, regular aileron operation is maintained. In most cases left and right deflection angles can be adjusted separately, making it easy to apply differential to the ailerons. The flap throws can be adjusted independently to trim out any adverse rolling when the function is activated, and you can also program in some elevator trim to counter any unwanted pitching.
Ailevators. This function requires the use of two elevator servos connected to individual elevators, which can then operate in unison to control pitch, and differentially to control roll. It’s a useful tool, particularly in 3D or aerobatic aircraft, where it provides aileron control even at low speeds due to the propeller wash that passes over the tail surfaces. In most cases Rx channels 2 and 5 are used; this means that retractable gear [CH5] is possibly sacrificed.

All the combined control deflections (left and right ailerons; left and right elevators) can be adjusted. With most transmitters its also possible to add differential to the ‘rear ailerons’, making this a versatile and highly adjustable function. This system also has the benefit of redundancy – very handy should the aircraft lose primary aileron control.  

Rudder coupling. Probably one of the first channel mixes developed, this function links rudder and aileron controls and is useful to help keep the fuselage facing the right way when making ‘coordinated turns’; especially effective when flying scale models or trainers. You can set up the left and right mixing rates independently, and mixing can be turned on and off during flight by setting a switch (assuming your Tx provides this facility). It may also be possible to adjust the amount of coupled rudder by setting a volume control on one of the auxiliary knobs. 

Rudder-to-aileron mix. If your model displays unwanted rolling tendencies during knife-edge flight then this is the mix for you. Typically used with aerobatic aircraft, the Rudder-to-Aileron mix allows you to define corrective aileron to be automatically input when rudder is being used. The rates for left and right rudder travel can be set independently. Most transmitters allow the mixing to be switched on and off via a switch, which is particularly useful because you may not wish for the mixing to interfere with other manoeuvres.

V-tail. This is a simple mixing function thats almost identical to the previously discussed Elevon, and is used to set up V-tail aircraft that have combined elevator and rudder functions. It requires two servos, one connected to Rx channel 2 (elevator), the second plugged into channel 4 (rudder). The elevator and rudder deflections can be adjusted independently for maximum tweaking. This is a straightforward and practical alternative to a potentially headache-inducing mechanical mix!  

Snap roll.
Many transmitters have a sprung function switch either at the right, left or on both top shoulder positions that can be employed (amongst other uses) to perform a preset snap roll. Early computer transmitters (like the first generation Futaba Field Force 7) often featured a pair of rudimentary slide switches to select up / down and left / right as desired. In the newest generation of computer transmitters, Snap roll (like most other functions) is now completely adjustable, allowing the user to enter the values for elevator, rudder and aileron.  

Trainer. The buddy-box training system is a great way to teach others how to fly a model aeroplane. The ability to take control of a pupils model by the flick of the above-mentioned spring-function (return) switch is far more desirable than the throw the box at me when youre in trouble approach of yesteryear! Mind you, as with most things, theres always room for some improvement.

With the advent of more advanced computer radios, the classic trainer function has received an overhaul. This being the case its now possible for the tutor to dedicate the controls that hell be surrendering to the pupil. For example, to familiarise the pupil with the basics of flight during the first few lessons you can allocate him only the primary functions of aileron and elevator whilst you control the throttle, rudder and any other ancillary items.

In all honesty there isn’t a conclusion to this subject. There are many more functions available for the advanced user and, of course, the technology is constantly evolving. Todays computer transmitters are so capable, it’s unlikely that normal club flying will take advantage of every function available. However, one thing is certain. The computer Tx is well and truly here to stay and will continue to be developed, bringing us more exciting features and functions in years to come. Currently theres the emergence of 2.4GHz, which although developed some time ago and already extensively used in R/C cars, is now taking the aeromodelling world by storm.

Shopping for a transmitter today, youd be hard pressed to find one without even the most simple computer system on board. In the words of that famous aviator Howard Hughes: Its the way of the future!

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