Radio Equipment - advanced functions.

R/C Basic functions

One of the most useful developments in radio technology is the increased use of microprocessors in the transmitter. This provides many extra facilities over the older discrete technology, from model memories to mixing of functions. There are also special requirements depending on whether you are flying fixed wing or helicopter, and many systems can be switched between fixed wing and helicopter mode - each mode providing mixers and functions appropriate to the intended use.

Because of the complexity of operations that computer controlled sets are capable of, they usually have some sort of digital display: this can be used to provide a menu system to select and adjust the various functions and is often capable of providing a graphical display of the shape of any non-linear response curves which may be selected.

It will probably look something like the set alongside - the LCD screen is the greenish rectangle in the bottom right, with a row of buttons underneath to select and adjust the various features. You may find the LCD positioned on top of the box.

Here are some of the extra functions you may come across in a computer set:

Model Type

Power, gliders and helicopters have differing requirements from a a radio control system. Helicopters, for example, require the ability to control throttle, idle settings and pitch settings in an interactive way: gliders often have flaps, spoilers or crow brakes to operate instead of a throttle and often have specific combinations of trim settings depending on the phase of flight. Computer radios usually offer the chance to select the type of model to be controlled and the transmitter will then use the existing switches in a different way depending on the model type selected.


Mixers enable one control movement to operate 2 surfaces on the model, for example, biplanes often need the rudder and ailerons to be used together to avoid slip in turns. This can of course be done by simply moving both sticks and judging the effect manually, but if a mixer function is available it can be set up so that when the aileron is moved, this also operates the rudder: sufficient control is provided so that the relative amounts of movement can be altered. The mixing effect can be switched on and off so that it is available during normal flight, but is switched to separate rudder and aileron control when landing.
Some other uses of mixers are:

- operation of flaps or retractable undercarriage often produces a pitch change, a suitable amount and direction of elevator can be mixed in to compensate for this change.

-V-tail or delta aircraft use the same control surfaces for both pitch and roll control: this has traditionally been achieved by using a mechanical system to combine the outputs from 2 servos, but it is much more easily achieved and controlled by using an electronic mixer in the transmitter.

Response curves

The usual response curve of a servo is linear: that is, the same amount of deflection of the stick will always cause the same amount of rotation of the servo; for example, 10mm of stick movement may give 10º of servo rotation. Sometimes it is an advantage to be able to make this response non-linear - often an exponential curve can be selected. What this means is that the amount of servo movement changes depending on how far the stick is moved. The graph alongside shows the ratio of control surface movement to stick movement for a standard linear response (in blue) and an exponential response (in red).

For example, if an exponential curve is selected, then the first 10mm of stick movement may give 5º of control surface deflection, the next 10mm give 8º of deflection and the last 10mm give 10º of deflection. This would have the effect of reducing the sensitivity of the model around the neutral stick point thus giving a smoother flight for small deflections while retaining full deflection for full stick movement. Note that the total amount of movement stays the same.

Helicopters require a non linear curve for throttle operation, and usually there are other functions mixed in and set to operate at different points on the curve.

Adjustable servo travel.

This enables you to adjust how far the servo moves for a given movement of the stick. The ratio can be adjusted mechanically by altering the physical connection between the servo and the control surface, but an electronic adjustment allows the user to increase or decrease the servo movement, and even to enable different amounts of movement either side of neutral - to give more up elevator than down for example. This facility is particularly useful when setting up a throttle servo, as the range of movement at the carburettor end is fixed, and it can be quite difficult to set up a mechanical connection so that it gives exactly the right amount of movement.


Setting up rates, curves, mixers and so on for a model can be quite complicated, and the set-up is unlikely to be useable on any other model. To get around this, most computer radios have a number of memories where all the details of the set-up for a particular model can be stored. When a different model is to be flown, the appropriate memory is selected, and all the settings for that model then become active. The number of memories can range from 2 to 40, and some sets can store the model settings on removeable cards, effectively allowing an unlimited number of model set-ups to be memorised.

Trim Memories

For gliders it is often required to alter the trim settings of the controls at different phases of flight. For example, in thermals the model is trimmed for maximum lift and minimum speed, but different trim settings may be used when speed is required when searching the area for lift. With switchable trim settings the relevant trims can be set up precisely and a switch on the transmitter used to select the most appropriate configuration for the task.

Fail Safe

This is a system which, in the event of loss of signal, or of interference, moves the servos to s pre-set position to enable the model to be brought to the ground as safely as possible., It is a legal requirement on large models, and can be a useful safety feature even on smaller ones.
The BMFA now has guidelines on the use of fail-safes in R/C models.