August 8

RC Transmitter Functions Reviewed

Today I want to discuss with you some of the more significant control functions on an RC Transmitter. As the range of rc transmitter functions become more sophisticated so the available range of adjustments and controls increases.

At one time the only variables on a transmitter were the two control stick gimbals and ratcheted trims at the side of and below these control sticks.

early futaba tx

On this early Futaba transmitter there are just four channels for the main control surfaces and throttle. It appears that the owner of this transmitter was using this with a very simple three control set up, throttle, rudder and an auxiliary control. In fact this could well have been for a boat but we will never know.

You will notice the four trim adjustment ratchet switches to the inside and below the sticks. The central switch is the ON/OFF switch. There does appear to be a fifth control knob on the top of the case the purpose of which is uncertain.

Gradually things like servo reversing switches and dual rate switches started to appear. Today we are faced with an array of buttons, knobs, switches and a liquid crystal display screen (LCD) that would do a space shuttle flight deck proud. Even a basic beginners six channel outfit will have quite a range of switches to contend with.

Transmitter Controls

Here is a modern six channel transmitter showing the principle control setup for Mode 2 operation. Let’s visit each of these controls in turn:

Left Gimbal stick – Throttle & Rudder Control

Right Gimbal Stick – Elevator & Aileron Control

Lefthand Upright Trim Switch – Throttle Trim

Lefthand Horizontal Trim Switch – Rudder Trim

Righthand Upright Trim Switch – Elevator Trim

Righthand Horizontal Trim Switch –  Aileron Trim

We have used this diagram in previous posts to discuss the effects of the main controls. If you look carefully you will see other switches, knobs and buttons arranged around the transmitter case. We will also come to these in due course.

I mentioned that this diagram showed a transmitter set up for Mode 2 operation. There are, in fact, four possible arrangements as illustrated here:-


Modes 3 & 4 are rarely used but there are a good number of more mature flyers, such as myself, who learned on Mode 1 especially in the UK.

Nowadays almost without exception Mode 2 is used to teach new pilots although it doesn’t really matter which mode you learn, all are valid and work. The choice will depend on your tutor’s preferred mode if you learn with a club. Should you be unhappy with your tutors mode, discuss the possibilty of changing to your choice of mode with the club training officer.

If you recall, the last series of posts covered our principle controls and we discussed in depth what each one does. If you refer back to the second image above, you will see the effects of applying these controls displayed alongside the transmitter. Throttle – Climb & Descent, Rudder – Left & Right Yaw, Elevator – Angle of Attack (AoA) and Aileron – Left & Right Roll.

Trim Levers

OK, now we need to understand the use of the Trim Levers. These are positioned alongside the appropriate main sticks and are used to adjust the centre point of each servo. As the name suggests, they are used to trim the plane in flight.

Let us imagine, for example, that just having taken off our plane wants to keep climbing when all controls except throttle are at a neutral position. A couple of clicks of down trim on the elevator should push the tail up and bring the plane into level flight.  Similarly, if the plane wants to roll to the left, a couple of clicks of right aileron trim should level it out for you.

In the early stages of your training program your tutor will make the necessary adjustments for you, eliminating the need for you to concern yourself with them.

On older transmitters these trim levers were of an analogue nature and the lever was left in the adjusted position. Unfortunately many people found that this position was easily changed accidentally after flying had finished. This meant that often adjustment was necessary at the start of each flying session. On modern transmitters these trims are switched digitally and cannot be changed inadvertently when the transmitter is switched off.


This is an adjustment that is made in the transmitters programming mode and allows fine adjustment of the servo centre point when setting up the model before flight. Sub-Trim adjustment is often used to ensure that the servo output arm is exactly at 90 degrees to the servo or that the control surface is exactly at neutral.

Ideally this should be done mechanically by adjusting a clevis at either end of the push-rod. Occasionally this is not possible so Sub-Trims can be used.

End Point Adjustment (EPA)

This is another programmable adjustment that can be used to reduce or extend the amount of servo travel either side of the neutral point. Most servos are set by the manufacturer to move 30 degrees either side of the neutral  or centre point, i.e. 60 degrees of total travel and is referred to as 100% movement.

Using the End Point Adjustment this movement range can usually be increased to 125% or decreased to 0%. This should not be used to correct for major errors in setting up the servo and control surface movement mechanically.

Whenever possible make as many adjustments to obtain the correct range of movement through physically setting the servo arms, pushrods and linkages.

Your End point Adjustments should always be as close to 100% as possible. Setting to much control surface deflection and then reducing it via the End Point adjustment results in lack of servo accuracy and a reduction of servo output torque (the power available to move the control surfaces).

One area where End Point Adjustment is particularly useful is in setting up “Aileron Differential” (more “up” than “down”).

Aileron Differential

Dual Rates (DR)

This facility is usually available via switches on the front of the transmitter, above and adjacent to the main control sticks. In the photo below you can see where this particular manufacturer has positioned a single switch to change rates on Ailerons, Elevator and Rudder for ease of use whilst continuing to control the model with the main sticks.

dual rates 1

The idea behind Dual Rate Switches is to enable small or large control surface movements to be set and available at the flick of a switch. This is most useful when setting up a model for first flights.

Large control surface movements will often make a model over sensitive and unduly responsive during a landing approach. On the other hand it is possible that for some models larger control surface movements are desireable when flying slowly.

On some transmitters the programming will allow this facility to be designated to just one switch for all three control surfaces (as in the above photo) or to single switches for each individual control.

Exponential (Expo)

Another programmable facility, Expo either increases or decreases the rate of response of the servo around the centre or neutral point. The overall range of movement is unaffected by this setting.


setting Expo

Manufacturers treat positive or negative exponential differently. Be sure to check the correct sense when using this facility on your chosen transmitter. Dual Rate and Exponential facilities are often grouped together on the same screen.

Model Memory

This is a facility common to most programmable transmitters and permits the storage of unique settings for a range of different models. Once a particular model’s settings have been entered into the programming they are stored in the model memory location selected.

Often there is also a facility to copy a set of parameters from one memory to another. This can be useful as the basis for setting up another model.

There are a couple of manufacturers that provide a desirable “Model Match” feature that prevents a receiver being activated unless the correct model has been selected at the transmitter.


Nearly all modern transmitters provide at least one timer. The prime purpose of this facility is to warn the pilot when a flight is nearing the end of its chosen duration.

This facility can normally be designated an “up” timer or a “down” timer. The “up” version counts from zero up to a pre-determined duration whilst the “down” setting counts down to zero from a pre-set chosen duration.  Each choice will usually provide audible warnings as the termination point is approached. There is often a choice of switch designation to initiate this facility.

Servo Reversing

This is simply a way to make the individual servo(s) operate in the opposite direction. It is useful where, having mounted a servo and connected it to the control surface, it operates in the wrong sense for your purpose.

Typically if your rudder turns to the right when you give left rudder at the transmitter then a quick reversal in the transmitter programme will rectify this problem.

Fail Safe Setting

Although this is set at the transmitter, it is actually controlled by the receiver. The idea is to set the servos and in the case of an electric model, the Electronic Speed Controller (ESC) so that they go to a pre-set state should signal connection between the transmitter and receiver fail.

Typically this could be to drop the throttle to idle or off in the case of an electric model, the rudder to a very gentle turn setting whilst aileron and elevator return to a neutral setting. This should reduce its impact speed and prevent it from flying away.

For the time being this should be sufficient knowledge for you and your tutor to ensure that your trainer plane is  eminently flyable.

There are more rc transmitter functions available within the programming facilities but these are more suited to advanced flying techniques. Over time you will be able to learn about your radio gear and familiarise yourself with these sophistications.

If you are considering buying your Radio Control Gear soon, can I suggest you visit my page on the website covering Selecting the Best Radio Control Equipment for you.

See you next time.



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Posted August 8, 2015 by Colin in category "General Flight Training

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