RC sailplane and turbine

All about GVARs

6/4/2020. Para nuestros amigos hispano-hablantes: traducción al español en formato PDF. Muchas gracias a Jorge Brun.

Why use GVARs?

GVARs are a powerful feature of OpenTx. Used sensibly, they can simplify your setup, and make it easier to adjust in the field. In this article, I'll describe how to use them.

Differences between radios

Before we start, a quick note about the X-Lite and other similar small screen radios: these don't have a dedicated menu for global variables. Instead, GVARs are accessed via the Flight Mode menu. Each flight mode shows a list of GVARs designated G1 to G9.


GVARs on X-Lite

GVAR basics

GVAR stands for 'global variable'. A GVAR represents a number. You can specify a GVAR in many contexts where a number is used, e.g.

You can also dynamically alter a GVAR, and it will retain its value even after you switch off the transmitter. (In this and other respects, its similar to digital trims.)

One source of confusion is the term 'global'. In fact, GVARs are not truly global - they apply only to the active model (not to be confused with global functions which apply across all models).

Setting and referencing GVARs

In this section we'll examine how to

Setting a GVAR

OpenTx provides nine GVARS, named 'GV1', 'GV2'... 'GV9. They are managed in the GLOBAL VARS menu, where you can view and alter their values.

"GVARS menu"

GVARs menu (first 7 shown)

Each GVAR has nine columns labeled FM0, FM1, ... FM8, representing the nine flight modes.

You can enter a different number for each flight mode. When a GVAR is referenced, OpenTx retrieves the value for the flight mode which is active. For example in the screenshot above: if FM1 is active, then GV1 returns '20'. If FM2 is active, GV1 returns '40'. And so on.

Remember that FM0 is the default flight mode: if you don't define any flight modes explicitly, then FM0 is always active.

Using a GVAR's value

In this section we'll see how to use a previously assigned GVAR. As an example we'll reference a GVAR in the weight field in a mixer.

  1. In the mixer list, highlight a mix line
  2. Open the mixer editor
  3. Highlight the weight field
  4. Press [ENTER long]
  5. The display changes from a number to 'GV1'. You can choose another GVAR using the +/- keys. You can also select the negated value by selecting the corresponding '-' entry.

    Long press to display GVAR

Simple glider example

OK, so that's the theory... let's see how GVARs are used in practice. As an example, create a simple glider setup, with channels 5 and 6 assigned to ailerons:

CH5:left ail



CH6:right ail


Setting diff the old way

Now add some differential for each aileron servo.

To set differential: open the mixer editor, and to skip to the curve field. Specify 'diff' as the curve type, and enter a value. The diff value may be entered directly, or it may reference a GVAR.

Note: we are assuming that the mixer behave perfectly symmetrically. You can achieve this, regardless of mechanical differences, by calibrating the outputs

First, let's specify diff by typing the value in directly:

CH5:left ail

Src=Ail, Diff = 30


CH6:right ail

Src=Ail, Diff = -30

There's one problem with this: In order to alter diff, we have to make two identical data entry operations, which is both tiresome and error prone. Let's see how to improve it!

Setting diff via a GVAR

Instead of specifying the value of diff directly, let's store the diff value in a GVAR and then reference it in the diff field.

  1. Go to the GVARs menu
  2. In the 'GV1' line, go to the FM0 column, and enter '30'.
  3. In the Mixers menu, set diff = GV1 and -GV1 for the two aileron channels.


GV1=30 -- stores diff value


CH1:left ail

Src=Ail, diff = GV1


CH2:right ail

Src=Ail, diff = -GV1

To change diff in both mixes, only one adjustment is required, to GV1.

Making diff flight mode specific

So far we've only used the default flight mode (FM0). Suppose our setup uses several flight modes.

We saw earlier that a single GVAR can provide different values for different flight modes - so we simply assign the required diff values to the appropriate flight modes in the GVAR menu.


GV1=30(FM0), 45 (FM1)-- stores diff value for FM0 and FM1


CH1:left ail

Src=Ail, diff = GV1


CH2:right ail

Src=Ail, diff = -GV1

Compare this with the solution without using GVAR's:

CH5:left ail

Src=Ail, Diff = 30 Flightmode = FM0

Src=Ail, Diff = 45 Flightmode = FM1


CH6:right ail

Src=Ail Diff = -30 Flightmode = FM0

Src=Ail Diff = -45 Flightmode = FM1

The benefits of GVARs should be clear!

Setting a GVAR dynamically via a Special Function

So far we've set our GVARs by typing in the value directly in the GVARs menu. However, OpenTx provides an alternative and very powerful mechanism for setting GVARs, using the 'Adjust GVAR' action in special functions. This opens up exciting possibilities for varying expo, diff etc. dynamically.

'Adjust GVAR' action

The Adjust GVAR action provides a way of altering the value of a GVAR in response to some event. Let's see how it works:

  1. Open the Special Functions menu, and find an empty slot
  2. Select 'Adjust GVAR' from the list of actions
  3. Choose the type of action:
    VALUE: Sets GVAR to a number
    SRC: Sets GVAR to the value of a channel, switch or input
    GVAR: Sets GVAR to value of another GVAR (copy GVAR)
    INC: Add or subtract 1 from GVAR
  4. Finally, specify a switch (physical or logical) to trigger the action

The action is executed the moment that the switch in step 4 becomes true.

You can also specify 'ON' as the switch, in which case the special function is always active - this effectively binds the GVAR to the source. We'll see why this is useful in the next section.

Binding a GVAR to its source

Sometimes it's useful for a GVAR to track the value of something, in other words to be 'bound' to a source. For example, you may wish to a GVAR to track the value of a channel output.

To implement binding,

  1. Create a special function
  2. Choose the "Adjust GVAR" action
  3. In the src field, set the source to the channel you want to track
  4. In the switch field, select "ON"

In the screenshot below, GV1 is bound to CH8. In other words, GV1 will always report the value of CH8.

Special funcs menu

Binding GV1 to CH8

Adjusting diff or expo dynamically

Say we want to vary aileron differential in flight, depending on the position of the rudder trim (TrmR). We can do this by binding GV1 to the rudder trim, then setting Diff = GV1.

The code is in three parts:

CH8:Diff value

Src=TrmR wt=20 offset=40



switch="ON" Action="Adjust GV1" Parameters="Src=CH8"



Src=Ail, Diff=GV1

Summary: benefits and drawbacks of GVARs

The benefits of GVARs are as follows

The drawbacks

Using GVARs with cascading mixers

GVARs can be used in conjunction with cascading mixers to create powerful and elegant solutions.