Choosing Servo Functions

Note

this page only applies for plane firmware version 3.8.0 and later.

The most fundamental setup for any plane is the servo output functions. Flight boards vary in the number of outputs they support, some with as few as 6 outputs and others with up to 16. Each of these outputs can be configured as needed for your airframe.

The default outputs are very simple:

  • servo output 1 is aileron
  • servo output 2 is elevator
  • servo output 3 is throttle
  • servo output 4 is rudder

This is fine for many simple aircraft, but will need to be changed if you have elevons, a vtail, flaps or any other more complex setup.

Overview

Each of the servo output functions is settable with the corresponding SERVOn_FUNCTION parameter. The defaults are:

ParameterValueMeaning
SERVO1_FUNCTION4aileron
SERVO2_FUNCTION19elevator
SERVO3_FUNCTION70throttle
SERVO4_FUNCTION21rudder

Note that this is very different from firmware versions prior to 3.8, where the function of the first 4 outputs was fixed, and outputs above 4 could be set using the RCn_FUNCTION parameters.

Available Output Functions

Each of your servo outputs can be configured to a wide range of possible functions. The key options for traditional fixed wing aircraft are:

FunctionValue
FLAP2
AILERON4
ELEVATOR19
RUDDER21
STEERING26
THROTTLE70
THROTTLE_LEFT73
THROTTLE_RIGHT74
ELEVON_LEFT77
ELEVON_RIGHT78
VTAIL_LEFT79
VTAIL_RIGHT80

in addition, there are servo functions for vtol lifting motors on quadplanes, control of internal combustion engines, pass-thru of controls from RC inputs, camera controls, control of motor tilt functions for tilt-rotors and many others. Those functions are discussed in other parts of the documentation.

The guide below will concentrate in the primary flight control surfaces of fixed wing aircraft.

Controlling Servo Throw, Trim and Reversal

In addition to the function, each servo output also has parameters to control the servo throw, the trim and servo reversal.

The parameters are:

  • SERVOn_MIN is the minimum PWM output value
  • SERVOn_MAX is the maximum PWM output value
  • SERVOn_TRIM is the trim value for output types that have trim
  • SERVOn_REVERSED controls servo reversal

For example, if you have:

ParameterValue
SERVO5_FUNCTION77
SERVO5_MIN1000
SERVO5_MAX2000
SERVO5_TRIM1504
SERVO5_REVERSED1

then that would setup servo output 5 as a reversed left elevon, with a throw from 1000 to 2000, and a trim of 1504.

Note

The reversed parameter is different from the REV parameters in firmware versions 3.7 and earlier. In 3.7 the RC2_REV value was 1 for not reversed and -1 for reversed, which means it was a multipler. In 3.8 the REVERSED parameter is a boolean, where 1 means reversed, and 0 means not reversed.

Multiple Outputs for One Type

The SERVOn_FUNCTION values can be assigned to any output, and you can have multiple output channels with the same output function. For example, you can have multiple output channels marked as aileron (function 4) and set the throws and trim on each aileron separately. This allows you to trim your left and right ailerons differently to account for differences in servo trim.

Setup of Specific Aircraft Types

Now that you understand the basics of the servo output functions you should read the guide for your particular type of aircraft.

Before you do that however, you should make sure you have correctly setup your RC inputs.

Note that you can combine setups for more complex aircraft. So for example, you could setup a vtail tiltrotor quadplane by combining functions from the guides above.

Final Setup

After completing the above you should move onto the final setup of your aircraft.