Choosing Servo Functions¶
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. Each type of plane frame type will require different servo output functions to controls its motor(s) and control surfaces. 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 for a standard plane frame 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, such as QuadPlane or Tail-sitter.
Each of the servo output functions is settable with the corresponding SERVOn_FUNCTION parameter. The defaults are:
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. See the Plane Functions section of Autopilot Output Functions section.
In addition, there are servo functions for 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 also discussed in the Autopilot Output Functions section.
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. Normally, no adjustment is needed, but can be modified to match your servo’s capabilities and range. While these values can be used to adjust throw and trim center of a flying surface, it is best to do so mechanically. SERVO_AUTO_TRIM can be used to fine adjust the TRIM value automatically in flight, instead of manually changing the mechanical trim. NEVER try to trim a plane with the transmitter trims as this will destroy the RC Calibration values (See Radio Control Calibration).
When using reversing ESCs, these values may be needed to be adjusted (See Reverse Thrust Setup )
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:
then that would setup servo output 5 as a reversed left elevon, with a throw from 1000 to 2000, and a trim of 1504.
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 multiplier. 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.
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.