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Defining Custom Airframes in PX4-XPlane

This guide walks you through defining custom airframe configurations in PX4-XPlane using the config.ini file. We’ll cover key concepts, provide detailed examples, and guide you through the entire process—from configuring the file to testing your setup in X-Plane.

Overview

To define a custom airframe configuration, follow these steps:

  1. Select or Build a Plane Model: Choose an existing X-Plane model or create one from scratch using X-Plane’s Plane Maker.
  2. Select a Similar Airframe in PX4 or Start from Scratch: Choose a pre-existing PX4 airframe that closely matches your custom setup, or define a new one from scratch. You can double-check channel numbers, servos, and functionalities in the dynamic mixing section of QGroundControl’s Actuator section under SITL.
  3. Configure the config.ini File: Map PX4 channels to X-Plane datarefs.
  4. Understand Datarefs and Their Types: Learn how to select and configure datarefs.
  5. Build and Test the Configuration: Test your configuration in X-Plane and PX4.

Step 1: Select or Build a Plane Model

Using Existing Models

  • Pre-existing Models: Use existing X-Plane models that closely match your custom airframe.
  • Modify Models: Modify an existing X-Plane model to fit your requirements.

Creating a New Model

  • Use Plane Maker: Create a new aircraft using X-Plane’s Plane Maker tool if no existing model fits your needs.

Step 2: Select a Similar Airframe in PX4 or Start from Scratch

Using Pre-existing Airframes in PX4

  • Choose a Similar Airframe: If your custom airframe is similar to an existing setup, use that as a starting point (e.g., xplane_alia250).
  • Dynamic Mixing in QGroundControl: Verify and adjust channel mappings in the dynamic mixing section of the Actuator settings in QGroundControl’s SITL section.

Starting from Scratch

  • Define in PX4 Source Code: For complex airframes, you may prefer to define the aircraft directly in the PX4 source code and compile it.
  • Dynamic Mixing in QGroundControl: Verify and adjust channel mappings in the dynamic mixing section of the Actuator settings in QGroundControl’s SITL section.

Step 3: Configuring the config.ini File

The config.ini file defines how PX4’s outputs (e.g., motors, control surfaces) map to X-Plane’s inputs.

Example Configuration for Alia 250 eVTOL

; PX4-XPlane Configuration File
; File Version: 1.3
; Last Updated: December 26, 2023
; This file configures the mapping of motor and servo channels from PX4 to X-Plane.

; Configuration Name (Displayed in UI)
config_name = Alia250

[Alia250]

; Auto-Prop Brakes: Specifies which motors have an auto-prop brake system.
; Format: Comma-separated list of motor indices in X-Plane.
autoPropBrakes = 0, 1, 2, 3

; Quadcopter Motors (1-4)
channel0 = sim/flightmodel/engine/ENGN_thro_use, floatArray, [0], [-1 1]
channel1 = sim/flightmodel/engine/ENGN_thro_use, floatArray, [1], [-1 1]
channel2 = sim/flightmodel/engine/ENGN_thro_use, floatArray, [2], [-1 1]
channel3 = sim/flightmodel/engine/ENGN_thro_use, floatArray, [3], [-1 1]

; Fixed-Wing Control Surfaces (Ailerons, Elevator, Rudder)
channel4 = sim/flightmodel/controls/wing2l_ail1def, float, 0, [-20 10] | sim/flightmodel/controls/wing2l_ail2def, float, 0, [-20 10] | sim/flightmodel/controls/wing3l_ail2def, float, 0, [-20 10]
channel5 = sim/flightmodel/controls/wing2r_ail1def, float, 0, [-20 10] | sim/flightmodel/controls/wing2r_ail2def, float, 0, [-20 10] | sim/flightmodel/controls/wing3r_ail2def, float, 0, [-20 10]
channel6 = sim/flightmodel/controls/hstab1_elv1def, float, 0, [-15 20] | sim/flightmodel/controls/hstab1_elv2def, float, 0, [-15 20]
channel7 = sim/flightmodel/controls/vstab1_rud1def, float, 0, [-15 20] | sim/flightmodel/controls/vstab2_rud2def, float, 0, [-15 20]

; Additional Forward Thrust Motor
channel8 = sim/flightmodel/engine/ENGN_thro_use, floatArray, [4], [-1 1]

Key Sections Explained

  • Configuration Name: Displayed in the PX4-XPlane UI; this helps you identify the active configuration.
  • Auto-Prop Brakes: Reduces drag by braking the propellers when they’re not needed, useful for eVTOL configurations.
  • Channel Mappings: Defines how each PX4 channel maps to X-Plane’s datarefs. Each channel can control a motor, control surface, or other aircraft function.

Step 4: Understanding Datarefs and Their Types

Dataref Types

  • float: Single floating-point value, typically used for control surfaces like ailerons or rudders.
  • floatArray: An array of floating-point values, often used for multi-engine aircraft or multiple control surfaces.
  • int: Integer values, typically used for binary states such as landing gear.
  • intArray: An array of integers, similar to floatArray but used for integer data.

Selecting Datarefs

Choosing the correct dataref ensures accurate simulation behavior:

  • Throttle and Motors: Use sim/flightmodel/engine/ENGN_thro_use with floatArray.
  • Control Surfaces: For ailerons, elevators, and rudders, use datarefs like sim/flightmodel/controls/wing2l_ail1def with float.
  • Auto-Prop Brakes: Define motor indices that should have auto-prop brakes to minimize drag during flight.

Use the Dataref Tool plugin in X-Plane to find the correct dataref for your setup, or refer to the X-Plane Dataref Documentation for more details.

Step 5: Building and Testing the Configuration

Testing

  1. Load and Test in PX4-XPlane: Load the configuration in X-Plane and test the controls.
  2. Adjust as Needed: Modify the config.ini file or PX4 parameters to fine-tune control and simulation accuracy.

Final Notes: Custom Airframe Setup in PX4

Remember, the channel numbers should be defined in the SITL Actuator section of QGroundControl. You can either:

  • Use Dynamic Mixing: Adjust actuator outputs directly within QGroundControl for flexibility.
  • Define in Source Code: For more complex setups, define the airframe in the PX4 source code, build it, and then test.

By following these steps, you’ll ensure your custom airframe behaves correctly in both PX4 and X-Plane, allowing for realistic and accurate simulations.