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# JSBSim External forces

JSBSim provides a framework for external reactions, or rather "arbitrary forces", because this feature can be used to model a wide variety of forces that act on a vehicle. Some examples include: parachutes, catapult, arresting hook, pushback, moorings and hydrodynamic reactions for seaplanes.

## Contents

### Frames

JSBSim incorporates several frame of reference. For external forces the chosen frame of reference is (only) used to determine the direction of the force. The location of the force, i.e. it's point of application, is always defined in the structural frame.

• Body XYZ The body frame uses the X axis for forward and aft, with + to the front. The Y axis is side to side with + to the right. The Z axis is up and down, with + being down. The origin of this frame is the Center of Gravity (CG), about which the aircraft forces and moments are summed and the resulting accelerations are integrated to get velocities.
• LOCAL XYZ The local frame is a world-based frame, with X positive north, Y positive east and Z completing the right handed system positive down towards the center of the Earth.
• Wind XYZ The wind frame X-axis points directly into the relative wind. The Z-axis is perpendicular to the X-axis, and remains within the aircraft body axis XZ plane (also called the reference plane). The Y-axis completes a right hand coordinate system. The origin of this frame is the AeroRP.

### Configuration file format:

```<external_reactions>

<force name="{string}" frame="{BODY | LOCAL | WIND}">
<location unit="{FT | M | IN}">
<x> {number} </x>
<y> {number} </y>
<z> {number} </z>
</location>
<direction>
<x> {number} </x>
<y> {number} </y>
<z> {number} </z>
</direction>
</force>

</external_reactions>
```

### Notes

• You can adjust the magnitude of the external force by setting the /fdm/jsbsim/external_reactions/{name}/magnitude property. This value is in pounds.
• Direction of the force can also be adjusted on the fly, by adjusting the /fdm/jsbsim/external_reactions/{name}/{x|y|z} properties.
• Multiple forces can be added.

### Usage examples

External forces can be used to model many phenomena. Some examples: