JSBSim Mass and balance

Configuration File Format

    <mass_balance>
        <ixx unit="{SLUG*FT2 | KG*M2}"> {number} </ixx>
        <iyy unit="{SLUG*FT2 | KG*M2}"> {number} </iyy>
        <izz unit="{SLUG*FT2 | KG*M2}"> {number} </izz>
        <ixy unit="{SLUG*FT2 | KG*M2}"> {number} </ixy>
        <ixz unit="{SLUG*FT2 | KG*M2}"> {number} </ixz>
        <iyz unit="{SLUG*FT2 | KG*M2}"> {number} </iyz>
        <emptywt unit="{LBS | KG"> {number} </emptywt>
        <location name="CG" unit="{IN | FT | M}">
            <x> {number} </x>
            <y> {number} </y>
            <z> {number} </z>
        </location>
        [<pointmass name="{string}">
            <form shape="{tube | cylinder | sphere | ball}">
               <radius unit="{IN | FT | M}"> {number} </radius>
               <length unit="{IN | FT | M}"> {number} </length>
            </form> 
            <weight unit="{LBS | KG}"> {number} </weight>
            <location name="{string}" unit="{IN | FT | M}">
                <x> {number} </x>
                <y> {number} </y>
                <z> {number} </z>
            </location>
        </pointmass>
        ... other point masses ...]
    </mass_balance>

Parameter definitions

i..	Inertia. Higher values result in slower rolls than lower values. A technical summary of moments of inertia is here or on Wikipedia here. Generally the ixz, iyz, and ixy terms will be zero (because the x, y, and z axes are principal axes); the exception would be in the case of an extremely asymmetrical aircraft design or if your x, y, and z axes don't line up with any of the axes of symmetry of the aircraft.
emptywt	The mass weight of the empty aircraft.
pointmass	There can be any number of pointmasses. Each can also have a form which - if present - causes an associated moment of inertia to be calculated based on the shape.

Notes

• emptywt + pointmasses + fuel = total weight
• Fuel weight is defined in the propulsion section.
• In the form tag, cylinders and balls are solid, while tubes and spheres are hollow.

You can add the pointmasses to the Equipment > Fuel and payload dialog, by adding the following code (for each single pointmass) to your aircraft's -set.xml file:

<payload>
	<weight>
		<name type="string">Passengers and crew</name>
		<weight-lb alias="/fdm/jsbsim/inertia/pointmass-weight-lbs[0]"/>
		<min-lb type="double">3000.0</min-lb>
		<max-lb type="double">90000.0</max-lb>
	</weight>
</payload>