Adding Payload Stations

Plane Maker 12 has a new tab “Payloads” in the weight & balance setup. It allows to configure 9 CG stations to distribute payload mass in the aircraft. Each station can be assigned a role “PAX” or “Cargo” and labelled with a descriptive name. Each station has a nominal maximum weight in lbs that can be placed there (though X-Plane obviously allows to overload each station with more weight than allowed).

Limit Radii of gyration overrides

Mass in a payload station not only increases the mass of airplane and adds a moment, but the added mass also increases the moment of inertia around all axes if an offset from the empty CG location exists. Since loading a payload station increases MoI and thus radius of gyration dynamically (depending on how much weight the user actually loaded) it is preferable to use payloads with CG offsets rather than increase the radius of gyration.

Since X-Plane 11 considered payload a point-mass, authors often increased the radii of gyration to achieve a more “heavy” feel to the aircraft by adding more moment of inertia.

In X-Plane 12, the preferred way is to create multiple payload stations throughout the airframe that add moment of inertia dynamically as they get loaded.

Loading payload stations by datarefs

Each configured payload station can have mass added to it with the array dataref sim/flightmodel/weight/m_stations[]. The dataref is in kilograms.

The result of loading the stations is the total weight sim/flightmodel/weight/m_fixed and the CG offsets sim/flightmodel/misc/cgz_ref_to_default (in pitch) and sim/flightmodel/misc/cgx_ref_to_default (in roll), though the latter is usually only relevant to helicopters.

Writing to these datarefs will try to distribute the payload throughout the stations to achieve the requested CG and total mass, but it is not guaranteed that the requested CG can be achieved with shifting the available mass between the stations. For example, no negative mass will be assigned to a station to achieve more CG offset. It is thus recommended to read back the CG dataref after writing it to see the actually achieved CG after moving payload.

It is thus preferred to write the mass to the station and read the resulting CG.

Legacy aircraft with no configured stations

Upon loading an X-Plane 11 aircraft, X-Plane 12 will still consider the payload weight a point-mass and not alter moment of inertia. Two “virtual” stations in the nose and tail of the aircraft will be created to allow for a maximal arm to change the CG location. Payload mass will be distributed between nose and tail to achieve the requested CG. Note that these virtual nose and tail stations are not counted for moment of inertia.

Programmatically, you can detect these virtual stations by their fixed max weight sim/aircraft/weight/acf_m_station_max reading 1kg.