Physics Objects Information

With Microsoft Flight Simulator 2024, each of the parts of an aircraft can be defined as specific physics objects permiting much greater control over their properties and their flight model behavior. This page contains various notes related to some of the functionality derived from the flight_model.cfg sections and parameters related specifically to these physics objects within the simulation (for example [OBJ_EA1_FUSELAGE.N] or [OBJ_AIRGEO_VTAIL]).

 

 

Note On Surface Cx

When it comes to defining the surfaces of your aircraft using the various [OBJ_EA1_*] and [OBJ_AIRGEO_*] sections of the flight model (for example [OBJ_EA1_FUSELAGE.N]), there are a number of parameters available which are used to create the lift and drag curves for the surfaces of the object being created. Each of these parameters have X (lateral), Y (vertical) and Z (longitudinal) components, since the surface can be positioned in 3D space, and the values used will be one or the other (or interpolated) depending on the position of the surface within the object definition. Note that these positions are all calculated relative to the Datum Reference Point. For example, if we consider a fuselage object:

• The X-axis (lateral, $C_y$) component defines how the surface will behave when on the sides of the fuselage.
• The Y-axis (vertical, $C_n$) component defines how the surface will behave when on the top and bottom of the fuselage.
• The Z-axis (longitudinal, $C_a$) component defines how the surface will behave when on the front and back of the fuselage.
• Any surface that is between these axis will use interpolated values, based on it's position.

 

The 4 different surface $C_x$ values will use the appropriate component (X, Y, or Z) to calculate how the surface should react to the incoming airflow, depending on the position of the surface. These parameters are:

 

• surface_cx - The friction - or drag coefficient - of the surface when the airflow / relative wind direction is close to the tangent to the surface. During normal flight, for the front facing surfaces of the fuselage, this parameter plays no role. For the side and top / bottom facing surfaces however, this will impact the global drag and sideforce of the aircraft when the aircraft slips or pitches up or down. For the top / bottom surfaces of the object, this also plays the role of the lift coefficient. In the case of the top-down surfaces of the fuselage, this describes the lift coefficient of the fuselage surfaces.

 

• surface_cx_tangent - The friction - drag coefficient - when the airflow is at a perfect tangent to the surface. This is used to model imperfections on the surface, old paint, etc... This parameter plays no role when the air is coming from a normal / perpendicular direction.

 

• surface_cx_normal - The friction - or drag coefficient - of the surface when the airflow / relative wind direction is perpendicular to the surface. During normal flight, for the forward facing surfaces of the fuselage, this would be the drag coefficient. For the side-facing surfaces, this would be the drag coefficient when the airflow comes from the side, ie: when still on the ground or taxiing in a strong side wind. The same principle applies to wind coming from above or below the aircraft and hitting the top or bottom surfaces.

 

• surface_cx_efficiency - this parameter works with the surface_cx parameter. When the surface starts to have some incidence with the airflow, the surface_cx applies to lift but will also generate some drag. The surface_cx_efficiency parameter allows you to define how much drag will be generated. This is the equivalent of induced drag for wings, but occurs for all surfaces of the aircraft with airflow coming from any direction.

 

 

 

Note On Physics Groups

Most of the available physics objects that can be created as part of the Microsoft Flight Simulator 2024 can be assigned to one or more groups. This is done through the group parameter (as listed in the object parameters for example: [OBJ_EA1_FUSELAGE.N]) of each object. By themselves, the group tags will have no direct impact on the simulation and are instead designed to be used in debugging and user-made tools. The built-in group tags are as follows:

 

Group Tag	Used By
aircraft	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N], [OBJ_AIRGEO_VTAIL.N], [OBJ_AIRGEO_HTAIL.N], [OBJ_AIRGEO_WING.N],
airframe	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N], [OBJ_AIRGEO_VTAIL.N], [OBJ_AIRGEO_HTAIL.N],
blade
center	All aircraft have a "center" point at the Datum Reference Point of the aircraft. This point has no physics surfaces attached to it and has no aerodynamic impact, however it allows the simulation to gather information at the center of the aircraft. This group is used as a fallback if no surface has a group assigned.
elevator	[OBJ_AIRGEO_VTAIL.N],
extairframe	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N], [OBJ_AIRGEO_VTAIL.N], [OBJ_AIRGEO_WING.N],
exttank
fuselage	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N],
gear
hstab	[OBJ_AIRGEO_VTAIL.N],
htail	[OBJ_AIRGEO_VTAIL.N],
inner	[OBJ_AIRGEO_WING.N],
left	[OBJ_AIRGEO_WING.N],
middle	[OBJ_AIRGEO_WING.N],
outer	[OBJ_AIRGEO_WING.N],
pN	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N], [OBJ_EA1_SURFACE.N], [OBJ_AIRGEO_VTAIL.N], [OBJ_AIRGEO_HTAIL.N], [OBJ_AIRGEO_WING.N],
right	[OBJ_AIRGEO_WING.N],
root	[OBJ_AIRGEO_WING.N],
rudder	[OBJ_AIRGEO_HTAIL.N],
spN
surface	[OBJ_EA1_SURFACE.N],
tail	[OBJ_AIRGEO_VTAIL.N],
tip	[OBJ_EA1_FUSELAGE.N], [OBJ_AIRGEO_FUSELAGE.N], [OBJ_AIRGEO_WING.N],
typical
up	[OBJ_AIRGEO_WING.N],
vstab	[OBJ_AIRGEO_HTAIL.N],
vtail	[OBJ_AIRGEO_HTAIL.N],
wing	[OBJ_AIRGEO_WING.N],

 

The following debugging tools will use these groups as part of the way they gather debug information:

• The plane engine simulation measures propeller absorbed torque in the Engines debug window by looking at surfaces in the blade group.
• The helicopter engine simulation measures rotor absorbed torque in the Engines debug window by looking at surfaces in the blade group. The main rotor and secondary/tail rotor are differentiated using the pNgroup, where p0 is the main rotor and p1 is the secondary or tail rotor.
• The plane propeller thrust force and moment debugging in the Engines debug window looks for surfaces with the blade group.
• For aircraft with surfaces that you want included in the data of the Sim Wind Tunnel debug window should be included int he group tag extairframe.
• To get debug data on helicopter rotor operations using the Engines debug window, you should include the following group tags: root, typical, tip, and blade.
• To include physics object surfaces in the Stall debug window data, use thewing, tip, left, inner, right, and extairframe group tags.

 

 

Groups And SimVars

The following SimVars will return information based on the groups assigned to the various surface objects:

 

• WINDSHIELD WIND VELOCITY - This SimVar will derive it's value a surface that belongs to the fuselage and tip groups. If there are no such surfaces available, it will use the center group.

 

• INCIDENCE ALPHA - This SimVar will return the angle of attack of different surfaces based on the supplied group ID value where:
  • 0 = the center group
  • 1 = the surface objects that belongs to wing, left, and tip groups (ie: the left wing surface), or the center group if there are no surfaces that meet the initial group criteria.
  • 2 = the surface objects that belongs to wing, right, and tip groups (ie: the right wing surface), or the center group if there are no surfaces that meet the initial group criteria.