Understanding override_prop_pitch and override_prop_mode

X-Plane features datarefs that let you override parts of the flightmodel. While these datarefs are often named for the dataref whose value you control when the override is enabled, they really act by removing logic blocks from X-Plane’s flight model and systems simulation. This article explains the exact effects of overriding the prop_pitch and prop_mode overrides.

Prop Modes In X-Plane for Constant RPM Props.

X-Plane’s prop model features four prop modes for constant-RPM props in sim/flightmodel2/engine/actuators/prop_mode:

• Feathered (prop_mode = 0): the prop goes to its feathered pitch as set in the .acf file.
• Alpha range (prop_mode = 1): this is normal operation – a target RPM is set with the blue prop levers.  This is visible via sim/cockpit2/engine/actuators/prop_rotation_speed_rad_sec. The governor tries to adjust pitch within the range of the min and max pitch as set in Plane-Maker to maintain commanded RPM if possible.
• Beta range (prop_mode = 2): prop pitch is commanded directly with the throttle as it moves through the beta range – the engine is at idle and the prop angle changes. On some planes the prop may even go into a reverse range while in beta mode – this is set in Plane-Maker.
• Reverse range (prop_mode = 3): prop pitch is commanded directly with the throttle and goes from the beta pitch to reverse pitch, and throttle commands the actual throttle. This can be used to simulate reverse-by-throttle or a range of reverse angles or both.

X-Plane will set the prop target speed with the prop joystick axis if assigned, and the throttle with the throttle axis if assigned. The prop mode is set between reverse/beta/alpha by commands, and feathering happens via a series of on-board systems, e.g. feather when mixture is pulled, auto-feather, or feather when the prop lever is pulled past a detent.

Overriding prop_mode

When sim/operation/override/override_prop_mode is set to 1, X-Plane will not move the prop mode between alpha, beta and reverse when (1) the panel 2-d throttles are dragged or (2) a joystick axis moves into a new range on an aircraft with beta and reverse ranges built into the throttle.  (This is a .acf setting.)

X-Plane will change modes if you write to the dataref or when commands like sim/engines/thrust_reverse_toggle and sim/engines/beta_toggle are run; to ensure complete control of the dataref, override commands that affect beta or reverse.

When prop_mode is overridden and prop_pitch is not overridden, X-Plane will control feathering automatically, and writes of 0 to the prop_mode will be rejected.  Automatic feathering in the sim will still work.

Overriding prop_pitch

When setting sim/operation/override/override_prop_pitch to 1, X-Plane will not change the prop pitch. A bunch of code is bypassed:

1. The joystick prop axis will not set the prop pitch (for variable pitch props) and will not set the target RPM (for constant RPM props). You must read the axis yourself using sim/joystick/joy_mapped_axis_avail and sim/joystick/joy_mapped_axis_value – array index 8 for a single global prop lever or 24-27 for individual prop levers.
2. Constant mach and thrust-vectored props will not auto-adjust the target RPM.
3. Feathering is disabled; you must set the prop mode to feather, and you must set the actual prop angle. Setting a prop mode of 0 and not setting the angle will not result in feathering!
4. Constant RPM props will not set their pitch.

You can set the prop angle directly via sim/cockpit2/engine/actuators/prop_angle_degrees.