1. Trim & Autopilot Following¶

TelemFFB can move your stick and pedals in response to the simulator’s trim and autopilot, so that trimming the aircraft relieves control pressure and relocates the stick’s resting position, just like in a real aircraft.

Because neither MSFS nor X-Plane have any concept of force feedback, they treat any axis movement as a deliberate control-surface command. If TelemFFB simply let the stick move with the trim and reported that movement to the sim, the sim would read it as additional elevator input stacked on top of its own trim, and the aircraft would pitch far more than it should. To prevent this, TelemFFB controls the axis position the sim receives, delivering only a fraction of the physical stick’s movement. Getting that fraction right is what the settings below (and the automatic calibration) are for.

1.1 Enabling the Feature¶

Under Axis Control, enable Trim Following and/or Autopilot Following as desired.

1.2 Trim Following Gains¶

Trim following is tuned with two gains per axis (X = aileron, Y = elevator):

• X / Y Trim Gain Physical - how far the physical stick moves in response to trim. At 100%, full trim travel moves the stick through its full travel. This is a feel setting; it controls how much the stick relocates as you trim.

• X / Y Trim Gain Virtual - how much of that physical movement is passed on to the simulator. At 20%, only 20% of the stick’s trim-induced movement is reported to the sim as control input. This is the setting that determines whether the aircraft pitches when you trim: if it is too high the nose follows the trim too strongly, too low (or too negative) and it fights the trim.

1.3 Calibrated Trim Curve (Elevator Only)¶

Some aircraft do not respond to trim in a simple, straight-line fashion, and a single Y Trim Gain Virtual value cannot hold the nose steady across the whole trim range. For these, TelemFFB can measure the aircraft’s actual trim response and store a curve instead of a single value.

• Use Calibrated Trim Curve - when enabled, the calibrated curve is used for the elevator axis in place of the static Y Trim Gain Virtual value. Leave it off to use the single value.

• Trim Curve Calibration → Calibrate… - opens the calibration tool, which flies the aircraft and measures the curve for you. See Automatic Trim Calibration.

1.4 How Trim Following Works¶

Each time you trim, three things happen:

1. TelemFFB reads the new trim position from the simulator.

2. The stick’s spring center relocates by the trim amount, scaled by the physical gain. If you were holding pressure against the spring, that pressure relieves as the center moves toward your hand, exactly what trimming does in a real aircraft.

3. An axis value is sent to the simulator. Your own stick input passes through normally, but the trim-induced part of the stick’s movement is first scaled by the virtual gain, or replaced by the calibrated curve. This is the critical step: the simulator still applies its own trim internally, so if the stick’s full trim movement were reported as control input, it would stack on top of the sim’s trim and the aircraft would pitch away from where you trimmed it. The virtual gain delivers only the fraction that keeps the two in balance.

When the balance is right, trimming while holding the stick still does not change the aircraft’s pitch: the force in your hand relieves and the nose stays put. Where that balance point sits differs per aircraft, which is exactly what the Automatic Trim Calibration measures.

1.5 Autopilot Following¶

With autopilot following enabled, the physical controls move with what the autopilot is flying:

• MSFS:

  • The elevator follows through the trim value, because autopilots fly pitch with elevator trim.
  • The aileron and rudder follow the control-surface deflection the autopilot commands.

• X-Plane:

  • The controls follow the simulator’s autopilot servo datarefs.

• The movements are dampened to prevent oscillation in turbulence or on aircraft with very sensitive controls.

• While you hold a control past the deadzone, its spring center stays where it was. It follows the autopilot again when you let go.

Two settings govern how your hands and the autopilot share the controls:

• Deadzone - you must move the control further than this before your input is sent to the sim. This lets the autopilot fly without your resting hand fighting it.
• Gain - the ratio of physical control movement to in-sim control movement once you do move past the deadzone.

1.5.1 Custom Autopilot Variable¶

Autopilot following uses AUTOPILOT MASTER on MSFS and sim/cockpit2/autopilot/servos_on on X-Plane to determine the autopilot state. Some advanced or non-standard aircraft implementations may not properly set these standard values and as a result, TelemFFB cannot determine the correct AP state. For those, enable Custom Autopilot Variable and enter a variable that shows whether the autopilot is engaged: on MSFS a SimVar, an L:Var or an input event (B:), on X-Plane a dataref path. A value of 0 means off; any other value means on.

• Set it on each device that follows the autopilot.
• If the variable needs converting first, turn this setting off and use the SimConnect/Dataref Overrides Editor instead: override APMaster (MSFS) or APServos (X-Plane) with the variable and a scale.

1.5.2 Custom Follow Variables¶

Autopilot following reads the control-surface deflection the autopilot commands, and the trim for pitch. Some addons move their surfaces in steps, or fly through their own systems and publish the yoke and pedal positions in variables of their own. For those, enable Custom Roll Follow Variable, Custom Pitch Follow Variable or Custom Yaw Follow Variable and enter the variable the control should follow: on MSFS a SimVar, an L:Var or an input event (B:), on X-Plane a dataref path, reading from -1 (full left, or full forward) to 1.

A variable on another scale needs the Transform field under it, where x stands for the value. Leave it blank for a variable that already reads -1 to 1; enter x - 1 for a variable that reads 0 to 2, x/50 - 1 for a percentage, or -x to reverse the sign. The syntax is the same as the Scale field of the overrides editor. The Monitor tab shows the raw value as APFollowX, APFollowY and APFollowRudder.

The pitch variable is read with a calibrated trim curve in use as well. The PMDG 737 profiles are an example of these settings in use; see Aircraft with Special Treatment.

1.5.3 Disconnect Autopilot on Override¶

Some autopilots do not let go when you overpower the controls. With Disconnect Autopilot on Override enabled, TelemFFB sends a disconnect once you hold the stick past the Disconnect Deflection: the stick’s travel away from the autopilot’s command, as a share of full travel. Keep it above the autopilot following deadzone, or a resting hand sends it.

On MSFS, Disconnect Event is what TelemFFB sends: a key event, an L:Var, an input event (B:) or a SimVar. AUTOPILOT_DISENGAGE_SET is the yoke disconnect switch; AUTOPILOT_OFF is the master switch. Disconnect Value is sent with the event, and Release Value, when set, is sent 100 ms later, for an addon that expects the switch to be let go.

On X-Plane, Disconnect Command is the command TelemFFB sends. sim/autopilot/servos_off_any disconnects the autopilot servos and leaves the flight director; sim/autopilot/servos_fdir_off turns the flight director off as well.