BFF CL Software - Short User Guide


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Appendix H. Elevator Axis Stick Pusher.

A basic anti-stall stick pusher behaviour is implemented in the system. Use Tab 11 in the Configuration Manager (v1.14+) to make the settings.

The Stick Pusher acts in the elevator axis only.

The Stick Pusher can be enabled using three methods:

  1. Set Tab 11 Item 1: Stick Pusher Master Enable = 1
  2. Set an Enable hot button using Tab 11 item 2. The hot button (on the Pilot joystick card) will toggle the feature On/Off when pressed.
  3. Set a custom FSUIPC offset using Tab 11 Item 3. When a non-zero value is written to the offset the Stick Pusher is enabled, writing zero will disable the feature.

Enable method 1. will override methods 2 & 3.


When enabled the feature will induce a nose down force on the elevator axis when the aircraft relative angle of attack rises above the onset threshold set in Tab 11 Item 4.

Relative AoA is the ratio of the aircraft current AoA to the maximum AoA for the aircraft (as specified in FSUIPC offset 0x11BE).

The nose down force will then increase linearly with relative AoA up to a peak AoA as set in Tab 11 Item 5 at which point the force will reach its maximum value as set in Tab 11 Item 6.

The sign of the force set in Item 6 is significant. If the pusher force is acting in the wrong direction (ie pulling rather than pushing) then the sign can be inverted to reverse the force direction.

The rate of force application and the maximum force can therefore be set using the AoA thresholds and the maximum force value.

Force Smoothing

In addition Tab 11 Item 7 allows a force delay/smoothing factor to be applied.

When the Item 7 Force Smoothing Factor is disabled (=0) the pusher force will follow the relative AoA variations directly. In some lighter aircraft pitch oscillations may result if rapid attitude changes during stall occur which then feed back stick pusher force oscillations to the elevator control axis.

This tendency can be reduced by applying the force onset delay/smoothing factor.

Increasing the factor will increasingly smooth the stick pusher force whilst also delaying its response to changes in relative AoA. It will also delay the removal of the force when the AoA drops. The effect is similar to causing the stick pusher force to follow a running average of the relative AoA - short sharp peaks in AoA tend to be ignored whilst sustained values more strongly increase the push force.

Excessively high values may however introduce too much lag in the force onset/offset so some experimentation may be needed to set the optimum value for a given aircraft.

The factor is actually the time constant in a low pass filter applied to the pusher force.

Auto-Pilot Interaction

If the stick pusher force becomes active when A/P is active then the system will automatically disengage A/P.

Custom FSUIPC Offset Values (Tab11 Item 3)

With version 1.540+ of the Background Process the offset value becomes bit-significant. Bit0 still controls the main ON/OFF switch. However the additional bits can be set to simulate various test procedures on the stick pusher.

The full bitwise significance is:

Bit0 = Stick Pusher ON/OFF
Bit1= Max Force ON/OFF
Bit2 = 1 to Trigger Stall Vibs (Rel AoA goes to 100% for stall vib calc only)
Bit3 = 1 to Drop OnSet Rel AoA (value drops to fraction specified in bits4-7, only for stick pusher calc )
Bits4-7 = Fraction of original OnSet AoA to drop too when Bit3 is set, fraction specified in 10ths

Eg bits 4-7 = 0001 = New Onset AoA = 1 tenth of Tab11 Item 4 value
Bits 4-7 = 0101 = 5 tenths
Bits 4-7 = 1010 = 10 tenths etc
Value is limited to 10.

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