Version: 1.8.16.0
Frequency: Consistently
Severity: High
Bug description:
We are seeing an issue with the default/legacy MSFS autopilot Flight Level Change / IAS mode when using OBJ_EA1_SURFACE entries for the elevator, rudder, and lower fin.
We are not having any issues with OBJ_EA1 fuselage objects. We use several of those successfully for the main fuselage, nacelles, and tip tanks.
The problem appears specifically related to these EA1 surface objects.
Current configuration:
[obj_ea1_surface.0]
name = ELEVATOR
position = 0.010571,8.594038,-22.89499
size = 15,0.1,1.8
group = aircraft
element_number = 10,2,2
surface_relative_position = 0,0,-0.9
surface_cx = 0.5,0.25,0.05
surface_cx_tangent = 0.09,0.01,0.01
surface_cx_normal = 0.5,0.25,0.05
surface_cx_efficiency = 0.2,0.2,0.2
surface_cx_nscaler = 4,4,4
surface_cx_npower = 1,1,1
modifier.0 = elevator_angle
modifier_angle_scalar.0 = -1.4,0,0
modifier_position_scalar.0 = 0,0,0
modifier_surface_relative_position_scalar.0 = 0,0,0
[obj_ea1_surface.1]
name = RUDDER
position = -0.000046,3.557908,-20.556206
size = 0.2,4.2,1.8
group = aircraft
element_number = 2,10,2
surface_relative_position = 0,2.1,-0.9
surface_cx = 0.5,0.25,0.05
surface_cx_tangent = 0.09,0.01,0.01
surface_cx_normal = 0.5,0.25,0.05
surface_cx_efficiency = 0.1,0.1,0.1
surface_cx_nscaler = 3,3,3
surface_cx_npower = 1,1,1
modifier.0 = rudder_angle
modifier_angle_scalar.0 = 0,-1.5,0
modifier_position_scalar.0 = 0,0,0
modifier_surface_relative_position_scalar.0 = 0,0,0
[obj_ea1_surface.2]
name = LOWER_FIN
position = 0.0,1.720623,-17.848277
size = 0.05,0.8,7
group = aircraft
element_number = 2,2,10
surface_relative_position = 0,0,0
surface_angle = -15,0,0
surface_cx = 0.5,0.25,0.05
surface_cx_tangent = 0.09,0.01,0.01
surface_cx_normal = 0.5,0.25,0.05
surface_cx_efficiency = 0.1,0.1,0.1
surface_cx_nscaler = 3,3,3
surface_cx_npower = 1.5,1.5,1.5
With all three EA1 surface entries enabled, engaging IAS/FLC causes a much larger nose-up response than expected. The aircraft can lose approximately 6-10 knots and then continue to fall behind the selected IAS instead of recovering properly.
Removing one or two of these surface entries progressively improves the behavior. When all three are removed, the default FLC/IAS mode holds within approximately 1-2 knots and the pitch response becomes much smoother.
We have tested the same aircraft with the GNS 530 and PMS GTN 750 configurations. Those autopilots behave correctly and hold IAS very well. The important difference is that those configurations do not use the legacy/default MSFS autopilot path used by the TDS GTNXi configuration.
We have also tested both the pitch PID and the flightLevelPID. Both are stable and behave correctly with the other autopilot configurations, so we do not believe the basic PID tuning is the root cause.
Summary:
- OBJ_EA1 fuselage objects work normally
- EA1 elevator/rudder/lower-fin surfaces cause excessive pitch-up and IAS loss with the legacy/default FLC controller
- Removing the surfaces progressively improves the behavior
- Removing all three allows IAS to hold within approximately 1-2 knots which is normal
- GNS 530 and PMS GTN 750 configurations behave normally with those entries
- Pitch PID and flightLevelPID appear stable
Could you please investigate whether OBJ_EA1_SURFACE forces are interacting incorrectly with the legacy/default Flight Level Change / IAS autopilot controller?