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Overview

The STAB parameter group (specifically STAB_PITCH_DOWN) configures the assisted flight behaviors in Stabilize and FBWA modes.

Key Concepts

1. Low Throttle Pitch Trim

In a manual airplane, reducing throttle typically causes the nose to drop as airspeed decreases. In an autopilot-stabilized mode, the autopilot might try to hold the nose up, which can lead to a stall.

  • STAB_PITCH_DOWN: Automatically adds a small amount of down-elevator (pitch down) when the throttle is at zero or idle.
    • Purpose: Keeps the nose down in a glide to maintain airspeed and prevent stall.
    • Typical Value: 2 to 5 degrees.

Developer Notes

  • Library: ArduPlane/Parameters.cpp.
  • Context: Only relevant for ArduPlane.

STAB_PITCH_DOWN

deg
Default 2.0f
Range 0 15

Low throttle pitch down trim (STAB_PITCH_DOWN)

Description

This parameter improves flight safety in stabilized modes (like FBWA) by automatically pitching the nose down when the pilot reduces the throttle. This mimics the natural behavior of a well-trimmed aircraft and helps maintain airspeed during glides or landing approaches, preventing accidental stalls.

The Mathematics

$$ \text{Pitch}{\text{bias}} = \text{STAB\_PITCH\_DOWN} \times \frac{\text{TRIM\_THROTTLE} - \text{Throttle}{\text{in}}}{\text{TRIM\_THROTTLE} - \text{THR\_MIN}} $$
(Applied only when Throttle < TRIM_THROTTLE)

The Engineer's View

Defined in ArduPlane/Parameters.cpp. This bias is added to the pitch controller's setpoint.

Tuning & Behavior

  • Default Value: 2.0 degrees
  • Increase: If the aircraft floats too much or slows down dangerously when you cut the throttle.
  • Decrease: If the nose drops too aggressively.
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