MAVLINKHUD

ACRO Mode (Plane)

Executive Summary

ACRO Mode for fixed-wing aircraft provides a rate-stabilized flight experience. Unlike "Manual" mode, where the sticks directly move the servos, ACRO mode uses the gyroscope to maintain the commanded rotation rate. This makes the aircraft feel "locked in," smoothing out turbulence and wind gusts automatically.

Theory & Concepts

1. The Stability vs. Neutrality Spectrum

Flight modes exist on a spectrum of control authority.

  • Manual: Neutral. The plane does exactly what the servos say. If wind hits a wing, the plane rolls.
  • Stabilize: Stable. The autopilot constantly "pulls" the plane back to the horizon.
  • Acro: Semi-Neutral. The autopilot uses the gyro to lock the current rotation rate. If you are in a 30-degree bank and a gust hits you, the autopilot moves the servos to resist that gust, keeping you at exactly 30 degrees.
  • The Benefit: Acro is the most efficient way to fly because you don't fight the autopilot's desire to level out, which saves battery and allows for perfect cinematic curves.

Hardware Dependency Matrix

ACRO mode relies on the rate gyros to function.

Sensor Requirement Code Implementation Notes
Gyroscope CRITICAL The core of the Rate Controller. Without it, stabilization is impossible.
Airspeed RECOMMENDED While not strictly required, airspeed data helps scale the PID gains (scaling control surface deflection based on speed) for consistent handling.
GPS NONE Not used.

Control Architecture (Engineer's View)

Plane ACRO is significantly more complex than Copter ACRO because it supports three distinct sub-behaviors defined by ACRO_LOCKING.

  1. Rate Only (ACRO_LOCKING = 0):
    • Behavior: Sticks command Rate (deg/s). Center stick command 0 deg/s.
    • Effect: The plane stops rotating when you let go, but it does not hold the angle. It will slowly drift due to trim/gravity.
  2. Attitude Lock (ACRO_LOCKING = 1):
    • Behavior: When sticks are centered, the controller "locks" the current Roll/Pitch angle.
    • Effect: If you bank 30 degrees and let go, the plane fights to hold exactly 30 degrees.
  3. Quaternion Lock (ACRO_LOCKING = 2):
    • Behavior: Uses a full 3D Quaternion controller.
    • Effect: This allows "3D Heading Hold". You can point the nose straight up (Vertical Hover) and let go, and it will try to hold that vertical attitude. Standard Attitude Lock fails at 90 degrees pitch (Gimbal Lock); Quaternion does not.

Pilot Interaction

  • Sticks Active: You are commanding the rotation speed of the aircraft.
  • Sticks Centered:
    • Rate Mode: The plane stops rotating but may drift.
    • Lock Mode: The plane holds its current attitude "like it's on rails."
  • Difference from Manual: In Manual, if a gust hits the wing, the plane rolls uncommanded. In ACRO, the servo will instantly move against the gust to keep the plane steady.

Failsafe Logic

  • Stall Prevention: ACRO mode does not natively prevent stalls. It will happily try to hold a pitch angle that results in a stall. However, if STALL_PREVENTION is enabled, the controller may limit elevator output at low speeds (if an airspeed sensor is present).

Key Parameters

Parameter Default Description
ACRO_ROLL_RATE 180 (deg/s) Max roll rate at full stick.
ACRO_PITCH_RATE 180 (deg/s) Max pitch rate.
ACRO_LOCKING 0 0=Rate Only, 1=Attitude Lock, 2=Quaternion (3D) Lock.
RLL_RATE_P ... Roll Rate PID gains (Main tuning knob for responsiveness).

Source Code Reference