MAVLINKHUD

Heli Autorotate Mode (Copter)

Executive Summary

This mode is exclusive to Traditional Helicopters. It automates the complex "Autorotation" maneuver required to land a helicopter safely after a motor failure. Instead of crashing, the helicopter uses the energy stored in the rotor blades and the airflow during descent (gliding) to maintain rotor RPM, flaring at the bottom to cushion the landing.

Theory & Concepts

1. The Physics of Autorotation

Autorotation is the process of extracting energy from the air to keep the rotor spinning.

  • The Inflow: As the helicopter falls, air rushes UP through the rotor blades.
  • The Angle of Attack: By reducing the collective pitch (blade angle), the upward air flow actually "pushes" the blades to spin faster (like a windmill).
  • The Flare: Just before the ground, the pilot increases pitch. This converts the spinning rotor energy (Inertia) into Lift, stopping the descent for a soft landing.

2. Energy Management

Autorotation is an energy-management game.

  • Potential Energy (Altitude): Your fuel.
  • Kinetic Energy (Airspeed): Helps keep the rotor spinning.
  • Rotational Inertia (RPM): Your "Battery" for the final landing.
  • The autopilot's job is to trade Altitude for RPM and Airspeed, ensuring the RPM stays within the "Green Zone" until the very last second.

Hardware Dependency Matrix

Requires specific setup for helicopters.

Sensor Requirement Code Implementation Notes
RPM Sensor CRITICAL The controller needs precise Rotor Speed (Head Speed) feedback to manage the collective pitch during glide.
Airspeed RECOMMENDED Improves the glide efficiency significantly.
GPS RECOMMENDED Required for position-controlled glide and flare. Without it, the mode relies on inertial estimates which drift.
Rangefinder CRITICAL Essential for determining the exact moment to trigger the Flare and Touchdown phases.

Control Architecture (Engineer's View)

This mode implements a multi-stage State Machine in mode_autorotate.cpp.

  1. Entry Phase:
    • Triggered when the motor interlock is disengaged (motor failure simulation) or Mode Switch is used.
    • The collective pitch is reduced immediately to preserve rotor RPM (AROT_COL_ENTRY).
  2. Steady-State Glide (SS_GLIDE):
    • The controller manages two main variables:
      • Head Speed: Controlled via Collective Pitch. (Low RPM -> Lower Collective to speed up).
      • Forward Speed: Controlled via Pitch Attitude.
    • Goal: Maintain efficient airspeed for maximum range or minimum descent rate.
  3. Flare Phase:
    • Triggered by Altitude (Rangefinder).
    • The vehicle pitches up aggressively to convert forward airspeed into lift and rotor energy.
  4. Touchdown:
    • The remaining rotor energy is used to cushion the final meter of descent.

Failsafe & Bailout

  • Bailout: If the pilot re-engages the motor interlock (decides to abort the landing), the Bailout logic immediately ramps the motor back up to flight idle speed to recover powered flight.
  • Bad RPM: If the rotor RPM drops too low during glide, the controller sacrifices altitude (lowers collective) to recover RPM, prioritizing control authority over flight time.

Key Parameters

Parameter Default Description
AROT_ENABLE 0 Master switch for the feature.
AROT_COL_ENTRY -2 Collective pitch (degrees) used during entry phase.
AROT_HS_TARG 1500 Target Head Speed (RPM) during glide.
AROT_FLARE_TIME 3.0 Duration (seconds) of the flare maneuver.

Source Code Reference