I am simulating a custom copter tailsitter in Gazebo Harmonic + ArduPilot SITL. VTOL takeoff and transition to fixed-wing complete successfully.
However, if I set Q_ASSIST_SPEED = -1 (or a low value), the aircraft pitches down after transition, appears to lose lift, and crashes. If I set Q_ASSIST_SPEED to a high value (e.g. 20 m/s), the aircraft flies normally because VTOL assistance remains active.
My understanding is that Q_ASSIST_SPEED should only control when VTOL motors assist based on airspeed. Why would lowering it make the aircraft unable to sustain fixed-wing flight?
Could this indicate an aerodynamic modeling issue (LiftDrag plugin, wing lift, CP/CG location), or am I misunderstanding how Q_ASSIST_SPEED works on a copter tailsitter?