Analysis and Optimization of Hanging Stability of Heavy-Duty Coaxial Unmanned Helicopter

This study examines the hovering and forward-flight stability of a heavy-lift coaxial unmanned helicopter with a suspended payload. A six-degree-of-freedom dynamics model is developed, coupling the rotor, fuselage, and a rigid-body sling via a universal joint, while neglecting rotor–payload aerodynamic interference for sling lengths exceeding eight rotor diameters. Trim calculations for hover and steady forward flight are compared with F-500 flight data, showing better agreement for the longitudinal trim quantities than for the lateral quantities. Stability analysis using static derivatives and eigenvalue decomposition reveals that the suspension system degrades airspeed stability but has little effect on angle-of-attack stability. At the reference flight condition, the Dutch-roll eigenvalues have a positive real part, indicating an unstable lateral-directional oscillation. Parametric studies indicate that increasing forward speed worsens the longitudinal phugoid mode, whereas heavier payloads enhance damping in both longitudinal and lateral-directional modes. Enlarging the payload’s cross-sectional area improves non-oscillatory mode stability but barely affects lateral dynamics. These findings provide theoretical guidance for design and safe operation, highlighting the importance of speed regulation and payload configuration optimization to enhance flight safety.