Passive Solution for Improved Stability and Drag of Cuboid Underslung Payloads

Cuboid helicopter sling loads (HSLs) impose severe operational constraints on rotary-wing aircraft due to high aerodynamic drag and flow-induced instabilities. These constraints limit maximum flight speed, operational range, fuel efficiency, and mission effectiveness while substantially increasing pilot workload and flight risk. This study investigates passive aerodynamic modifications to improve the stability and reduce the drag of cuboid underslung payloads through combined leading-edge and wake-based flow control strategies. Dynamic wind tunnel testing was conducted using a 1/16-scale model of an ISU-90 cargo container suspended from a ball joint to permit realistic six-degree-of-freedom motion. An onboard inertial measurement unit (IMU) measured angular kinematics while a force balance quantified aerodynamic loads across a velocity range scaled to operational helicopter speeds via Froude scaling. Building on prior work demonstrating that double-chamfer leading-edge modifications reduce drag but provide insufficient stability alone, whereas large trailing-edge fins achieve stabilization but induce excessive drag, this study systematically optimized fin geometry to minimize drag while maintaining stability. Systematic variation of fin geometry parameters—including planform area, sweep angle, taper ratio, aspect ratio, and deflection angle—revealed that deflection angle exerts the dominant influence on drag, followed by planform area. Sweep angle, taper ratio, and aspect ratio variations produced converging drag coefficients at higher velocities with no clear trends. The double chamfer corner profile on the forward faceplate was found to most effectively increase fin control authority, resulting in smaller area fins maintaining stability with zero deflection. A novel perturbation-based testing methodology was developed to assess dynamic stability, demonstrating that increased fin deflection improves damping of yaw disturbances. The results indicate that the minimum fin area capable of maintaining stability at zero deflection provides optimal performance, reducing drag coefficient by approximately 35% compared to large deflected fins while maintaining stability across the operational velocity range. These findings provide actionable design guidance for field-deployable passive stabilization systems that enhance HSL operational capabilities without active control complexity.