Flight Test Results for the Motions and Aerodynamics of a Cargo Container and a Cylindrical Slung Load
ARMY AVIATION-MISSILE RESEARCH DEVELOPMENT-ENGINEERING CENTER MOFFETT FIELD CA AEROFLIGHTDYNAMICS DIRECTORATE
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The stability of loads slung beneath a helicopter has been an ongoing problem. Many loads are limited by stability to speeds well below the power limited speed of the helicopter-slung-load configuration. Load motions are forced by steady and unsteady aerodynamics and are rich in complex interactions between the dynamics and aerodynamics. The present report examines the motions and aerodynamics of a cargo container and a cylindrical engine canister slung load using a flight database. The loads were instrumented with a military-grade inertial navigation unit and Global Positioning System and with sling-leg load cells to measure cable tensions. Data were collected for various load weights, sling configurations, slings suspended with and without a swivel, and for both offset and centered center of gravity in forward flight out to the limiting airspeed for all configurations. Load motions are analyzed for the directional and pendulum degrees of freedom. A variety of steady-state behaviors were found, depending on configuration. When suspended without a swivel, the cargo-container sling winds up and pendulum excursions limit operational speeds to 60 kts. With a swivel, the cargo container spun up to a steady yaw rate and the spin suppressed the pendulum motions and allowed the load to be carried to the power limit of the configuration. The spin also produced a small but measurable Magnus effect. An extreme cg offset produced stability around small end into the wind. The engine canister generally oscillated around broadside to the wind. Cable tensions and hook force are analyzed for both the swiveled and unswiveled slings. For the swiveled sling, the sling-leg tension variations occurred at harmonics of the spin rate and the harmonics were cancelled or reinforced when summed into the hook force. For the unswiveled sling, cable tension variations are dominated by the windup cycle.