Using Wind Tunnel Tests to Investigate Dual Lift Trim, Maneuvers, Stability and Control

The paper presents results of wind tunnel tests of a pendant dual lift system. A rig has been designed and built to represent the helicopter hooks and the suspension and load below the hooks. Four of the six degrees of freedom of movement of the hooks are modeled and the hooks are operated by open or closed loop control systems. At first quasi-steady maneuvering is studied. A special analytic model is derived and then used to study a typical maneuver and system sensitivities to servo inputs and drag estimation errors. A good agreement is shown between analytic and test results. Then a nonlinear simulation model is validated via comparisons of its results and test results for various frequency sweeps. The validated model is used to design an equal load distribution control system that is implemented in the wind tunnel test system. Wind tunnel tests show the capability of this control system to reduce significantly load differences during maneuvers.