In many cases helicopters carry external loads that are connected by slings to a hook on the fuselage (slung loads). The main advantages of this method are the capability to transfer big loads and the ease of handling those loads. When the load is too heavy to be carried by a single helicopter, a multi lift technique can be applied: Connecting a single load to the hooks of two (dual lift) or more helicopters. Dual lift presents a complex dynamic system. The possibility of dual lift was presented by limited flight tests and investigated by various numerical analyses. The present research will present the use of wind tunnel tests to investigate various aspects of dual lift operations.
A rig has been designed and built to represent the helicopters hooks, the suspension cables and the load below the hooks. The rig includes a bar, where the tips of this bar represent the hooks of the two helicopters. During the wind tunnel tests each tip can move along the bar axis, thus representing changes in the distance between the two helicopters. In addition, the pitch and yaw angles of the bar can be changed during the test, representing changes in the relative altitude and flight formation of the two helicopters. In the present study the load is a model of a CONEX container with two stabilizing fins. A numerical simulation of the test was also developed where the coupled dynamics of the system (suspension cables and the slung load) is modeled. The simulation includes the influence of aerodynamic, gravity and inertia effects. Results of the wind tunnel tests will be presented. The test results will be compared with results of the numerical model. The validated numerical model will be used to study the influence of various parameters on the dual lift operation.
