M.Sc Thesis

M.Sc StudentFogel Omer David
SubjectWind Tunnel Tests to Investigate the Behavior of a Dual
Lift System
DepartmentDepartment of Aerospace Engineering
Supervisor PROFESSOR EMERITUS Aviv Rosen
Full Thesis textFull thesis text - English Version


In many cases helicopters carry external loads that are connected by slings to a hook under 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. There are three main methods of investigating such a system: flight tests, numerical simulations and wind tunnel tests. Flight tests address directly the actual problem but usually they are very expensive, take long time, and can be risky. Numerical simulations are the most economic approach but require verification and validation. Wind tunnel tests offer an intermediate tool which can be used to validate the simulation and reduce the risk of flight tests.

The present research will present the use of wind tunnel tests to investigate various aspects of dual lift operations and verify a numerical simulation tool.

In order to perform the wind tunnel tests, a new rig has been designed and built to represent the helicopters hooks, the suspension cables and the load below the hooks. The tests were focused on a pendant configuration, in which the load is connected to the hooks using two suspension cables and a four-legged sling (FLS).

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.

The tests included trim and dynamic runs. The current research is focused on the trim characteristics of the system. A numerical simulation of the test was developed, where the entire system is modelled: Bar, suspension cables and the slung load (container, stabilizing fins, and FLS). The simulation includes the influence of aerodynamic and gravity effects on each component. Results of the wind tunnel tests and comparisons with the numerical results are presented. The validated numerical model is used to study the influence of various parameters on the dual lift operation. Configurations where equal load distribution between the two helicopters hooks is obtained are of special interest, since they maximize the carrying capability of the dual lift system.