M.Sc Thesis


M.Sc StudentMalka Klein-Frucht
SubjectInvestigation of a Slung Load Model in a Wind Tunnel
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Rosen Aviv
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

It is common for helicopters to carry an externally suspended load, in what is known as slung-load operation. The combination of a helicopter and a slung-load is a very efficient technique of moving different kinds of loads from one location to another.

Yet, in many cases the helicopter slung-load combination suffers from severe stability problems, which result in significant limitations on the performance and may also cause accidents. Usually the instability occurs at medium or high flight speeds, wherein the load develops yaw vibrations accompanied by lateral diverging vibrations.

The purpose of the present research is to investigate the option of a simple, cheap and practical stabilization method that will stabilize a slung load that is attached to a helicopter through a single suspension point. The investigation is focused on a CONEX load (a container used by the US Army). The stabilization system includes two stabilizers that are attached to the back narrow side of the load. While the CONEX is on ground, the stabilizers are folded, and while being carried by the helicopter the stabilizers are opened and secured at a certain angle relative to the CONEX walls.

The investigation of the aerodynamic characteristic of the CONEX includes detailed static wind tunnel experiments. An extensive data base was obtained including all the aerodynamic force and moment coefficients, as functions of the side-slip angles and the angles of attack.

A new test set-up was designed and built for performing dynamic tests in the wind tunnel. The model is hanged from the tunnel ceiling by a pendulum-like mechanism, allowing rotations in three angles: roll, pitch and yaw.

A theoretical model that describes the dynamic wind tunnel tests was derived. This model includes a system of three nonlinear equations. In order to investigate the stability of the load, a technique of analyzing small perturbations about a basic steady state is applied.

A comparison between the steady state results of the wind tunnel tests and results of solving the nonlinear equations that represent the same case, showed a good agreement between the two cases.

Using the theoretical model, a detailed stability investigation of the CONEX was carried out. As expected, in the case of a stabilized load there was an improvement in the load stability. Yet, the system still suffered from instability problems over a wide range of flight speeds. Similar trends were obtained during the wind tunnel tests.