Carriage of external slung loads by helicopters is a common method of transporting large and heavy loads. In many cases slung loads that are connected to the helicopter through a swivel, tend to develop steady rotation as a result of the aerodynamic loads that act on them during flight. This phenomenon is called autorotation. Previous research has shown that autorotation may be very beneficial since in many cases it stabilizes the slung load. Without rotation the same load may become unstable, exhibiting large pendulum vibrations that endanger the helicopter. Wind tunnel and flight tests have shown that the autorotation phenomenon depends on the: geometry of the load, its mass distribution, the orientation of the load relative to the incoming flow, and other parameters. There is not a simple method of predicting the autorotation speed of a body when the above parameters are known. Detailed CFD calculations predict the phenomenon, but the differences between the predicted autorotation speed and measured speed are significant in many cases.
Previous studies at the Technion showed good agreement between the autorotation speed of actual slung loads during flight tests and small wind tunnel models. In those tests the models were hung from the tunnel ceiling and performed pendulum motions as well as autorotaion. The goal of the present research is to study the autorotation of box-like models during wind tunnel tests. During the tests rotation about a fixed axis is the model sole degree of freedom. The angle of rotation is measured continuously and the angular speed and acceleration are calculated. In addition the forces and moments that act on the model during autorotation are also measured.
The seminar will start with a short review of previous research on the autorotation phenomenon. Then the wind tunnel test set up and test techniques will be described. Selected results will be presented and discussed. These results will include autorotation of different models at different wind tunnel speeds. An effort to describe the phenomenon by a simple empirical formula will also be presented and discussed. The presentation will end with suggestions for further research.
