טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentKfir Arbel
SubjectInvestigation of the Autorotation of a Cargo Container
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Emeritus Rosen Aviv


Abstract

As a result of a flow around them, various bodies develop a steady rotation. This phenomenon is called autorotation. 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, exhibit autorotation as a result of the aerodynamic loads that act on them during flight. 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 exhibit 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 (Computational Fluid Dynamics) calculations can predict the phenomenon, but the difference between the predicted autorotation speed and the measured speed is significant in many cases.

Previous studies at the Technion showed good agreement between the autorotation speed of full size slung loads during flight tests and small wind tunnel models (taken into account scale factors). In those tests the models were hung from the tunnel ceiling and performed pendulum motions as well as autorotation.

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 (in contrast to previous investigations in the Technion). 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 measured by a sting balance.

Based on the experimental investigation, an empirical formula to calculate the autorotation speed is examined. A similar approach was reported in the past, using only the results for a single model. This formula is based on the assumption that the aerodynamic yaw moment that acts on the model during autorotation, as a function of the angle of the model, is basically the same as the moment that acts on the model that does not rotate, while the rotation causes a certain time lag (as compared to the moment without rotation). The time lag can be translated into an angular lag.