Geometry effects on autorotation of rectangular prisms

• Autorotation of low aspect ratio rectangular prisms has been studied in a wind tunnel.
• Previous correlations of geometry effects have been shown to perform poorly.
• A well-behaved variation of rotation rate with aspect ratio and thickness is seen.
• A 3D surface fit permits effects of geometry to be determined graphically.
• Two new parameters for characterising geometry effects have been proposed.

This paper presents an analysis of published data on the autorotation of rectangular prisms, supplemented by additional wind tunnel tests of container-like shapes typical of helicopter slung loads. Previous widely used correlations for effects of geometry on autorotation rate are shown to perform rather poorly, particularly for higher aspect ratios and thickness/chord ratios. A smooth and well-behaved 3D surface has been fitted to the data, permitting the likely autorotation rate of any rectangular prism to be determined graphically. Of particular significance to the behaviour of slung loads is a well-defined region where autorotation does not occur. Two new parameters for presenting autorotation data have been proposed. A normalised aspect ratio has been used which allows 2D and 3D data to be plotted on a single figure, and gives a more nearly linear variation of autorotation rate with aspect ratio. The geometric mean aspect ratio is a simple single parameter for the autorotation rate, which gives an approximate collapse of trends in the data. This parameter can be interpreted as a ratio between the strengths of driving vortices shed from spanwise and streamwise edges respectively, suggesting that the 3D structure of the flows driving autorotation would repay further investigation.