The tumbling is one of autorotations, and can be used for a novel windmill and waterwheel. The authors conduct a series of subsonic wind-tunnel experiments with the least support-friction loss, in order to investigate the effect of cross-section geometry upon the tumbling of a simple two-dimensional rectangular-cross-section prism. On the basis of the freely-rotating rate of the prism, the tumbling can be classified into two; namely, a flat-plates mode for depth-to-width ratio λ<0.6 with reduced rotating rate Ω*max≃0.4-0.6, and a rectangular-prism mode for λ≥0.6 with Ω*max≃0.4. In the former mode, Ω*max monotonically decreases with increasing λ.
