Technical terminology: what’s it called? A cylindrical shaft which rotates in a fluid medium and carries a set of radially-arranged blades (?splines) attached by hinges. The blades fold flat against the shaft when it rotates in one direction (say clockwise) but they sweep out automatically to resist rotation in the opposite direction (counterclockwise). Viewed from either end it would look like a spiral galaxy, rotating smoothly in one direction but not in the opposite direction. No hard data (only hypothesis), but any help with terminology would be greatly appreciated!