The Savonius Rotor: Two Hollow Vanes, Any Wind, Always Starts

Cut a cylinder in half lengthwise, slide the halves apart so they overlap, and stand them on a vertical shaft: that is the rotor Sigurd Savonius of Finland patented in 1924–29. The wind pushes harder on the hollow of one vane than on the back of the other, and air flowing through the overlap from one hollow into the other helps it along. It turns in wind from any direction without steering, starts itself in the lightest breeze, and gives strong torque at low speed — for pumping water, ventilating, or driving a current meter. It is also slow and inefficient, because it is pushed by drag. This rung works out the drag machine's ceiling and builds a Savonius rotor from a PVC pipe, measuring its torque with a string brake.