The Darrieus Rotor: Wings Round a Vertical Shaft

US 1,835,018, 'Turbine Having Its Rotating Shaft Transverse to the Flow of the Current', Georges Jean Marie Darrieus of Paris, assignor to the Société Anonyme pour l'Exploitation des Procédés Maurice Leblanc-Vickers. The blades have a *"stream line section analogous to that of the wings of birds"*, and the machine is built *"to offer a speed which much exceeds the speed of the current"*. Darrieus cites *"the modern theory of the Lanchester-Prandtl lifting wing"* — the blade is a wing, pulled by lift. The blades may be straight, or *"curved or twisted in the form of a skipping rope"* — the 'eggbeater' — *"in order to reduce or annul the bending stresses due to centrifugal force"*. This rung builds the straight-bladed form.

Print three straight blades 300 mm long of a symmetrical wing section with a 40 mm chord (a NACA 0018 profile prints well), with a hole through each near the quarter chord. Mount each on a pair of arms of balsa or printed PETG at a radius of 150 mm, blades vertical and set with their chord exactly tangent to the circle. Carry the shaft vertically in two skateboard bearings in a plywood frame, and couple a hobby motor to the bottom as a generator with a 330 Ω load.

Put the rotor in front of the box fan at its highest setting. It sits still or rocks. Flick it round hard by hand: if you get it fast enough it catches, accelerates and runs on its own. Measure its speed with the tachometer and the wind with the anemometer; the tip-speed ratio is (rpm × 2π ÷ 60 × 0.15) ÷ wind speed. Compare with the notebook. Now fix a small Savonius rotor (from the previous rung, half size) on the same shaft inside the circle of blades. Stop everything and start the fan: the Savonius starts the shaft turning, the Darrieus blades gather speed and take over. Hybrid rotors like this are built for exactly that reason.

**US 1,835,018, 'Turbine Having Its Rotating Shaft Transverse to the Flow of the Current', Georges Jean Marie Darrieus, Paris; filed 1 October 1926 (Serial 138,844), claiming a French filing of 9 October 1925; granted 8 December 1931.** The idea was little used until the 1970s, when vertical-axis turbines were revived for research and small power. **Honest limits.** It does not start itself. Its blades are loaded and unloaded twice every turn, which fatigues them and the shaft. Near the ground the wind is slow and turbulent, where a vertical-axis rotor usually sits. And a blade that leaves its arm at speed travels a long way: run the model behind a guard.