A vertical axis wind turbine with a symmetric airfoil having a concavely curved tail portion defining a trailing edge angle of between 0° and 5° on either side of the chord, a maximum thickness center point between 26% and 34% of the length of the chord from the leading edge, a leading edge curvature ratio (D1/D2) from 0.42 to 0.50, and a ratio of maximum thickness to chord length T/C from 0.12 to 0.19, preferably 0.16. Advantageously, the rotor may have helical blades defining a cylindrical swept envelope with a positive angle of attack and a solidity ratio NC/D from 0.125 to 0.270, and is governed at a tip speed ratio from 2.25 to 4.00.