Vertical axis wind turbines or VAWTs have some advantages compared to the better known horizontal axis turbines or HAWTs. VAWTs based on the Darrieus rotor principle are more efficient and economical than the Savonius type, but fixed pitch Darrieus VAWTs lack starting torque. This can be overcome with variable pitch blades, but active pitch control is complex and impracticable for stand-alone, non-electrical applications. Passive or self-acting variable pitch allows blades to pitch under the combined action of aerodynamic and inertial forces such that that a favourable blade angle of attack...