United States Patent 4427343 is a Efficient wind turbine design for low velocity air flow.
Abstract: Six rows of radial blades, arranged to extend in a spiral (in the direction of axial rotation) covering 55 degrees of arc about a hollow support section, constitute an optimum blade arrangement for maximum efficiency in low velocity airflows. Each blade in the rows is contoured to receive both direct flow pressure as well as airfoil lift in order to provide maximum energy transfers from low velocity airflows.
