Why are the wind propellers of wind generators always in the vertical position and never horizontal? It would seem they could be stacked horizontally, could be enclosed easily and would be more aesthetic for building tops. — Bill V., San Clemente, CA.Bill, Horizontal “axis” wind turbines (vertical blades) are the traditional conventional design. They consist of a rotor with one to twenty blades driving a generator or a pump either directly or through a gearbox, chain or belt system. A tail vane or fantail is required to direct the machine. These turbines are usually more efficient than vertical-axis units. Savonius and Darius are two designs of vertical-axis machines, and these types of units do not have to be directed into the wind. The Savonius windmill was the brainchild of Sigrid Savonius of Finland, racecar driver of the 1930s. The design produces a low-speed high-torque unit that can be used for pumping water through a gearing mechanism, generating electricity — and the design also has the advantage of an aerodynamic effect called the “Magnus principle,” whereby suction is formed by the air moving over the convex face of the rotor. The Darius windmill was named after its French inventor. It is also known as “catenary” because of its profile when operating. Darius units, also known as “egg beaters,” will often not start to turn by themselves and need either an electric start or use a small Savonius unit attached to the top. As the blades revolve they lose some energy as they head into the wind, reducing the output. Some newer versions are coming on the market that can “self start” but they are not as widely commercialized. Southwest Windpower (AZ) and Bergey Windpower (OK) have tens of thousands of horizontal-axis wind turbines globally, and you can be sure they would have adopted vertical-axis designs if they were more viable. — Scott Scott Sklar is President of The Stella Group in Washington, DC, a distributed energy marketing and policy firm. Scott, co-author of “A Consumer Guide to Solar Energy,” uses solar technologies for heating and power at his home in Virginia.