and diameter of the rotor and is the wind speed. However, in practice the extractable power is about half that (one can argue that only one half of the rotor — the scoop co-moving with the wind — works at each instant of time) and depends also on the efficiency of the given rotor. Thus, for the theoretical ideal rotor, one gets , but the average maximum efficiency of the Savonius wind turbine is around 20% (), making the real extractable power of the typical Savonius . The angular frequency of a rotor is given by , where is the radius and is a dimensionless factor called the tip-speed ratio. λ is a characteristic of each specific windmill, and for a Savonius rotor λ is typically around unity. For example, an oil-barrel sized Savonius rotor with h= and r= under a wind of v=, will generate a maximum power of and a maximum angular speed of (190 revolutions per minute). Use Savonius turbines are used whenever cost or reliability is much more important than efficiency. Most anemometers are Savonius turbines for this reason, as efficiency is irrelevant to the application of measuring wind speed. Much larger Savonius turbines have been used to generate electric power on deep-water buoys, which need small amounts of power and get very little maintenance. Design is simplified because, unlike with horizontal axis wind turbines (HAWTs), no pointing mechanism is required to allow for shifting wind direction and the turbine is self-starting. Savonius and other vertical-axis machines are suited to pumping water and other high torque, low rpm applications, and are not usually connected to electric power grids. In the early 1980s, Risto Joutsiniemi developed a helical rotor (wiki:fi) version that does not require end plates, has a smoother torque profile and is self-starting in the same manner as is a crossed pair of straight rotors. The most ubiquitous application of the Savonius wind turbine is the Flettner ventilator, which is commonly seen on the roofs of vans and buses and is used as a cooling device. This rotor was developed for ventilation by the German aircraft engineer Anton Flettner in the 1920s. It uses the Savonius wind turbine to drive an extractor fan. The vents are still manufactured in the UK by Flettner Ventilator Limited. Specifically constructed Savonius wind turbines have been used to provide power to autonomous neutrino detector stations of the ARIANNA experiment