warm weather. Other post-World-War-I work Following World War I, Flettner was named Managing Director of the Institute for Aero and Hydro Dynamics in Amsterdam. He held that post until 1931. With assistance from German physicist Albert Betz, Swiss aeronautical engineer Jakob Ackeret, German engineer Ludwig Prandtl and German theoretical physicist Albert Einstein, Anton Flettner constructed an experimental rotor vessel, the Buckau, later renamed the Baden-Baden, in October 1924 at the Friedrich Krupp Germaniawerft. The Buckau was a schooner refitted with two rotating 50-foot cylinders and was the first vessel built with a propulsion system based on the Magnus effect. Flettner came upon the idea while at the beach with his wife. Flettner used sand, flowing over his rotating hand, to describe the Magnus effect and realized its potential for sail propulsion. The Baden-Baden crossed the Atlantic in 1926. It could outsail normal schooners under moderate to heavy winds, but was destroyed by a storm in 1931. A commercial rotor ship, the Barbara, was also built, and sailed to the United States. In 1926, Anton Flettner shifted his focus to aviation in founding his own company, the Anton Flettner Aircraft Corporation in Berlin, aiming at the application of the Flettner-rotor as a wing replacement on large wind turbines. In 1935, Flettner built a German night reconnaissance and anti-submarine autogyro called the Fl 184. Flettner followed this accomplishment by building the Flettner Fl 185 in 1936, an experimental German gyrodyne, which could fly as both a helicopter and as a gyroplane. In 1938 Flettner, together with Kurt Hohenemser, built the Flettner Fl 265. The Flettner Fl 265 as the likely pioneering example of a twin-lift rotor synchropter, addressed the problem of torque compensation as the first helicopter with intermeshing rotors (Flettner double rotor). World War II During World War II, Anton Flettner headed Flettner Flugzeubau GmbH, which specialized in reconnaissance helicopters for the German Luftwaffe (Air Force). Flettner's helicopter inventions were financed from wealth acquired from his ventilator business, a company that was enhanced by the skill of his wife, Lydia Freudenberg Flettner. Anton Flettner built his helicopters for the German military, primarily for navy spotter use. Although his wife was Jewish, he held a personal relationship with the head of gestapo, Heinrich Himmler. Himmler had Flettner's wife and family escorted safely to Sweden for the duration of the war. Anton Flettner's partner and confidant was Dr. Kurt Hohenemser, a brilliant and