Following World War I, Flettner was named Managing Director of the Institute for Aero and Hydro Dynamics in Amsterdam.[1] He held that post until 1931.[4] In the 1920s, Flettner also invented the famous Flettner rotary ventilator, which was widely used on buses, vans, boats, railroad cars, campervans, and trucks to assist cooling without the use of energy.[2] Modern derivatives of his ventilator are still manufactured in Britain by Flettner Ventilator Limited.[2]