The FW-61, Henrich Focke’s Masterpiece

More than 70 years ago, on June 26th, 1936, the FW-61 developed by Professor Henrich Focke and his team started its maiden flight in Bremen, Germany, see Fig.1. It is well accepted that this event was the start of a new area in helicopter development. The first practical helicopter was born and only one year later this machine won all the official world records for helicopters. This great success was not received as a fortunate coincidence. A lot of work had been done prior to the final design and construction of this helicopter. As Focke has pointed out himself a lot of scientific work was necessary to understand the complicated aerodynamic environment of helicopter rotors in both hover and forward flight conditions. Stability and control issues were of crucial importance to successfully operate the helicopter in any circumstances. The later show-flights of the FW-61 inside the “Deutschland Halle” in Berlin piloted by Hanna Reitsch show impressively that stability goals had been fully achieved. At the beginning of Focke’s studies on helicopters, in the early 1930’s, a considerable knowledge base was already available and has intensively been studied. At this point the existing constructions of the autogiros developed by Juan de la Cierva, a Spanish engineer who had a company in England, have to be highlighted in particular. The Focke-Wulf company has acquired the license to build both C19 and C30 autogiros. These machines have been flown intensively and their flight qualities were studied. Focke assumed the autogiros as interim solutions which were not able to start and land vertically, stand still in the air or move up and down vertically. He was ready to close the gap to a real helicopter. Beside the already existing flight vehicles corresponding theoretical approaches were available to calculate aerodynamic forces on rotating blades. The Glauert-Lock-Wheatly theory was developed specifically for autogiros and Focke and his co-workers extended the theory to be applicable for helicopters as well. Furthermore the wind tunnel was a tool extensively used for research. The main objective of the present paper will therefore focus on the scientific background of Focke’s work.