Friedrich von Doblhoff
Baron Friedrich von Doblhoff was an Austrian helicopter pioneer.
Shortly after WW2 began, the German navy held a competition to design and build a rotary-wing aircraft to be used on small ships and submarines for surveillance. Friedrich von Doblhoff proposed a helicopter with tip-jet driven rotor, and was authorised to start work with a team of engineers which included Dipl.-Ing. Theodor Laufer and Dipl.-Ing. August Stepan. Support came from Prof. Henrich Focke.
Doblhoff rotor blades were driven round by the reaction of 'hot' jets using a fuel/air mixture fed through the rotary blades using an engine-driven compressor. Tip jets avoided the need for gearing to drive the rotor, and avoided torque reaction which would otherwise have to be counteracted by a tail propellor or by contra-rotating blades. The drawback was high fuel consumption, and this restricted the jets to take-off and landing, while in normal flight the engine would drive a pusher propeller while the rotor 'windmilled'.
The Doblhoff WNF-342 was the first helicopter in the world to take off by the use of jets, but the end of the war curtailed development by Doblhoff. The WNF-342 V4 was captured by the Allies and taken to the U.S.A.
The above information is drawn mainly from the website of the Hubschraubermuseum in Bückeburg, Germany.
The jet tip concept was taken up with mixed success by a number of firms. The successful Djinn Helicopter, made in France used compressed air only (no fuel) for the tip jets. The large Fairey Rotodyne showed promise, but did not get beyond the prototype stage.