October 2016

OctoberDoblhoff Wienerer-Neustädter Flugzeugwerke WNF 342

The world’s first tip jet powered helicopter, the WNF 342 V1 was flying in the spring of 1943. It was designed to meet a German Navy requirement for an observation helicopter to be carried by submarines and small naval vessels. It featured a frame of uncovered metal tubing with a small twin finned vertical tail and tricycle landing gear. An Argus As 411 supercharger was adapted as a compressor to provide air to the rotors, an arrangement that would then be employed on all of Friedrich von Doblhoff’s prototypes. The V1, slightly damaged in an allied bombing raid on August 13, 1943, was soon followed by the WNF 342 V2, which added a sail-like rear fuselage faring with a single fin and an upgraded 90 hp Walter Mikron engine. It was constructed in Obergraffendorf, where the WNF development program had been relocated after the bombing. Experience with the first two models convinced its inventor that the high fuel consumption of the tip-jets would make the WNF 342 prohibitively costly to operate, so the decision was made to power the rotors only on takeoff and landing. The rotor would be unloaded in flight and the craft would then fly as an autogyro. The resulting WNF 342 V3 was constructed with twin tail booms, each of which supported an oval shaped vertical fin and rudder with a horizontal stabilizer linking the booms. A BMW-Bramo Sh 14A 140 hp engine both provided forward thrust with a pusher propeller and the compressor for the jet-tip rotors. During forward flight power (air and fuel) was cut off from the rotor jet-tips as the engine was declutched from the compressor and power redirected to the propeller — lift was obtained from autorotation. The final model of V3 weighed 1208 lbs and had flapping and dragging rotor hinges — vertical control was achieved by varying the rotor speed. Unfortunately, the innovations incorporated in Doblhoff’s third model were not enough to ensure success and after only a few flights. Fortunately no one was injured as it literally destroyed itself from ground resonance vibration. An additional prototype was constructed before the war ended, the WNF 342 V4, the largest of Doblhoff’s prototypes, and in many ways, the most significant although not for any intended reasons. The V4 could carry a crew of two in side-by-side open cockpits and the fuselage was now faired. It retained the twin boom layout but the two verticals were replaced with a single vertical mounted on top of a horizontal tail that connected the booms. Heavier than its predecessors, the V4 weighed 1411 lbs and had a 32.68 ft. rotor, just slightly larger than the V3. It also innovatively used air pressure to control the collective pitch of the 32.42 feet diameter rotor blades - the blades could be pitched for helicopter-powered takeoff and landings, and then changed to allow for autogyro flight. Testing of the V4 began in the Spring of 1945, with 25 hours of flight time having been accumulated by early April, although it was not tested in forward flight over 25 – 30 mph. But it was too late – on April 7, 1945, Doblhoff and his colleagues could hear the artillery of the approaching Russian forces moved into Vienna, 18 miles to the east. After some discussion, the decision was made to load the WNF 342 V2 and V4 prototypes on a trailer and flee westward to the Americans and British. For almost twelve days the truck carrying the designers and mechanics, and towing the trailer, moved westward over roads often clogged with refugees and others doing likewise. Eventually Doblhoff and his colleagues surrendered to American forces at Zell am See and were quickly interrogated by engineering officers who recognized the importance of the prototype and its designers. The model was crated and shipped to the United States for evaluation – followed quickly by Doblhoff who eventually went to work for McDonnell Aircraft as Chief Helicopter Engineer and significantly contributed to development of the McDonnell XV-1 compound helicopter convertiplane. Of perhaps equal importance, August Stepan, who had done the structural design and most of the test flying of the prototypes, joined Fairey Aviation as Chief Tip-Jet Engineer and contributed to the design of the Fairey Gyrodyne and Rotodyne which employed the rotor tip-jet technology for takeoffs and landing but flew as an autogyro.

(Ty C. Connors, U.S. Navy photo; text by Bruce Charnov)

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