Apr 062012
 

The photos above depict the Doblhoff Wnf 342 V4, an experimental German helicopter which employed tip jets to drive the rotor. It was built by a group of Austrian engineers at the Wiener Neustädter Flugzeugwerke headed by Friedrich von Doblhoff and was originally designed to fulfill a German Navy requirement for an observation platform to be used from small ships and submarines. Unlike the first three prototypes of the Wnf 342, the V4 had a clutched propeller for forward thrust and could fly in autorotation like a gyrocopter. It accumulated 25 flight hours in Austria before being captured by US forces and shipped to the US on July 19, 1945 under Operation Lusty on the HMS Reaper. It eventually arrived at Wright Field along with Doblhoff, its creator.

On September 9, 1946, the General Electric Company (specifically, the Thermal Power Systems Division of its General Engineering and Consulting Laboratory) was permitted by the USAAF to evaluate the Wnf 342 V4 in connection with that company’s work on the power plant of the XR-17 helicopter, which would later become the Hughes XH-17. It was shipped by a Fairchild C-82 to Schenectady, New York on December 6, 1946, where it was tested and studied by General Electric under Army Air Force Contract No. W-33-038-AC-16283. This article presents their evaluation report dating from April 30, 1948; the following paragraphs reproduce the introduction to the document.

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The object of this study was primarily to evaluate the outstanding engineering contributions of this aircraft and to make recommendations for future developments. Particular emphasis was placed on the novel power cycle employed in the helicopter, its thermodynamics, economics and its mechanical embodiment. Results of this investigation indicated that, based on contemporary standards, the ship had marginal performance; with the piston engine rated at 135 hp, only 73 air horsepower reached the tip burners. Additional combustion boosted it to 96 rhp; the rotor lifted a gross weight of 1400 pounds. Specific fuel consumptions under maximum lift conditions were: 0.62 for the engine and 3.13 for the tip burners; a total of 3.75 pounds per hp-hr, based on the rotor horsepower. These figures were further qualified in the thermodynamic study presented tn the second half of the report.

However, the analysis revealed that the poor showing of the jet reaction drive of this machine, compared to the conventional drive, was not inherent in the power cycle itself. It could be traced to a number of engineering compromises, obviously forced upon the designers by war handicaps, which introduced disproportionately large losses throughout the cycle.

Both theoretical analysis and practical considerations based on the study of the Doblhoff machine clearly indicated a number of outstanding advantages in favor of the jet reaction drive, especially if coupled with a gas turbine prime mover. While these advantages may seem to be insignificant in the 130 hp class, they gained serious prominence at and beyond the 500 hp class.

To read the report in depth, please click through the gallery above. Igor Bensen, one of the primary authors of the report, would go on to perform many test flights of the Wnf 342. Unfortunately, he suffered severe spinal injuries when the helicopter was destroyed in a ground resonance accident; these injuries would plague him throughout his life. However, they did not stop him from playing a significant role in the development of the Hughes XH-17 and eventually founding his own company, Bensen Aircraft, which produced a successful line of gyrogliders and autogyros over the course of several decades. Doblhoff was also involved with the XH-17 program, as well as the McDonnell XV-1 convertiplane, among others.

All images from NARA Archives II, College Park, MD, RG 111 & 341

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