The Doblhoff WNF 342 prototypes were the world's first jet-driven helicopters built by the Wiener Neustadter Flugzeugwerke (WNF) in the suburbs of Vienna, four machines being built representing progressive experimental steps in a research programme instituted in October 1942. The operating principle was to use a conventional piston engine driving a compressor to provide a compressed air supply, which, after mixing with fuel, was fed as a combustible mixture up through the rotor hub and out through the three hollow rotor blades to be burnt in tip-mounted combustion chambers, thereby generating thrust. Each of the first three machines (V1, V2, and V3) was provided with only a small rear propeller to blow air at the tail surfaces for steering, but the last machine (V4) had a second propeller mounted co-axially to provide thrust for forward flight when clutched to the compressor motor. Thus, by gradual development, the rotor jets (which had a high fuel consumption) were only used for take-off, hovering and landing, and the rotor blades turned by autorotation for forward flight in the manner of an autogyro.

In order, V1 thru V4: http://rareaircraf1.greyfalcon.us/picture/aus3.jpg http://rareaircraf1.greyfalcon.us/picture/aus4.jpg http://rareaircraf1.greyfalcon.us/dh1.jpg http://rareaircraf1.greyfalcon.us/dh2.jpg