Defense Media Network

Newest Defense Media Network Promotion

World War II German Helicopters – Doblhoff WNF 342

Part 4

In 1941, Friedrich von Doblhoff (1916-2000) a junior engineer at Wiener Neustädter Flugzeugwerke (Vienna-Neustadt Aircraft Factory, which assembled Bf 109 fighters,) built a static test rig on his own time from scrounged materials in order to prove his radical helicopter concept. He would eliminate the complex gearbox by spinning the rotor blades with tip-mounted jets. Factory compressed air was mixed with aviation gasoline and piped to rotor-tip combustion chambers fitted with car spark plugs.

Luftwaffe officials were invited to witness a demonstration. With a roar and a burst of flame, the rotors whirled, and the test rig lifted off the floor.  An anvil was attached to hold it down. It lifted the anvil, tilted over and crashed spectacularly. The officials quickly gave von Doblhoff a development contract.

With a roar and a burst of flame, the rotors whirled, and the test rig lifted off the floor.  An anvil was attached to hold it down. It lifted the anvil, tilted over and crashed spectacularly. The officials quickly gave von Doblhoff a development contract.

Von Doblhoff’s small team built four prototypes, with a first flight in 1943. The German Navy hoped to use the craft as a light observation and anti-submarine platform (the same role as Flettner’s Fl 282.)  A second airframe was destroyed in a crash, and Allied bombing of the factory complex forced the team to relocate. But development work, with increasingly powerful engines, continued until the war’s end.

Von Doblhoff’s WNF 342 was originally designed as a single seat aircraft with a tubular steel frame and a fabric covered tail section. The fourth prototype added a second seat for an observer in a partially enclosed cockpit. The engine drove a compressor that fed a fuel-air mixture to the rotor hub, then through three hollow blades to tip-mounted jets.  A pusher propeller behind the engine provided airflow, allowing a large rudder to control the direction of flight.

 

Doblhoff WNF 342 (V4)

The torque problem was largely eliminated, since the rotor disc spun itself, imparting very little twist to the fuselage. The design team experimented with various tail configurations, including twin oval tail fins, before settling on a conventional single rudder.

It was found that the enormous centrifugal force at the rotor tips interfered with proper fuel-air mixing. A special heat-resistant alloy had to be found from which to fabricate the tip jets. Fuel consumption was so high that von Doblhoff decided to use the jets only for takeoff and landing.  In flight WNF 342 relied on its pusher propeller, and the rotor spun freely, as with Cierva’s autogyro.

By 1945 the fourth prototype had completed 25 hours of flight testing.  It was captured by the Americans near Salzburg, Austria and shipped to the United States for evaluation, along with von Doblhoff himself. He enjoyed a successful postwar career in the aviation industry, helping to develop the radical experimental McDonnell XV-1, (1954) an ancestor of today’s successful V-22 Osprey.

  • Crew: 2
  • Length: 5.1 m (16 ft. 7.5 in)
  • Height: 2.4 m (7 ft. 10.5 in.)
  • Main rotor diameter: 10.00 m (32 ft 9¾ in)
  • Main rotor area: 78.54 m2 (845.42 ft2)
  • Empty weight: 430 kg (948 lb)
  • Gross weight: 640 kg (1411 lb)
  • Powerplant: 1 × BMW-Bramo Sh.14A radial piston engine, 104 kW (140 hp)
  • Maximum speed: 48 km/h (30 mph)

Conclusions

A generation of brilliant engineers, trained under the German Empire and the Weimar Republic, provided the Third Reich with a stream of innovative and creative aircraft designs, including helicopters of several types.

The lack of any long-term strategy, and a divisive command structure led to constant changes in operational requirements.  The Reich Air Ministry micro-managed design specifications. Development projects that had little hope of producing workable systems were maintained largely to provide draft deferments for young engineers, scientists and technicians. The proliferation of prototypes and variant models, as firms competed for contracts, led to duplication of effort.  Germany failed to mobilize its industrial capacity until too late in the war:  running factories on a single 8-hour shift, and (for ideological reasons) refusing to employ large numbers of women as workers.

A captured Focke-Wulf Fa 223 Drache undergoes flight testing after the war. San Diego Air & Space Museum photo

A captured Focke-Achgelis Fa 223 Drache undergoes flight testing after the war. San Diego Air & Space Museum photo

Irrational leadership, and the increasingly bizarre politics of the regime, doomed German aircraft production, just as it undermined every other aspect of the war effort.  As the war dragged on toward defeat, the regime increasingly believed its own propaganda about Wunderwaffen (“Wonder Weapons”) that would restore hopes for victory.  Compared with jet fighters like the Me 262 and rockets like the V-2, the helicopter was a minor footnote in the aviation history of the Third Reich, but the rotary wing technologies explored by German engineers would have significant impact on American, Soviet, British, and French helicopter designs in the postwar world.