German Helicopter development.

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German Helicopter development.

Berichtdoor Tandorini » 17 feb 2009, 19:43


Doblhoff WNF-342 Unit: unknown Serial: WN342

Experience with the first two machines showed that the high fuel consumption of the tip-jets would make the WNF-342's operating costs prohibitive, and so a major design change was introduced in the V3 and V4 prototypes. The tip-drive system was retained for take-off, hovering and landing only, a selective clutch enabling the engine (now a 140hp Siemens-Halske Sh.14A radial) to drive a conventional pusher propeller for forward flight while the rotor blades 'free-wheeled' in autorotative pitch. To clear this propeller the rotor pylon was raised above the cabin and the tail unit was redesigned as a twin-boom assembly, that of the V3 carrying two end-plate oval fins and rudders while that of the V4 had a single fin and rudder mounted on the tailplane centre. Gross weight of the V3, a single-seater with 9.88m diameter rotors, was 548kg. The V4 had side-by-side open cockpits for a crew of two. The V3 was destroyed early in its test programme by ground resonance vibration, but the V4 had completed 25 hours of testin g before, in April 1945, it was hastily taken westwards to prevent its capture by the advancing Soviet forces. It eventually fell into American hands, Doblhoff later accompanying it back to the United States to assist with further tests before joining McDonnell to work on development of the XV-1. Stepan, who had done most of the test flying on the WNF-342, joined Fairey in the United Kingdom after the war, while Laufer went to work for the SNCA du Sud-Ouest in France.


Fl.282V-21 (Fl.282B) Unit: unknown Serial: CI+TU
One of the 24 helicopters (probably 21st), during evaluation tests in 1943.


Fa.223E-0 Unit: Luft-Transportstaffel 40 Serial: GW+PA (W.Nr.22300052)

May 1945. On the direct order of Hitler, this machine was used for a series of trial flights in East Prussia shortly before the end of the war. Probably owing to the semi-operational nature of these flights, the machine is finished in what appears to be overall RLM76 or RLM65 with a dense mottle of green (RLM81 or RLM83). Crosses and swastikas are outline type in white, codes are black. The Werk Nummer, apparently abbreviated to 300051, was painted in white across the top of the fin just above the swastika, the last three digits being twice the size of the others.

In 1924, Dr. Heinrich Focke and George Wulf formed a new aircraft firm in Germany. One of the nation’s most brilliant aeronautical engineers, Focke possessed a doctorate in engineering, and Wulf was a veteran test pilot. By 1931 the autogyro had become a proven vehicle. Professor Focke, noted for forward thinking and a willingness to explore unorthodox avenues of flight, recognized the inestimable value of operating an aircraft into and out of a small area. As a result of this vision, Focke-Wulf purchased a manufacturing license from the Cierva Autogyro Company. Under that license Focke-Wulf built more than thirty Cierva C-19 and C-30 autogyros.

In 1935, under Adolph Hitler’s accelerating rearmament program, the German Air Ministry issued a requirement for a utility/ liaison aircraft. Focke-Wulf decided to enter the competition with an autogyro. The company built and flight tested the FW-186 autogyro, which was similar to—but more streamlined than—the Cierva autogyros. A competitor, the Fieseler Storch (Stork), won the competition, ending further development of the FW-186.

In the early 1920s, Focke concluded that all previously built helicopters were just the result of trial-and-error experimentation by inventors with little or no systematic scientific thought. To prevent random errors in his developmental venture, Focke initiated a detailed research program before any design or prototype construction began. Focke defined his design objectives to eliminate several major limitations of previous helicopters, and his conclusions are still relevant to contemporary helicopter production. He outlined a plan to study the theoretical aspects of rotary-wing flight and to develop test models to verify his concepts. Only after intense testing would construction of a full-size helicopter begin (Ross 1953, 63–64).

Focke realized that autogyro technology was the starting point for his endeavor. Unfortunately, Focke-Wulf ’s license from de la Cierva granted rights only to manufacturing, not to Cierva’s theoretical data. However, the British company Avro produced the Mk I liaison autogyro, also under license from de la Cierva, and the British government published the technical report of Glauert and C. H. N. Lock, two of the British Aeronautical Research Committee’s most preeminent engineers, who had just completed a critical study of the autogyro. The British data became the basis for Focke’s helicopter engineering team.

To confirm their analytical calculations, Focke and Gerd Achgelis, another leading aircraft engineer, directed the construction of a three-bladed rotor model, powered by a 3-horsepower electric motor. They tested the model under both powered and autorotational conditions. They also evaluated flight characteristics both in and out of ground effect. When a rotor system operates in ground effect, roughly half the diameter of the rotor disk above the ground, the rotor blades become more effective. The whirling blades pull air down, creating a “cushion” of air below the helicopter, thus requiring less power to hover. Few previous experimental helicopters had produced the power to rise out of ground effect, so little was known about this aerodynamic phenomenon.

Before the construction of a full-size helicopter began, the Focke- Wulf team built and flight-tested another scale model that weighed almost 11 pounds. A 0.7-horsepower, two-cylinder gasoline engine powered the rotor system. In November 1934 the model attained an altitude of 59 feet, which was then equal to the world’s record for full-scale manned helicopters (Ross 1953, 92–95).

Following the scale model’s successful flights, Focke-Wulf constructed and ground-tested a full-size rotor system and gearboxes. During these tests engineers measured the rotor’s lifting capacity by hanging weights below the rotors. The Germans also ascertained the reliability of the gearboxes by conducting a fifty-hour test run, and then tearing down the boxes to determine wear and fatigue. With satisfactory results, construction began on a full-size aircraft.

For his full-size helicopter Focke selected the fuselage and engine of a training biplane already produced by the company. The FW Stieglitz was a two-cockpit aircraft with a fuselage of welded steel covered by fabric. The helicopter retained the conventional-looking vertical tail and rudder, but Focke’s engineers replaced the wings and front cockpit with two three-bladed plywood rotors mounted on tubular steel outriggers. The outriggers also included a swashplate assembly, and the blades were attached to the hub with flapping and lead/lag hinges. The counter-rotating rotors eliminated torque effects and allowed the rotor system to operate without the fuselage affecting air flow through the rotors. Separating the rotors also eliminated interference from vortices created by the rotors themselves. Inclining the rotors slightly inward also increased lateral stability of the newly designated Fa-61.

A single 160-horsepower radial engine drove both rotors through a BMW transmission and a series of complicated shafts and gearboxes. A shortened propeller remained to provide cooling air for the engine during hover and slow flight. Unlike an autogyro, the propeller provided negligible forward thrust to the aircraft.

Although the Fa-61’s configuration created the appearance of an autogyro with two rotors instead of one, the aircraft was a true helicopter. A robust control system allowed the pilot to tilt the rotor disks forward and backward simultaneously, causing the aircraft to move in the desired direction. By moving the cyclic stick sideways, the pilot increased the angle of the blades on one rotor and reduced the pitch on the other, thus controlling the roll of the craft. The rudder pedals tilted the rotors in opposite directions to control yaw. In forward flight the conventional tail added both longitudinal and yaw stabilization. Unlike most previous helicopters, the Fa-61 exhibited excellent stability and was capable of hovering, ascending, and descending vertically, as well as flying forward and backward.

After several tethered tests, Ewald Rohlfs piloted the Fa-61 on its first free flight on June 26, 1936. The first flight lasted less than a minute. After the initial test flight, Focke restricted the Fa-61 to only a few more indecisive flights of short duration. He then suspended the test flights in order to make several small improvements to the prototype.

On May 10, 1937, Rohlfs flew the Fa-61 on a record-breaking flight. He reached an altitude of 1,130 feet (344 meters). After setting a new altitude record he reduced the engine power to idle, entering autorotation. Although the Fa-61’s control system did not provide full collective pitch control, the helicopter’s freewheeling rotors allowed pilot and machine to descend safely to the ground, demonstrating conclusively that a helicopter would not inevitably crash if its engine failed.

During the next several weeks Rohlfs broke every previous helicopter record with the Fa-61. One flight extended to one hour and twenty minutes; on another the helicopter reached a speed of 76 mph (122 kph); still another extended the record to a distance of 143 miles (230 kilometers). The Fa-61 ultimately set an altitude record of 11,243 feet (3,427 meters).

The success of the Fa-61 heightened the Luftwaffe’s interest in Focke-Wulf ’s unusual aircraft. In September 1937, Karl Francke, chief test pilot of the Reichlin experimental center, and Germany’s famed female pilot, the twenty-five-year-old Flugkapitan Hanna Reitsch, received orders to conduct several flights in Focke’s prototype. In subsequent flights, Reitsch broke several of Rohlfs’s previous records. With the introduction of the German Focke-Wulf Fa-61, the first practical helicopter became a reality, and vertical flight was no longer a dream.

Adolph Hitler’s government immediately recognized the propaganda value of having its famous female pilot fly such an advanced aircraft in public. In February 1938, Reitsch flew the Fa-61 for fourteen consecutive nights inside the huge, enclosed Deutschlandhalle sports stadium in Berlin. Although Reitsch had less than three hours flying experience with the aircraft, she adeptly demonstrated the helicopter in a confined area before large crowds, proving the exceptional maneuverability of the Fa-61.

Focke’s helicopter so impressed the Luftwaffe command that it awarded his company a contract for an enlarged version of the aircraft that could lift 1,500 pounds (680 kilograms) of cargo. The new helicopter, designated the Fa-223 Drache (Dragon), made its first flight in the spring of 1940. A 1,000-horsepower engine drove two triple-bladed 39-foot (12-meter) rotors. The Fa-223 measured slightly more than 80 feet (24 meters) wide and 40 feet (12 meters) long and, unlike the Fa-61’s open cockpit, sported a four-seat enclosed passenger cabin. Eight additional seats could be installed outside to carry passengers at low speeds. The company built seventeen prototypes and one production version before the Allies bombed the assembly line, and Focke fell out of favor with the Nazi government, effectively ending Focke-Wulf ’s helicopter production.

The Fa-223’s performance records included a top speed of 115 mph (185 kph) and an altitude of 23,400 feet (7,132 meters). At low speeds and altitudes the aircraft had the capability to transport loads weighing a full ton slung under the airframe with cables. The Drache operated in a coastal reconnaissance role and carried limited amounts of cargo during World War II. Several Fa-223s survived the war, and one became the first helicopter to cross the English Channel.

Another helicopter, a small, single pilot craft manufactured by Anton Flettner, the Fl-282 Kolibri (Hummingbird), also participated in limited combat operations with the German military during World War II. Flettner’s rotor system was also a side-by-side design, but his 1939 prototype differed significantly from the Fa-223. Flettner’s design, known as a synchropter, placed two shafts in close proximity but angled outward at a significant angle. A synchronizing gear system ensured the exact phasing, or intermeshing, of the overlapping, counter-rotating rotor blades.

In October 1942 two Fl-282 helicopters, accompanied by Kaptain Claus von Vinterfeldt, Flettner’s pilot (a man named Fuisting), and another unknown pilot, along with three technicians, arrived in Trieste. The unarmed Fl-282 achieved a maximum speed of 93 mph (150 kph) and a range of 186 miles (300 kilometers). From November 1942 until January/February 1943, one Fl-282 flew reconnaissance missions over the Aegean Sea from the improvised helicopter carrier, Kriegsmarine’s minelayer Drach 50. Records indicated that the Fl-282 was the first operational shipboard helicopter. The second Fl-232 remained ashore in reserve.

Although the German Fa-61 was the first practical helicopter, and the Fa-223 the largest helicopter introduced into limited production during World War II, the most important helicopter advancements took place not in Europe but in the United States. The German concept of counter-rotating rotors mounted side by side did not prove very effective, but the design did, nonetheless, solve the instability of previous helicopters and prompted other inventors to pursue vertical lift technology.


just prior to World War II, France nationalized its aviation industry, resulting in the formation of the Societe Nationale de Constructions Aeronautiques Sud-Est (SNCASE) and the Societe Nationale de Constructions Aeronautiques Sud-Ouest (SNCASO). After the war, both companies, subsidized by the French government, initiated a myriad of projects to develop rotorcraft to meet a wide variety of requirements. In early 1947, Sud-Est began flying a rotorcraft, albeit of antiquated design. The SO-3000, a twin-rotor six-seat model, was copied directly from the German Fa 223. As in the Fa 223, cantilever outriggers extended outward and upward from each side of the welded steel-tube fuselage, supporting the two three-bladed rotors. A German Bramo Fafnir BMW 323 R2 1,000-horsepower radial engine powered the French machine. The two-man crew sat in a Plexiglas-covered cockpit. A cabin behind the pilot’s position accommodated four passengers, and a small hoist provided power for lifting cargo and rescue work, but the SO 3000’s lack of success generated little interest and fewer orders.

The previous year Sud-Ouest had introduced its first helicopter, the SO 1100 Ariel, also derived from experiments conducted by German aeronautical engineers during World War II. A two-seat, allmetal aircraft, the Ariel featured small tip jets powering the three-bladed main rotor, but again buyers did not appear.
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