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The DFS 346 (Samolyot 346) was a Germanmarker rocket-powered, high-speed research aircraft of World War II. It was designed by Felix Kracht at the Deutsche Forschungsanstalt für Segelflug (DFS), the "German Institute for Sailplane Flight". The prototype was still unfinished by the end of the war and was taken to the Soviet Unionmarker where it was rebuilt, tested and flown.

Concept

Concept art
Test pilot Rolf Mödel tries out the prone position
The DFS 346 was a parallel project to the DFS 228 high-altitude reconnaissance aircraft. While the DFS 228 was essentially of conventional sailplane design, the DFS 346 had highly-swept wings and a highly streamlined fuselage that its designers hoped would enable it to break the sound barrier. Like its stablemate, it also featured a self-contained escape module for the pilot, a feature originally designed for the DFS 54 prior to the war. The pilot was to fly the machine from a prone position, a feature decided from experience with the first DFS 228 prototype, mainly because of better g-force handling and aim of reducing the fuselage cross-section as much as possible.

The 346 design was intended to be air-launched from the back of a large aircraft, the baseline being the Dornier Do 217. After launch from the bomber the plane's Walter 509B/C engine, which consisted of two superimposed combustion chambers, one above the other, with the lower unit of 400 kg (880 lb) thrust for cruising and the upper unit used only for takeoff, in a so-called "sharp start" directly from the ground, or for short periods when the maximum thrust was necessary, would accelerate the craft to proposed speed of Mach 2.6 and altitude of 30,500 meters, at which point the engine would turn off. In an operational use the plane would then glide over Englandmarker for a photo-reconnaissance run, descending as it flew but still at a high speed. After the run was complete the engine would be briefly turned on again, to raise the altitude for a long low-speed glide back to a base in Germanymarker or northern Francemarker.

Design

The DFS-346 was a midwing design of all-metal construction. The front fuselage of the 346 was a rotation body based on the NACA-Profile 0,0121-0,66-50. The middle part was cylindrical and narrowed to the square in the back. Probably for capacity and weight reasons the DFS-346 was equipped with landing skids, both in the original German design and in the later Soviet prototypes; this caused trouble several times. The wings had a 45° swept NACA 0,012-0,55-1,25 profile of 12% thickness. The continuously varying profile shape caused a stall in certain flight conditions, which caused complete loss of control. This was later corrected by use of fences on the top of the wings.

Development

First model in Siebel windtunel

World War II

Felix Kracht and his team at DFS began selecting a suitable airframe design in the late World war II period. Since the aircraft was to be of all-metal construction, the DFS lacked the facilities to build it and construction of the prototype was assigned to Siebel Werke located in Hallemarker, where the first windtunnel models and partially built prototype were captured by the advancing Red Army.

Post-War

On 22 October 1946, the Soviet OKB-2 (Design Bureau 2), under the direction of Hans Rössing and Alexandr Bereznyak, was tasked with continuing its development.
Escape capsule prepared for droptests
The captured DFS 346, now simply called "Samolyot 346" ("Samolyot" - Aircraft) to distance it from its German origins, was completed and tested in TsAGI wind tunnel T-101. Tests revealed some aerodynamic deficiencies which would result in unrecoverable stall at certain angles of attack. This phenomenon involved a loss of longitudinal stability of the airframe. After the wind tunnel tests, two wing fences were installed on a more advanced, longer version of the DFS-346, to correct the airstream separation. This solution was used on the majority of the Soviet planes with sweptback wings of the 1950s and 1960s. In the meantime, the escape capsule system was tested from a B-25J and proved promising. Despite results from studies showing that the plane would not have been able to pass even Mach 1, it was ordered to proceed with construction and further testing.

Operational use

In 1947, an entirely new 346 prototype was constructed, incorporating refinements suggested by the tests. This was designated 346-P ("P" for planer - "glider"). No provision was made for a powerplant, but ballast was added to simulate the weight of an engine and fuel. This was carried to altitude by a B-29 Superfortress captured in Vladivostokmarker and successfully flown by Wolfgang Zeise in a series of tests. This led to the construction of three more prototypes, intended to lead to powered flight of the type.

First accidents

Engine test on ground
Minor landing difficulties resulted with landing on the snow without landing gear
Newly built 346-1 incorporated minor aerodynamic refinements over the 346-P, and was first flown by Zeise on September 30, 1948, with dummy engines installed. The glider was released at an altitude of 9700 m, and the pilot realised that he hardly could maintain control of the aircraft. Consequently, while attempting to land, he descended too fast (his speed was later estimated at 310 km/h). After first touching the ground he bounced up to a height of 3-4 m and flew 700-800 m. At the second descent, the landing ski collapsed and the fuselage hit the ground hard. The pilot seat structure and safety-belt proved to be very unreliable, because at the end of a rough braking course Zeise was thrown forward and struck the canopy with his head, losing consciousness. Luckily, he wasn't seriously injured, and after treatment in hospital he was able to return to flying. Accident investigation research team came to the conclusion that the crash was a result of pilot error, who failed to fully release the landing skid. This accident showed that the aircraft handling was still very unpredictable, as a result, all rocket-powered flights were postponed until pilots were able to effectively control the aircraft in unpowered descent, requiring further glide flights. The damaged 346-1 was later repaired and modified to 346-2 version. It was successfully flown by test pilot P.Kazmin in 1950-1951 winter, but nonetheless these flights also ended "on fuselage". Furthermore, after the last flight of these series, the airframe again required major repairs. On 10 May in 1951, Zeise returned to the program, flying final unpowered test flights with the 346-2, and from 6 June, unpowered tests of the 346-3 without accidents.

Final flights

By the mid-1951 346-3 was completed, and Zeise flew it under power for the first time on 13 August 1951, using only one of the plane engines. Continuing concerns about the aircraft controlling at high speeds had led to a limitation of Mach 0.9 being placed on test flights. Zeise flew it again on 2 September and 14 September. On these last flight, however, things went drastically wrong. Separating from the carrier plane at 9,300 meters (30,500 ft) above Lukovici airfield, pilot fired his engine and accelerated to a speed of 900 km/h (560 mph). The rocket engine worked as expected, and 346-3, quickly accelerating, started ascending and soon had flown in very close proximity to its carrier aircraft. Zeise then reported that the plane was not responding to the controls, and was losing altitude. Ground control commanded him to bail out. He used the escape capsule to leave the stricken aircraft at 6,500 meters (21,000 ft) and landed safely by parachute. With the loss of this aircraft, the 346 program was abandoned.

Variants

  • DFS-346 - First prototype built by Siebel Werke in the early 1940s. Later taken to USSRmarker where the newly formed OKB-2 tested it in TsAGImarker wind tunnel.Later scrapped, because it was not flyable.
  • 346-P - This airframe was first post-war build of this plane, and was completed in 1948 by German engineers. Visually 346-P was identical to the earlier design, excepting a landing gear cowl which was removed primarily for lightening the airframe. This prototype also featured mounted under wing supports, to help stop the plane when landed.
  • 346-1(A) - On the 5 may 1949 construction of 346-1 was finished. It had a rocket engine mock up installed, and incorporated some minor changes in the rudder and tail design.
  • 346-2(D) - The same as 346-1, but the rocket engines fitted.
  • 346-3 - Only plane that flew rocket-engine powered, and twice went subsonic.


Operators



Specifications (346-3)

346-3 Cross-section


References

  1. Deutsche Flugzeuge in russischen und sowjetischen Diensten

See also


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