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Ju 287

A startlingly prescient encapsulation of the advantages of forward swept wing [FSW] technology informed the design of the experimental Junkers Ju 287 jet bomber. Starting in 1943 with a rearward swept wing design intended to exceed Mach 0.8, thereby outrunning Allied fighters, the Ju 287 design team quickly forecast that the wing sweep would yield unacceptable slow-speed flight characteristics. The Junkers team, led by Hans Wocke, interpreted available German data on wing sweep and concluded a forward sweep would give the Ju 287 manageable low-speed characteristics.

Any adverse traits would be shifted to the high-speed end of the bomberís FSW envelope, where it was believed these issues would be easier to tame. Key to the advantage of forward wing sweep is the airflow migration inboard as it passes over the wing. With sweptback wings, airflow moves outboard and to the rear. The inboard flow of a forward swept wing has the aerodynamic effect of retaining attached airflow at the outboard sections of the wing even after the wing root has stalled, yielding greater aileron controllability at slower speeds.

German wind tunnel tests confirmed the Junkers teamís radical ideas while also revealing a major hurdle of early high-speed-FSW designs: the wing structure was prone to potentially disastrous twisting loads lifting the leading edge of the tips at higher speeds. For a bomber design, the FSW geometry allowed for an unobstructed, large bomb bay ahead of the wingís forward spar in an area that experienced little center of gravity (c.g.) shift with bomb release.

Research on the swept wing as a drag-reducing mechanism for high subsonic and transonic speeds during the late 1930ís and early 1940ís resulted in some of the first conventional aft-swept wing aircraft during World War II. At that time, it was also recognized that forward-swept wings (FSW) could produce the same beneficial effect for performance. Furthermore, the FSW also promised improved low-speed controllability. Stalls were expected to start at the wing root rather than the tip (in contrast to aft-swept wings), thereby maintaining the effectiveness of outboard ailerons and their contributions to roll control at low speeds.

The onset of shock waves at high speeds was expected to begin at the wing root, which again maintains aileron effectiveness at high speeds. With lateral control effectiveness assured across the operational envelope, there would be no need for drag-producing leading-edge high-lift devices. Finally, the general layout of the FSW resulted in a more aft location of the wing spar carry-through structure in the fuselage, which results in more fuselage internal volume.

During 1942, the German Junkers Design Bureau initiated studies of an FSW bomber designated the Ju-287. The Ju 287 was primarily intended to equip the German Luftwaffe with a Jet-powered bomber aircraft that could remain aloof to the enemy interception. It had a unique swept-forward wing design which was the brain child of, Dr Hans Wocke. He wanted to provide it with extra lift at low airspeeds. Starting in 1943 with a rearward swept wing design intended to exceed Mach 0.8, thereby outrunning Allied fighters, the Ju 287 design team quickly forecast that the wing sweep would yield unacceptable slow-speed flight characteristics.

The German-built Ju-287 had about 15į of forward, leading-edge sweep [other sources claim 23 degrees of forward sweep]. For a bomber design, the FSW geometry allowed for an unobstructed, large bomb bay ahead of the wingís forward spar in an area that experienced little center of gravity (c.g.) shift with bomb release. The Ju-287 concept was so promising that a full-scale forward swept wing was quickly mated to the modified fuselage of a Heinkel He 177A bomber to permit lowspeed flight exploration. A Ju 388 tail assembly was fitted, and in a quirky move to get the test bed flying, the prototype was said to have used dual nosewheels cannibalized by the Germans from a pair of downed American ConsolidatedVultee B-24 Liberator bombers. The landing gear was not retractable, to save the time and engineering necessary to adapt swinging gear to the patchwork test bed. The mainwheels came from the Junkers Ju 352 as another off-the-shelf expedient. Four Junkers Jumo 004 turbojets powered the Ju 287, with two slung under the forward swept wing and another pair riding the lower cheeks of the forward fuselage near the nose.

The first prototype Ju 287V1 was completed by construction in mid-summer 1944. It had four Jumo 004 engines with a thrust of 900 kg each. Two engines were under the wing consoles and two on the sides in the bow of the fuselage. To facilitate take-off, it used starting accelerators with liquid-reactive Walter engines HWK 501 with a thrust of 1200 kg and an operating time of 40 seconds. To get the Ju 287 airborne, auxiliary jettisonable rocket packs clung to each jet nacelle during takeoff. The Ju 287 was an early employer of a drag chute in the tail.

First flight of the Ju 287V1 was on August 16, 1944 from the Brandis airfield near Leipzig. That day the plane reached a speed of 645 km / h. During the next sixteen flights, a speed of 780 km / h was reached, and according to other information - 875 km / h, or maybe 645 km / h.

The FSW Junkers completed 17 flights from the Brandis airfield near Leipzig, Germany, and validated the slow-speed benefits of the wing planform. Tufted for full-scale flow visualization flights, the Ju 287 confirmed the nature of flow separation at high angles of attack for a FSW design, leaving the outer panels with attached airflow after the inboard airflow was degraded. One quirk of the wing design was the tendency to drop a wing when yaw was applied to the unorthodox Ju 287. Its slow-speed traits mapped, the big Ju 287 was next dived to 404 miles per hour with attendant aeroelastic issues as predicted. A fix was to build the second bomber with its intended six turbojet engines clustered three apiece under each wing, slightly inboard of midspan and jutting forward to provide mass balance.

Four Heinkel HeS 011 engines were about to be placed in Ju 287 but this engine ran into some trouble which led to the BMW 003 engines to replace the earlier Heinkel engines. Later prototypes were to have six BMW 003 engines. The second prototype had a cluster of three under each wing whereas the third prototype had two under each wing and one on each side of the fuselage just like the first prototype.

The FSW Junkers completed 17 flights from the Brandis airfield near Leipzig, Germany, and validated the slow-speed benefits of the wing planform. Tufted for full-scale flow visualization flights, the Ju 287 confirmed the nature of flow separation at high angles of attack for a FSW design, leaving the outer panels with attached airflow after the inboard airflow was degraded. One quirk of the wing design was the tendency to drop a wing when yaw was applied to the unorthodox Ju 287. Its slow-speed traits mapped, the big Ju 287 was next dived to 404 miles per hour with attendant aeroelastic issues as predicted. A fix was to build the second bomber with its intended six turbojet engines clustered three apiece under each wing, slightly inboard of midspan and jutting forward to provide mass balance.

The war ended before extensive flight tests could be carried out. The Ju-287 exhibited several problems, the most serious of which was a tendency to increase g-loading during a turn without control inputs from the pilot. The analysis of the problem by Junkers revealed that the cause was wing structural deformation from the aerodynamic loads on the forward-facing wingtip panels. At high speeds, the deformation was predicted to become very severe, exceed structural limits, and result in wing failure. This potentially catastrophic phenomenon was referred to as aeroelastic divergence.

The JU-287 quickly revealed one of the major problems of any swept forward design: structural divergence. Lift forces on wings cause them to bend slightly upward. When the wings sweep forward, this force tends to twist the leading edge upward, increasing lift and the bending motion until the wing fails. One solution was to keep the wing absolutely rigid, but conventional metal construction made such wings so heavy they were impractical. Although swept forward wings occasionally appeared on various aircraft in the postwar era, construction and weight problems proved intractable. Further analysis indicated that the structural modifications required to avoid the divergence problem for the aluminum wing of the Ju-287 would result in excessive weight and unacceptable performance penalties. The solution appeared in the form of composites, affording wings of light weight but high strength.

Parallel to the tests, the prototype V2 was built with a new fuselage and retractable chassis. The second prototype Ju-287V2 was under construction by the end of the war. It was equipped with six turbojet engines BMW003A1. Two Junkers jet turbines of 28.5 kN each (about 2900 kp) were planned. Since these, like Heinkel HeS 011 engines, were not yet available, six BMW-003 engines combined to form triplets, each with a thrust of 7.8 kN thrust, were provided under the wings (triangle arrangement), but this was less favorable due to strong surface vibrations and low ground clearance.

The prototype Ju 287V2 was not finished until the end of the war. It was equipped with six BMW 003 engines with a total thrust of about 4,800 kg, located under the consoles in the form of two packages of three engines in each. Its design speed was 784-819 km / h, the bomb load - up to 4000 kg.

For the third prototype, the Ju 287 V3, the proven design with two engines on the fuselage and four under the wings was used again. The V3 received the trunk of the Ju 288 with a pressure booth for three men crew. The chassis was retractable into the hull. The rear armament was remote controllable from the cockpit.

The serial model Ju 287V3 was to have six BMW 003 turbojet engines, four of which were installed in pairs in the nacelles under the wing, and two - along the sides in the front of the fuselage. This aircraft was supposed to have a maximum flight speed of 860 km / h, and its bomb load would have been 3000 kg. The possibility of installing four Heinkel Hearth 011 engines with a total thrust of 5200 kg was also investigated. The design speed of this version of the aircraft was 797-832 km/h.

But the Germansí FSW bomber program suffered a reversal of priorities in the summer of 1944 when renewed emphasis was placed on fighter production to combat the increasing Allied bomber presence overhead. The Ju 287/He 177 hybrid test bed was seriously damaged in an Allied bombing raid at Rechlin, Germany, but a flicker of work continued on the second example, with the forward swept wing mated to a bomber fuselage with landing gear retracting into the fuselage to preserve the wingís structural integrity.

The backward sweep of the wing in Ju 287 was adopted with the aim of increasing the critical Mach number and simultaneously avoiding a stall flow at the wing ends occurring in the wings with a straight sweep (in Ju 287, the flow failure at large angles of attack occurred first in the root parts of the wing, this efficiency of ailerons). In addition, it was possible to place the bomb bay in front of the wing, near the center of gravity of the aircraft.

There were 3 prototypes which were produced and later the Russian Army confiscated them. Unfinished aircraft number two was confiscated by the Soviets in 1945 and taken, along with engineer Hans Wocke and members of his team who completed its assembly in time for a 1947 flight in Russia. Later, the Soviets continued with its further development.

The Ju 287ís wing spanned nearly 66 feet, mated to a 60-foot fuselage on the prototype that later was stretched about a foot on the production specification. The unfinished second and third Junkers Ju 287 prototypes, which far more accurately reflected the design of the eventual production bomber, were captured by the Red Army in the closing stages of World War II. Unfinished aircraft number two was confiscated by the Soviets in 1945 and taken, along with engineer Hans Wocke and members of his team who completed its assembly in time for a 1947 flight in Russia.

The design was further developed in the Soviet Union after the end of the war. The Soviet's nearly-complete version of the Ju-287 was designated the OKB-1 EF-131. It had two wing-root mounted engines and a flight deck styled after the older Ju-88. German designers were asked to bring the aircraft and pass it on to flight tests. However, the completion of the aircraft was subsequently deemed unnecessary, and in 1948 the works were terminated, since at that time new, higher TTTs were already compiled.

When that design went nowhere, it was transferred to the Alekseyev Design Bureau [not to confuse Semyon Alekseyev with Rostislav Alekseyev Ė the father of modern wing-in-ground effect aircraft]. Thus ended a most ambitious foray into the world of FSW technology. Certain advantages were evident, but structural and control hurdles remained for a later era to solve. Thus ended a most ambitious foray into the world of FSW technology. Certain advantages were evident, but structural and control hurdles remained for a later era to solve.

parameter Ju 287 V1
crew 1 pilot and 1 copilot
length 18,30 m
span 20.11 m
height 6.0 m
wing area 58,30 m≤
aspect Ratio 6.9
payload Approx. 3,000 kg
empty weight 12.510 kg
Max. off mass 19,974 kg
Wing surface loading 215 kg / m≤
cruising speed 511 km / h
top speed 558 km / h
Service ceiling Approx. 11,000 m
range 2,100 km
Max. flight duration 4 h 10 min
engines 4 ◊ Jet engines Junkers Jumo 004 B-1 "Hurricane" with 8,83 kN each Boom (approx. 910 kp)



 
Page last modified: 08-01-2021 13:53:14 Zulu