For my last entry this year, I have chosen a very unusual aircraft - the Belyaev DB-LK.
DB-LK means "long-range bomber - flying wing" ("long-range" only in contrary to light bombers like the Su-2, which was initially designated "BB", short-range bomber, RT). The concept of this plane was developed by TsAGI engineer Prof. Victor Belyaev in early 1930s. It was an attempt to find the unusual, most rational scheme of the plane. By the way, Belyaev himself defined the scheme of the plane as "flying wing". V.B. Shavrov considered the scheme of this plane being absolutely unique, definitely impossible to be called "flying wing". But Belyaev didn't only emphasize on the formal definition of the scheme, he was also interested in the results it may deliver. First of all, he carefully analyzed the experiments he had made with the BP-2 and BP-3 gliders. By that time the designer had shown a great interest in devices with a so-called "butterfly"(i.e. elastic) wing. In 1938 V. Beljaev and V. Juhanin designed an experimental aircraft with a similar swept-forward wing and an air-cooled 430 hp engine. Projected top speed was 510 km/h.
But the direct harbinger for designing DB-LK can be named V. Belyaev's project of the fast passenger plane which received the award for originality of design at the "Avianito" competition. Shavrov wrote: "The project engineer Belyaev dedicated himself to the maximal boldness of a plan. The plane - double fuselage, high-wing with two engines. It's distinctive feature - a lancet wing, swept forward. The ends of the swept-forward wing are very thin, and their pitch angle is less than in the basis of the wing... ". Two years later, this description could be applied to the new DB-LK bomber. Both projects have been assessed and approved by TsAGI. Prof. Belyaev was assigned to convert the passenger plane design into a long-range bomber.
It was 1938, and the DB-3 long-range bomber has already shown quite an adequate and even impressive performance. The DB-LK should surpass the DB-3 in every aspect. The plane had two duralumin fuselages formed by extended engine gondolas (with 950hp M-87B engines). The horizontal stabilizer was located very high. The gondolas, and it was the main advantage of a design, formed a special sort of gunner comparments which were extensively glazed and could rotate (!) around the fuselage axis. The vertical stabilizer was located on the rear part of the center wing section. NACA 23012 profile was initially chosen for the wing, later replaced with TsAGI profile in the center section and Göttingen-387 on the ends of the outer wings.
The angle of sweep on the forward edge of the outer wing parts was 5 degrees 42 minutes. Lengthening of a wing - 8.81, narrowing - about 7, cross-section V - 6 degrees 28 minutes. The unbent ends of a wing bore small ailerons. A balancing shield on the center wing section rose and fell at landings to insignificant angles. Fuselages of the polymonocoque type, had power structure of four longerons and three basic frames. Pilot and gunner seats were in the left fuselage, navigator and gunner/radio operator sat in the right one.
The welded tubular motor-frame was attached to a forward frame of a fuselage, and to its rear - a rotary frame with a Plexiglas "greenhouse" and machine-gun position. Four rear-firing machine guns were installed to protect the bomber. Depending on the circumstances, every gunner of the DB-LK had to operate two machine guns. The system of defensive fire, as a whole, was extremely dense. Test pilot Stepan Suprun said that "a fighter attack on this plane is possible only by incident". Besides, two remote-controlled coupled ShKAS machine guns were installed in the center wing section. The bomb load was 1000 kg of bombs on internal suspension brackets in the fuselages in various combinations - from four FAB-250 up to 58 small bombs. In a reloading variant it was possible to take two FAB-1000 or two FAB-500 on external suspension brackets. The plane was built at a factory Nr.156 and in April 1940 was delivered to the Air Force Scientific Research Institute (NII VVS) for tests. To the people at the airfield, the bomber looked weird: "Will it fly? It's all wrong side foremost!... "
Most of the tests were carried out by leading engineer G. Samarin and test pilot M. Nyukhtikov. In total 102 flights were performed.The first trials on the DB-LK were undertaken by the Chief of the Scientific Research Institute General A. Filin. On one of approaches at 240 km/h the venerable pilot has run into a tree stump and the aircraft lost a wheel. Fortunately, the wheel was quickly fixed, but the "rare bird" was plagued by bad luck. During one of the next approaches it lost balance and began a sharp and dangerous ascension. The rear slats saved the day (and the pilots): they had worked automatically, and the plane as though as if nothing had happened, smoothly lowered the nose, speeding up. This near-accident has considerably added to the credit of the new machine: not every plane could perform such a trick.
While a new commission studied the viability of further tests, Nyukhtikov finally decided he doesn't give a damn to all this bureaucratic fuss, and at his own risk lifted the DB-LK into the air. He has flown the aircraft from Chkalovskaya to a nearby airfield in Monino. From this point, things seemed brighter for the new bomber, and on May 1st, 1940 DB-LK even has awarded honour of flight over the Red Square during the parade attended with all big brass including Joseph Stalin.
Initially the Air Force put very high requirements for the DB-LK: top speed 550 km/h (at 7000 m), maximal range with a 1000 kg bomb load - 4000 km. But with 950 hp M-87B engines it was possible to "squeeze out" only 488 km/h. Range at tests was not defined because of the limited flight weight of the plane caused by conditions of centering and unsatisfactory take-off characteristics. The high commission specified that the DB-LK could not be tested with the maximal payload because of low durability of the chassis, high take-off, approach & landing speeds, greater lengths of start and run. At the same time, the commission expressed high interest in the new bomber: "It is necessary to complete the DB-LK regarding improvement of its controllability, perfection of runway properties, armament - and to bring the plane to repeated state tests to 15.10.1940 to perform a research necessary for construction of a new plane of this type."
What was modified before the new round of tests? The elevator area was increased from 4.27 sqm up to 4.8 sqm. The axial compensation of elevators, vertical rudders and ailerons was reduced. Rear part of the center wing section was transformed into an additional elevator (0.582 sqm). The area of vertical tail unit have increased up to 7.0 sqm. For proper balance, the engine cowls were loaded with 280 kg of lead. The test pilot has noted some interesting features of the plane: while ascending, it tended to be turned to the right; at speeds about 100 km/h the tail slipped up sharply. Take-off speed was high - 180 km/h, with an annoying tendency to jump in the end of start. The plane could adequately fly on one engine, gaining height at a speed of 1.15 km/s and easily changing the course. Proceeding was soft and smooth for the speed of 240 km/h. Landing speed was a bit high - 155 km/h.
There were serious complaints on cockpits/gunner compartments, incommodious and very uncomfortable, especially the navigator's cockpit. Forward and downward visibility proved absolutely unsatisfactory. And fire protection of the DB-LK was far from excellent, despite Suprun's optimistic remarks. The tests have shown that the gunner compartments were too narrow for service on two fire-critical points. Besides, the cabins were filled with engines exhaust fumes. To fly in such a cabin would be possible only with a gas mask. Firing tests have revealed unsatisfactory defense of the forward hemisphere because of small shooting angles, and also the bad visibility towards the forward hemisphere. The positioning of machine-guns in the rear fuselage cones did not allow to use their shooting angles effectively. And transition from the central machine-gun point to the cone and back is extremely inconvenient: gunners continually get stuck with straps of the parachute, intercom cords and the oxygen hose to parts of the machine gun and other equipment.
Eventually, the DB-LK was removed from tests in view of inaccessibility on the runway properties to pilots of average qualification. However, some features of it were still so tempting, that the new order has been issued - to convert the DB-LK into a dive bomber. Drafts were developed of an external suspension bracket for heavy bombs and the brake lattices necessary for decrease of speed at a dive. But all these efforts were in vain.
No doubt, this unusual aircraft is attractive. But some modern researchers have translated their affection for the DB-LK into fantastic statements like: "In its class, to the DB-LK no other plane appeared equal neither on speed, nor on height, nor on range of flight. "In fact, it surpassed machines of the similar weight only in speed, by mere 30 km/h. In terms of altitude and range the DB-LK was inferior to the SB and DB-3. Its calculated (and never tested!) range was only 2900 km, 1100 km less the Air Force requirement while the DB-3 had the maximal range of 3500 km - with the same M-87B engines!
The DB-LK was a transitional design from the "conventional" monoplane to the flying wing. Its builders lacked technologies and experience. Nevertheless, it remains in history as one-of-a-kind, unorthodox design.