B O K - 2 , RK / B O K - 5
BOK-2, RK Purpose: To test designer's experimental wing. Design Bureau: Aircraft constructed by BOK to design of S S Krichevskii. Sawa Syemenovich Krichevskii, called 'a talented designer' by historian Shavrov, spent the early 1930s trying to create the most efficient aeroplane wing. He made many tunnel models, eventually settling on a wing of high aspect ratio constructed in front and rear sections. The rear part was hinged to the front
with a small intervening gap acting as a slot. In flight, the intention was that the pilot would select the optimum angle for the rear portion, Shavrov commenting that 'this wing could always be flown in a drag-polar envelope'. Krichevskii secured funding to build a research aircraft, called RK (Razreznoye Krylo, slotted wing) and designated BOK-2 by the construction bureau. The BOK-2 was completed in 1935 and flew successfully, but Krichevskii died shortly afterwards. Documentation on this aircraft has never been found.
The BOK-2 was an extremely neat cantilever monoplane, with a single M-l 1 engine rated at 11 Ohp. Shavrov comments that 'The wing skin was polished to mirror brilliance [suggesting all-metal construction]...it is hard to say if its excellent performance was due to its drag-polar envelope or to its perfect aerodynamic shape'. Despite its apparently excellent performance the RK appears to have had no impact on the Soviet aviation ministry. No data available.
modified aircraft was tested by the Nil WS (air force flight-test institute), where it was flown by such pilots at P M Stefanovskii and M A Nyukhtikov. Stefanovskii is reported to have said that the BOK-5 could be 'flown by pilots of average or even below average ability' and to have been 'impressed by its acrobatic capability'. The BOK-5 was a basically simple aircraft, apart from the flight-control system. The airframe was made of duralumin. The wing was of CAHI (TsAGI) 890/15 profile (15 per cent t/c ratio), with two spars with tubular booms and sheet webs, and ribs assembled from channel and angle sections, with fabric covering. The short fuselage was a semi-monocoque, with some box-section longerons and
pressed-sheet frames, the vertical tail being integral. The main landing gears were described as 'U-2 type'. On the nose was a 1 00hp M-l 1 engine in a Townend-ring cowl, driving a two-blade metal propeller. Modifications concentrated on the trailingedge controls. According to Shavrov there were three movable surfaces on each wing, extending over 21 per cent of the chord. The outermost was a rectangular aileron, and the two inboard surfaces acted in unison as elevators. Most photographs and drawings show these surfaces as simple one-piece units hinged to brackets below the trailing edge and with a neutral setting of-5째. However, recently a drawing (reproduced here) was discovered showing the main surfaces operated
BOK-5 Purpose: To experiment with a tailless (socalled 'flying wing') design. Design Bureau: Bureau of Special Design, Smolensk. Design team led by V A Chizhevskii. The idea for this small research aircraft came from the BOK-2, though the two aircraft were completely unrelated. In 1935 Chizhevskii began studying tailless aircraft, and obtained funding to build a simple research aircraft. This was completed in early 1937, but was then modified and did not fly until September, the pilot being I F Petrov. It 'flew satisfactorily...but crashed during a landing'. After being repaired and modified its handling qualities were greatly improved. In 1938 the BOK-5
46