BOK-5

Purpose: To experiment with a tailless (so – called ‘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 satisfac- torily…but crashed during a landing’. After being repaired and modified its handling qualities were greatly improved. In 1938 the
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 ofCAHI (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 trailing – edge 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-5°. However, re­cently a drawing (reproduced here) was dis­covered showing the main surfaces operated

by servo action. The pilot’s control cables can be seen to drive a narrow-chord servo control which in turn moves the main surface. The neutral setting of the main surfaces can be seen to be adjusted by a longitudinal-trim wheel with cables to screw-jacks.

The BOK-5 was clearly a safe aircraft which impressed two of the Soviet Union’s best test pilots, but it remained a one-off which was soon forgotten.

Top and centre: Two views of BOK-5.

Bottom: BOK-5 servo control.