Tupolev TB-3 - Design

Design

The TB-3 was an all-metal aircraft of steel construction. The frame was composed of V-section beams covered with non-stressed corrugated skin ranging from 0.3 to 0.8 mm (1/64 to 1/32 in) in thickness. The corrugations were 13 mm deep (0.51 in) and 50 millimetres (2.0 in) apart. The cantilever wing was supported by four tube-section spars. In 1934, thanks to development of stronger steel alloys, wingspan was increased from 39.5 to 41.85 metres (130 to 137.3 ft) with a concurrent wing area increase from 230 to 234.5 m² (2,475 to 2,524 ft²). Any part of the aircraft could be walked on in soft shoes without damaging the skin, and the leading edges of the wings swung down to form walkways for engine maintenance. Controls were cable-actuated with a variable-incidence tailplane and a trim compensation system in case of engine failures on one side. Fixed main landing gear was not fitted with brakes. The fuel tanks did not have fire or leak protection, although the engines had an internal fire-extinguishing system. The M-17 engines were tuned to provide a maximum theoretical range of 3,250 kilometres (1,750 nmi; 2,020 mi) without spark plug or carburetor fouling. Defensive armament consisted of light machine guns in five turrets — one in the nose, two on top of mid-fuselage, and one retractable "dustbin" under each wing between the engine nacelles. Later variants moved one of the top fuselage turrets aft of the tail fin.