Tupolev ANT-20

Tupolev had already, in mid-1931, drawn a passenger derivative of the ANT-16 powered by four geared M-35R engines, designated Tupolev ANT-20. To meet the new demand, which was from the start named for the writer it honoured, Tupolev kept the ANT-20 designation but further extended the outer wings and made other minor changes. The most important modifications were that, lacking the desired engines, Tupolev had to add two more on the leading edge, outboard of the existing four. Then, finding that this was still only marginally enough power, he added a final pair, making eight in all, in a tandem push/pull nacelle.

Aerodynamically the new giant was almost an extended-span ANT-16, but with huge wheel spats. Almost the whole exterior was covered in skin with corrugations of 50mm (2in) pitch and 16mm depth, thickness being the usual 0.3-0.8mm. The enormous wing, by far the biggest ever designed up to that time, had CAHI-6 profile, thick­ness/chord ratio decreasing from 20 per cent at the end of the tapered centre section to 10 per cent near the tips which, unusually for Tupolev, were rounded. To achieve the best possible lift/drag ratio the aspect ratio was 8.2. This was the first time such an aspect ratio had been used on a heavy aircraft.

Each outer wing carried two engines and had a span of 26.18m (85ft l0 in), over which the chord tapered to 3.2m at the final rib, carrying the screwed- on tip. Including the double rib at the side of the fuselage there were ten A-ribs on each side of the aircraft, interspersed by lighter L-section ribs and the multiple spanwise stringers which inboard were built-up triangu­lar Warren girders. Experience with the ANT-16 indicated that the very long ailerons should be made in four sections, to prevent the hinges from binding due to flexure of the wing. For rail transport the wing could be unbolted into ten sections.

The fuselage had a cross section fractionally wider than that of the ANT-16, namely 3.5m (138in), the height being 2.5m (98.4in), or the same as the depth of the wing at the root. It was made in sections F-1 to F-5, with steel eye/lug bolted joints at the four principal tubular longerons. Most of the structure was made from standard profiles, but highly-stressed members were thick tube or special tetrahedral profiles built up from riveted sections, sometimes with lightening holes. Again the skin was mainly 50mm by 16mm corrugated, with small smooth areas of stressed skin.

Normal usable floor area was 109sq m (l.173sq ft), though this varied according to the reason for each flight. Depending on the requirement, equipment could include a ‘voice from the sky’ loudspeaker system, an audio recording studio, a pharmacy and a leaflet dispensing system. With most of the special equipment removed, the ANT-20 could comfortably seat seventy-two passengers, with a normal operating crew of eight. In this config­uration red carpet was laid down in the passenger areas. Entry was via a large section of F-2 which hinged down with integral stairs, or by doors on each side at the trailing edge.

A total of 9,400 litres (2,068gal) of fuel could be housed in twenty-eight riveted aluminium tanks arranged in groups of four and eight in each outer wing, feeding into a collector tank on each side. All throttle linkages were connected to the three engineer sta­tions. Moving on from the ‘fighter cockpit’ of the ANT-14, Tupolev gave the ANT-20 three such cockpits, each with four front windows, a metal aft fairing and hinged roof providing access to the aircraft upper surface. The wing engineers were just inboard of engines 2 and 7, and the fuselage engineer was under the nacelle on the right side of the fuselage.

Bearing in mind the enormity of the task, the design and construction of the ANT-20 was remarkably quick.

The monster aircraft joined the MG squadron on 18 August 1934. At one time it was fitted with a compli­cated system of electric lamps under the huge wing with which slogans could be displayed at night, but this proved predictably erratic in reliability and was removed. Later all the upper surfaces were painted red, adding a tonne to empty weight, and the regis­tration was changed to 1-20. A routine was established whereby a crew of twenty managed all the on-board equipment when no (or few) passen­gers were being carried, reduced to eight when the propaganda equipment was replaced by passengers. Many of the latter were high officials, but others were people being rewarded for public service or for breaking output records.

Early in the programme Tupolev schemed the ANT-20V heavy bomber. This would have had an unchanged wing and engines but a bulbous fully glazed nose, large chin gondola, a taller and more rounded vertical tail, internal and external carriage for up to 10 tonnes of bombs, and defensive armament of two Oerlikon 20mm cannon and six 7.62mm DA or ShKAS machine-guns. Limited to a gross weight of 41,000kg, it would have had a poor performance, the calculated maximum speed being 230km/h at 3,500m, the climb 27min to 3,000m and the range only 1,300km with a bombload of a mere 2 tonnes (4,4101b). This project was overtaken by the even bigger ANT-26, TB-6.