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Avro 684

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The Avro 684 was a prototype British heavy bomber of the Second World War, based on Avro's successful Lancaster.


In August, Avro completed the drawings for the 684 Stratosphere Bomber, a design based on the Lancaster which had then just entered production. Roy Chadwick, Avro's chief designer, was prompted to develop the 684 design based on reports from the Royal Air Force on their experince operating small numbers of the Boeing B-17 Flying Fortress, in this case the B-17C version. Chadwick decided to design a bomber that would dispense with defensive armament and rely on altitude and speed to evade enemy defences. The 684 was essentially identical to the Lancaster apart from the nose section which contained a pressure cabin (from the Vickers Wellington B Mark X) and a large 'chin' mounted air intake and radiator.

The projected performance for the 684 was far beyond that of the Lancaster upon which it was based. Its cruise speed was to have been 330 mph (520 km/h) with a maximum speed of 410 mph (660 km/h), both at a height of 42,000 ft (12,800 m), absolute ceiling would have been 50,300 ft (15,330 m). A range of 2,300 mi (3,700 km) was predicted and a maximum bombload of 12,000 lb (5,400 kg) could have been carried.

To achieve this performance, Chadwick used an engine arrangement called the 'Master-Slave' layout. The four wing-mounted Rolls Royce Merlin XX's engines were supercharged by a fuselage-mounted slave Merlin 45 engine which drove a large, intercooled supercharger. The Merlin 45's rpm could be adjusted so that the compressed air output from the slave's engine-driven supercharger was always at 20,000 ft (6,100 m) equivalent pressure at all altitudes between 20-40,000 ft (6,100-12,200 m), the propellors were to be four-bladed 13 ft (4 m)-diameter rotols.

Due to the pressure of constant developments to the Lancaster and design work on the York, work on the 684 was suspended and the design was never realised.


Butler, Tony. British Secret Projects. Fighters & Bombers 1935 - 1950. Hickney, England: Midland Publishing. 2004 ISBN 1-85780-179-2.
Holmes, Harry. Avro Lancaster. The Definitive Record 2nd Edition. Shrewsbury, UK: Airlife Publishing Ltd, 2001. ISBN 1-84037-288-5.

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