Avro 684

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Avro 684
Role High-altitude bomber
National origin England
Manufacturer Avro
Developed from Avro Lancaster

The Avro 684 was a proposed British high altitude heavy bomber of the Second World War, based on Avro's successful Lancaster.[1]


Due to the increasing vulnerability of contemporary heavy bombers, the need to fly at ever higher altitudes to avoid Anti-aircraft fire and fighter interception became paramount. Avro developed the Lancaster to fulfil the requirement for such a high altitude bomber completing a brochure in August 1941, for the Avro 684 Stratosphere Bomber. Chadwick decided to design a bomber that would dispense with defensive armament and rely on altitude and speed to evade enemy defences and was essentially a Lancaster with the nose section containing a pressure cabin (similar to the cabin from the Vickers Wellington B Mark X) and a large 'chin' mounted air intake and heat exchanger assembly. Pressure in the cabin was to be maintained at the 10,000 ft (3,048 m) equivalent up to 40,000 ft (12,192 m).[1]

To achieve this performance, Chadwick used an engine arrangement called the 'Master-Slave' layout. The four wing-mounted Rolls-Royce Merlin XX engines driving four-bladed 13 ft (4 m) diameter Rotol propellers, were to be supercharged by a fuselage-mounted slave Rolls-Royce Merlin 45 engine driving a large supercharger supplying pressurised air via an intercooler. The output from the slave engine was to be controlled to supply air at 20,000 ft (6,096 m) equivalent pressure between 20,000 ft (6,096 m) and 40,000 ft (12,192 m).[1]

Due to the pressure of constant developments to the Lancaster and design work on the York, the 684 was abandoned.[1]

Specifications (projected)[edit]

Data from British Secret Projects. Fighters & Bombers 1935 - 1950[1]

General characteristics

  • Fuel capacity: 2,130 imp gal (9,683 l)
  • Powerplant: 4 × Rolls-Royce Merlin XX V-12 liquid-cooled supercharged piston engines, 1,480 hp (1,100 kW) each at 3,000 rpm at 6,000 ft (1,829 m) (un-boosted by slave engine)
  • Powerplant: 1 × Rolls-Royce Merlin 45 V-12 liquid-cooled supercharged piston engine, 1,515 hp (1,130 kW) at 3,000 rpm at 11,000 ft (3,353 m) driving a large supercharger delivering pressurised air to the wing mounted engines via an intercooler.
  • Propellers: 4-bladed Rotol, 13 ft (4.0 m) diameter fully feathering constant-speed propellers


  • Maximum speed: 410 mph (660 km/h; 356 kn) at 42,000 ft (12,802 m)
  • Cruise speed: 330 mph (531 km/h; 287 kn) at 42,000 ft (12,802 m)
  • Stall speed: 300 mph (483 km/h; 261 kn)
  • Range: 2,300 mi (1,999 nmi; 3,701 km)
  • Service ceiling: 50,300 ft (15,300 m) absolute at 38,492 lb (17,460 kg)
  • Rate of climb: 940 ft/min (4.8 m/s)
  • Time to altitude: time to 40,000 ft (12,192 m) - 57 minutes


  • 12,000 lb (5,443 kg) of bombs


  1. ^ a b c d e Butler, Tony (2004). British Secret Projects. Fighters & Bombers 1935 - 1950. Hinkley: Midland Publishing. p. 107. ISBN 1-85780-179-2. 


  • 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.