The Armstrong Whitworth AW.55 Apollo was designed, like the Vickers Viscount, to the requirements of the Brabazon Committee Type II design calling for a medium size, mid-range and pressurised passenger aircraft. It featured four turbo-prop engines and a tricycle undercarriage which, due to the narrowness of the engines, retracted directly into the wings.
It was a low-wing monoplane with capacity between 26 to 31 passengers. Outwardly, there was a rounded line to the fuselage and the Armstrong Siddeley Mamba engines were installed in very slim nacelles.
Armstrong Whitworth AW55 Apollo G-AIYN taxying at the Farnborough Air Show.
In the event, only two aircraft were built (G-AIXN / VX220) with Mamba 1 engines and (VX224 / G-AMCH) with Mamba 504 engines, the first of these flying from the grass airstrip at Baginton, Coventry on 10th April 1949. Test flights soon established that the aircraft was both unstable and underpowered and it was grounded after just 9 hours. Although test flying restarted in August 1949, it was soon established that it still had major engine issues.
The second aircraft did not fly until 12th December 1952, by which time the success of the Vickers Viscount made it clear that the Apollo did not have a commercial future.
In this instance, Armstrong Whitworth’s preference for Armstrong Siddeley engines proved to be unhelpful as engine development issues, particularly in respect of reliable operation at their rated power, undoubtedly contributed to the Apollo’s lack of success.
The AW Mamba used an axial compressor, rather than the Rolls-Royce Dart’s simpler and more robust centrifugal compressor and in the end the Dart proved to be a far more outstanding engine. The Mamba eventually found its only production application in Double-Mamba form as fitted to the Fairey Gannet ASW aircraft.