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.
The Armstrong Whitworth design comprised four turbo-prop engines with 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.
Initially, two aircraft were actually built (G-AIXN / VX220) fitted with Mamba 1 engines whilst another two (VX224 / G-AMCH) were later built with Mamba 504 engines.
The first of these prototype flying from the grass airstrip at Baginton, Coventry on 10th April 1949. Test flights soon established however, that the aircraft was both unstable and underpowered, so 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 AW Apollo did not have any real commercial future.
In this instance, the company's preference for Armstrong Siddeley engines proved to be real problem 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 engine used an axial compressor, rather than the Rolls-Royce Dart’s simpler, more robust centrifugal compressor. In the end the Rolls-Royce Dart power unit proved to be a far more outstanding engine. The AW Mamba eventually found its only production application in a Double-Mamba form, as fitted to the Fairey Gannet ASW aircraft.