The Type 138A was a single engine, low-wing monoplane of large span and generous area.
It was designed to break the aircraft world’s altitude record at a time of intense competition within the British aircraft idustry and much prestige was awarded for record breaking. The Bristol 138A project also had the subsidiary benefit of achieving publicity for the Bristol Pegasus engine.
The early 1930's saw an active period of altitude record breaking, the height being raised to 41,790 ft in May 1929 by the Germans and then successively to 43,166 ft (US Navy 1930); 43,976 ft (Uwins, Vickers Vespa 1932); and 47,360 ft (Italy 1934).
Air Ministry interest following the successful first flight over Everest led to Specification 2/34 and Bristol were invited to tender. They proposed a special high-altitude monoplane using a highly supercharged Pegasus engine with a view to taking the height record to above 50,000 ft.
The aircraft, designed by Frank Barnwell, was of wooden construction to minimise weight and the Bristol Type 138A (K4897) was first flown on 11th May 1936. It featured a standard Pegasus engine, driving a three-bladed propeller and was piloted by Cyril Unwins, who was already well-known following his world record flight in the Vickers Vespa.
After 2 further test flights the aircraft was transferred for high-altitude flights from the RAE at Farnborough who commenced testing in September 1936.
On its very first flight from the Hampshire Research Facility the Type 138A acheived a new record of 49,967 ft on 28th September 1936 although the pilot (Squadron Leader FRD Swann) suffered from oxygen startvation and had to break the windows in his pressure helmet once he had descended to a safe height.
After this flight a number of small modifications were made to save weight and improve the supercharging and the Type 138A returned to 50,000 feet on 6 further occassions.
Italy moved quickly to recapture the record with a flight to 51,364 ft in early 1937 and so on 30th June 1937, the Bristol 138A increased the record yet again to 53,937 ft.
A number of issues occurred during the flight (which took 2½ hours) including a major crack appearing in the canopy with Flt Lt Adams only surviving due to the major development work that had been carried out on his pressure suit and helmet.
After this no further record flights were made although it is rightly acknowldged that the increase in knowledge regarding pressurisation during these early flight was immense.
The height record was broken a further 9 times within ten years from 1928, once using a Jupiter engine and five times with a Pegasus and this was a hugely significant achievement for Bristol engines.
A Type 138B was ordered in 1935 as a two-seater, Rolls-Royce Kestrel S engined project and although the airframe was delivered to Farnborough in 1937, the engine was never fitted and the airframe was used for ground instructional training.
|Powerplant||One 500 hp Bristol Pegasus PE6|
|Span||66 ft 0 in|
|Maximum Weight||5,310 lb|
|Maximum Speed||123 mph|
|Design ceiling||More than 50,000 ft|