The Bristol Type 138 High Altitude Monoplane was a British high-altitude single-engine, low-wing monoplane research aircraft developed and produced by the Bristol Aeroplane Company during the 1930s. It set nine world altitude records, with the maximum altitude achieved being on 30 June 1937, during a 2¼-hour flight.
A second aircraft, designated as the Type 138B, was ordered in 1935 but work was abandoned during 1937 without it having flown.
Development
The Type 138 was built during a period of intense competition between aviation manufacturers. Prestige and useful technological progress came from breaking major aviation records, such as airspeed, distance and altitude but by the 1930s, the resources and development work necessary to achieve these records was beyond individual companies, and required government assistance.
Bristol found themselves lagging behind other companies from Germany, Italy, the United States, and the United Kingdom. Between 1929 and 1934, altitude records established by rival aircraft included those set by a Junkers W.34, a Vickers Vespa and a Caproni Ca.113 biplane, as well as the first flight over Everest by a pair of Westland Wallaces in 1933. All of these aircraft had been powered by Bristol engines. Between 1928 and 1938, the altitude record was broken 10 times, once using a Jupiter engine and five times using Pegasus engines which was seen as a major achievement for Bristol's engines.
In November 1933, having observed British Air Ministry interest following the success of the Everest flight, aeronautical engineer Frank Barnwell proposed a purpose-built high-altitude research aircraft. This proposal, designated the Type 138, was a large single-engine, single-seat monoplane, equipped with a retractable undercarriage and a supercharged Pegasus radial engine. Nothing came of this until Italian pilot Renato Donati achieved a new world record during April 1934 prompting public opinion to swing in favour of a government-sponsored record attempt. In June 1934, the Air Ministry issued Specification 2/34, for a pair of prototypes capable of reaching an altitude of . Bristol was among the companies which were invited to tender proposals.
Barnwell revised the Type 138 proposal, producing the Type 138A whose size and configuration remained the same, but the retractable undercarriage was replaced with a fixed design to reduce weight and it would be powered by a two-stage supercharged Pegasus engine and provision for an observer was made. Using the Pegasus was expected to generate publicity and boost sales.
Considerable research was carried out by both the Royal Aircraft Establishment (RAE) and National Physical Laboratory to fine tune the design of the aircraft, as well as to develop a reliable pressure suit to be worn by the pilot. Sir Robert Davis of Siebe Gorman and Professor J.S. Haldane were instrumental in developing the helmet. During tests, the pressure suit was tested to the equivalent altitude of .
In early 1936, the airframe was completed and on 11 May 1936 the Type 138A was flown for the first time by Cyril Uwins, Bristol's chief test pilot, who had previously flown the Vickers Vespa on its world record flight. As the engine was not ready, it was powered by a standard Pegasus IV driving a three-bladed propeller for the early flights. Two additional flights were performed at Filton prior to the aircraft being delivered to the RAE at Farnborough where the pressure helmet was tested prior to the aircraft being returned to Filton for the installation of the special Pegasus engine and a four-blade propeller. On 5 September 1936, the Type 138A returned to Farnborough for more test flights.
Design
The Bristol 138 was a low-wing cantilever monoplane designed to fly at extremely high altitudes for the era. Aviation publication 'Flight observed of the aircraft that: "except for its size, reminds one very much of the little Bristol Brownie.... the machine is the largest single-seater aeroplane ever built". The pilot was seated in a spacious cockpit, which was heated by air directed from the oil coolers set within the wings, which could be adjusted. Instrumentation included fore-and-aft levels, oil pressure gauges, airspeed indicator and fuel gauge, engine speed indicator and a pyrometer. Purpose-built recording altimeters, developed by the RAE, were housed within the wings, while a separate altimeter was installed in the cockpit.