Bristol Type 138


The Bristol Type 138 High Altitude Monoplane was a British high-altitude research aircraft developed and produced by the Bristol Aeroplane Company during the 1930s. It holds the distinction of setting nine separate altitude world records, the ultimate of these occurring on 30 June 1937, during a 2¼-hour flight flown by Flight Lieutenant M.J. Adam, in which he achieved a record altitude, which was later homologated by the Fédération Aéronautique Internationale as having attained a maximum altitude of 53,937 ft.
As flown, the Type 138 was a single-engine, low-wing monoplane with a fixed, tailwheel undercarriage. From the outset it was designed as a dedicated research aircraft capable of reaching high altitudes; aeronautical engineer Frank Barnwell had first proposed the design to the British Air Ministry during November 1933. The revised design was produced in response to the issuing of Specification 2/34 by the Air Ministry during June 1934. The specification, which called for a pair of prototypes, sought an aircraft that would be capable of achieving the unheard-of altitude of 50,000 feet; it had been issued in response to public pressure in favour of government sponsoring of such record attempts.
In addition to Bristol's own work on the Type 138, the Royal Aircraft Establishment and National Physical Laboratory also made valuable contributions to the development effort on designing the type. On 11 May 1936, the Type 138A performed its maiden flight, piloted by Cyril Uwins. On 28 September 1936, Squadron Leader F.R.D. Swain set a new world record altitude record, having climbed to an indicated altitude of 51,000 ft, which was homologated as 49,967 ft. After this milestone, research flights continued, along with further attempts to break the altitude record. Further development of the aircraft after the first record-breaking flight led to a series of small modifications being implemented. The last record-breaking flight was flown on 30 June 1937. A second aircraft, designated as the Type 138B, was ordered in 1935 but work on it was abandoned during 1937 without ever having flown.

Development

The Type 138 was born of a period of intense competition between aviation manufacturers in the 1920s and 1930s. There was much prestige, as well as useful technological progress, to be gained from breaking any of the major aviation records; these being airspeed, distance and altitude. By the 1930s, the resources and development work necessary to realistically achieve absolute speed and distance records were becoming beyond the resources of individual companies, and required the involvement of national governments.
The Bristol Aeroplane Company was considered to be well placed to compete in this field but, in the event, they found themselves lagging behind other competing companies based in Germany, Italy, the United States, and at home in the United Kingdom. Between 1929 and 1934, there were a number of altitude records established by rival aircraft, including 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; of these aircraft, all had been powered by Bristol-produced or Bristol-designed engines. Between 1928 and 1938, the altitude record was broken 10 times, once using a Jupiter engine and five times using a Pegasus engine; this was considered to be a hugely significant achievement for Bristol engines.
In November 1933, having observed interest within the British Air Ministry as a result of 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 engine. Nothing came of this until Italian pilot Renato Donati achieved a new world record during April 1934; after this, public opinion swung in favour of a new, government-sponsored record attempt. In June 1934, the Air Ministry issued Specification 2/34, which sought a pair of prototypes that would be capable of achieving the never-before-reached altitude of 50,000 ft. Bristol was amongst the companies which were invited to tender.
In response to the Specification, Barnwell revised the Type 138 proposal, producing the Type 138A. This revision retained the original design's size and configuration, however, it was powered by a special, two-stage supercharged Pegasus engine and, although remaining basically a single-seater aircraft, had made provision for an observer's cockpit to be installed if deemed to be required. The use of the Pegasus engine on the proposed aircraft was viewed as being positive for gaining publicity and thus fostering wider sales. As a weight reduction measure, the original retractable undercarriage arrangement was replaced with a lightweight, fixed counterpart instead.
In support of the ambition, considerable research was carried out by both the Royal Aircraft Establishment and National Physical Laboratory for the purpose of establishing the most efficient design for 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 was reduced to the equivalent of being at an altitude of 80,000 ft.
In early 1936, the airframe was completed; on 11 May 1936, the Type 138A was flown for the first time, powered by a standard Pegasus IV which drove a three-bladed propeller. For this maiden flight, it was piloted by Cyril Uwins, Bristol's chief test pilot, who had previously flown the Vickers Vespa on its world record flight. Two additional flights were performed at Filton prior to the aircraft being delivered to the RAE at Farnborough. The RAE conducted a series of tests of the pressure helmet prior to the aircraft being returned to Filton for the installation of the special Pegasus engine and a corresponding four-blade airscrew. On 5 September 1936, the Type 138A returned to Farnborough to commence 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 relatively spacious cockpit, which was heated by air directed from the oil coolers set within the wings, which could be actively controlled. Instrumentation included fore-and-aft levels, oil pressure gauges, airspeed indicator and fuel gauge, engine speed indicator and pyrometer. Purpose-built recording altimeters, developed by the RAE, were housed within the wings, while a separate altimeter was installed within the cockpit for the pilot.
The 138 was powered by a single Bristol Pegasus engine. This engine featured a highly powered supercharger, which was a critical element enabling the engine to deliver the required performance in order to attain the envisioned altitude performance. The supercharger's installation, the detail work of which having been performed by Clifford Tinson, was a two-stage system; the first-stage compressor was permanently engaged, but the second-stage incorporated clutches which enabled it to be manually engaged by the pilot upon attaining the correct height. This arrangement was necessary to avoid administering an excessively dense mixture to the cylinders when flown at low altitudes. The supercharger arrangement employed an intercooler between the first and second stages.
Weight-saving was a priority across the aircraft; the airframe, other than the steel tube engine mount and cowling, featured a wooden rectangular monocoque structure. It had a glued plywood skin, fixed to the mahogany longerons and struts forming the internal structure, which was faired throughout to result in a low-drag extensions. A conventional fixed undercarriage was used; it had been deemed of greater importance to reduce the aircraft's weight than its drag, thus a retractable undercarriage was seen as counterproductive. The wings, which were constructed in three sections, had a centre section integral with the fuselage; the internal structure used three main spars with plywood webs and mahogany flanges, which were covered with plywood sheeting.
In order to cope with the extreme altitudes flown at, the pilot was provided with a specially-developed two-piece suit. This suit, which was principally composed of rubberised fabric, was joined at the waist using a form of pipe-clip. It was provided with a helmet, which featured a large forward window to provide view. It is complete with closed-circuit breathing apparatus, the oxygen jet being delivered via a small injector for circulation purposes. Once exhaled, the gas would travel via an external tube to a canister containing carbon dioxide-absorbing chemicals to restore it to a pure oxygen state prior to being re-circulated. The 138 had an internal fuel capacity of 82 gallons, spread across 70 gallons in the lower tank and 12 gallons in the upper tank. A specially-developed fuel, known as S.A.F.4, was used for the altitude record flight, derived from standard grade Shell Ethyl aviation gasoline. Of note, this fuel has a high anti-knock value; the high degree of supercharge involved results in the fuel mixture reaching high temperatures, which generally increases the potential for detonation, thus a high anti-knock value was viewed to be of critical importance.

Operational history

, who had joined the experimental division of the RAE in 1933, was selected to pilot the high-altitude flights. Both the general research programme and preparations for the first record altitude flight, were undertaken under the immediate direction of Mr H. E. Wimperis, the Director of Scientific Research at the Air Ministry.
On 28 September 1936, Swain took off from Farnborough in the Type 138A; he climbed to an indicated altitude of 51,000 ft, during which he engaged the auxiliary supercharger at 35,000 ft. Swain ran low on oxygen on the two-hour flight and had to break the window of his pressure helmet after descending to a safe height. The data from this flight were recognised by the Fédération Aéronautique Internationale as a world record of 49,967 ft.
After this flight, further development work resulted in a number of small modifications to the aircraft, the typical objective of these being weight savings and improving the performance of the supercharger. In this mildly revised form, the Type 138A conducted six further flights, during which a maximum altitude of around 50,000 ft. During this period, Italy had been able to recapture the record, achieving a recorded maximum altitude of 51,364 ft. In response, on 30 June 1937, Flight Lieutenant M.J. Adam undertook a 2¼-hour flight in which he achieved a record altitude, later certified as 53,937 ft. Despite the canopy cockpit suffering a major crack during this pioneering flight, Adam was protected from injury by his pressure suit and helmet.
Research flights continued, but there were no further attempts to break records. According to the British aerospace company BAE Systems, the test flights had resulted in invaluable flight data being obtained, particularly in the field of pressurisation.
During 1935, a second machine was ordered, designated the Type 138B. This was to be a two-seater aircraft, powered by a Rolls-Royce Kestrel S engine, which would be fitted with a similar two-stage supercharger installation, enabling it to generate 500 hp. In 1937, the airframe was delivered to Farnborough Airfield for completion, but the engine installation was never completed. The Type 138B was used as a ground instructional trainer instead, and accordingly never flew.

Variants

;Type 138
;Type 138A
;Type 138B

Operators