Bristol Type 138
| Industry | Aerospace |
|---|---|
| Founded | 1910 |
| Headquarters | Filton , United Kingdom |
The Bristol Type 138 High Altitude Monoplane was a British single-engine, low-wing monoplane developed in the 1930s by the Bristol Aeroplane Company specifically for high-altitude research. This aircraft established nine world altitude records, reaching a peak height of 53,937 ft (16,440 m) during a flight lasting two and a quarter hours on 30 June 1937.
A second model, known as the Type 138B, was ordered in 1935 but construction ceased in 1937 before it ever took to the air.
Development
The creation of the Type 138 occurred amid fierce rivalry among aircraft manufacturers, where breaking significant aviation records—such as those for speed, distance, and altitude—brought prestige and technological advancement. By the 1930s, achieving these milestones required extensive resources and development efforts that individual companies could not manage without government support.
Bristol found themselves trailing behind competitors from Germany, Italy, the United States, and other British firms. Between 1929 and 1934, rival machines such as the Junkers W.34, Vickers Vespa, and Caproni Ca.113 biplanes set new altitude records, as did a pair of Westland Wallace aircraft during the first flight over Mount Everest in 1933. Notably, all these aircraft were powered by Bristol engines. From 1928 to 1938, the altitude record was surpassed ten times; one instance involved a Bristol Jupiter engine and five involved Bristol Pegasus engines, underscoring Bristol's dominance in engine performance.
In November 1933, inspired by the British Air Ministry's interest following the Everest achievement, aeronautical engineer Frank Barnwell proposed designing a dedicated high-altitude research plane. This concept, called the Type 138, featured a large single-seat monoplane layout with a retractable undercarriage and a supercharged Pegasus radial engine.
Initial progress stalled until April 1934 when Italian aviator Renato Donati set a new world altitude record, prompting public demand for an official government-backed attempt. Consequently, in June 1934, the Air Ministry released Specification 2/34 calling for two prototypes capable of reaching 50,000 ft (15,240 m) altitude. Bristol was invited to submit proposals.
Barnwell refined his design into the Type 138A, maintaining its overall dimensions but replacing the retractable landing gear with a fixed undercarriage to reduce weight. The aircraft was powered by a two-stage supercharged Pegasus engine and included provisions for an observer. Selecting the Pegasus engine was anticipated to enhance publicity and stimulate sales.
Extensive investigations were conducted by both the Royal Aircraft Establishment (RAE) and the National Physical Laboratory to optimize the aircraft's design and develop an effective pressure suit for pilot safety at extreme altitudes. Key contributors to the pressure helmet included Sir Robert Davis of Siebe Gorman and Professor J.S. Haldane. The pressure suit underwent testing simulating altitudes up to 80,000 ft (24,384 m).
By early 1936, construction of the airframe was complete. The Type 138A had its maiden flight on 11 May 1936, piloted by Cyril Uwins, Bristol's chief test pilot who previously flew the Vickers Vespa on its record-breaking mission. Since the specialized engine was not yet available, initial flights utilized a standard Pegasus IV engine coupled with a three-bladed propeller. After two more flights at Filton, the aircraft was transferred to the RAE at Farnborough where the pressure helmet underwent evaluation before returning to Filton for installation of the custom Pegasus engine and fitting of a four-bladed propeller. The Type 138A resumed flight testing at Farnborough on 5 September 1936.
Design
The Bristol 138 featured a low-wing cantilever monoplane configuration, engineered specifically for exceptionally high-altitude flight during its time. Aviation journal 'Flight remarked that, aside from its dimensions, the aircraft bore a strong resemblance to the smaller Bristol Brownie, noting it as "the largest single-seat airplane ever constructed".[1] The pilot occupied a roomy cockpit, heated by air channeled from oil coolers embedded within the wings, with adjustable airflow.[2] The instrumentation suite comprised fore-and-aft levels, gauges for oil pressure, airspeed, fuel quantity, engine revolutions, and a pyrometer. Custom-designed recording altimeters developed by the RAE were integrated into the wings, while an additional altimeter was positioned in the cockpit.[3]
Propulsion was provided by a single Bristol Pegasus engine equipped with a high-pressure two-stage supercharger, essential for maintaining engine output at elevated altitudes.[4][5] The first-stage compressor operated continuously, whereas the second stage was engaged manually via a clutch once reaching the appropriate altitude to prevent excessive boost at lower levels.[5] An intercooler was installed between these two stages to improve efficiency.[6]
Minimizing weight was crucial; thus, apart from the steel tubing used in the engine mount and cowling, the airframe primarily consisted of a wooden shell.[5][4] This structure featured plywood skin bonded to mahogany longerons and struts forming the internal framework, which was smoothly faired to reduce aerodynamic drag. A fixed undercarriage was chosen over retractable gear to prioritize weight reduction over drag minimization, as retractable systems would have added unnecessary mass.[5] The wing assembly comprised three sections with a centre section integrated into the fuselage. Construction included three spars with plywood webs and mahogany flanges, all covered with plywood sheeting.[5]
To withstand extreme altitudes, the pilot wore a custom-designed two-piece suit.[7] This suit primarily consisted of rubberised fabric connected at the waist using a pipe-clip mechanism. It incorporated a helmet featuring a large forward-facing window for visibility.[8] The ensemble included closed-circuit breathing apparatus delivering oxygen through a small injector jet to maintain airflow. Exhaled air passed through an external tube into a canister containing chemicals that absorbed carbon dioxide, allowing purified air to be recirculated back to the pilot.[8] Fuel capacity inside the aircraft totaled 82 imp gal (370 L; 98 US gal), divided between a 70 imp gal (320 L; 84 US gal) lower tank and a 12 imp gal (55 L; 14 US gal) upper tank.[5] For record-setting altitude flights, a specialized fuel called S.A.F.4 was used; it was derived from standard Shell Ethyl aviation gasoline. Notably, this fuel possessed a high anti-knock rating which was critical due to the elevated temperatures caused by significant supercharging that increased detonation risk.[5]
Operational history
Squadron Leader F.R.D. Swain, who joined the Royal Aircraft Establishment's experimental division in 1933, was chosen to pilot the high-altitude missions.[9] The overall research initiative and the preparations for the initial altitude record attempt were overseen by Mr H. E. Wimperis, then Director of Scientific Research at the Air Ministry.[9]
On 28 September 1936, Swain departed from Farnborough in the Type 138A and ascended to an indicated altitude of 51,000 ft (16,000 m). During this climb, he activated the auxiliary supercharger at approximately 35,000 ft (10,668 m). Due to oxygen depletion during the two-hour ascent, Swain had to break his pressure helmet’s window after descending to a safer altitude.[4] The Fédération Aéronautique Internationale officially recognized the flight's altitude as a world record of 49,967 ft (15,230 m).[6][9]
Following this achievement, additional development led to several minor aircraft modifications aimed primarily at weight reduction and enhanced supercharger efficiency.[4] In this slightly updated configuration, the Type 138A completed six more flights, reaching a peak altitude near 50,000 ft (15,000 m). Meanwhile, Italy reclaimed the altitude record with a maximum recorded height of 51,364 ft (15,656 m).[4] On 30 June 1937, Flight Lieutenant M.J. Adam responded with a 2¼-hour flight that established a new certified record altitude of 53,937 ft (16,440 m), despite experiencing a significant canopy crack during the mission; Adam’s pressure suit and helmet safeguarded him from harm.[10][4]
Although research flights persisted, no further record attempts were made. According to British aerospace firm BAE Systems, these test flights yielded critical data, especially advancing knowledge in pressurisation.[4]
In 1935, a second aircraft was commissioned under the designation Type 138B.[4] This version was designed as a two-seater powered by a Rolls-Royce Kestrel S engine equipped with a comparable two-stage supercharger setup capable of producing 500 hp (370 kW). Delivered to Farnborough Airfield in 1937 for final assembly, the engine was never installed,[11] and consequently, the Type 138B served exclusively as a ground-based instructional trainer without ever taking flight.[4]
Variants
- Type 138
- Never constructed.
- Type 138A
- Single unit completed.
- Type 138B
- One airframe built intended for a Rolls-Royce Kestrel S engine; remained unflown and utilized only for ground instruction.
Operators
Specifications (138A)
See also
References
- ↑ Flight 1 October 1936, pp. 338–339.
- ↑ Flight 1 October 1936, p. 340.
- ↑ Flight 1 October 1936, pp. 339–340.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Cite error: Invalid
<ref>tag; no text was provided for refs namedbae herti - ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Flight 1 October 1936, p. 339.
- ↑ 6.0 6.1 Barnes 1964, p. 255.
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedflight 340 348 - ↑ 8.0 8.1 Flight 1 October 1936, p. 348.
- ↑ 9.0 9.1 9.2 Flight 1 October 1936, p. 338.
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedthet raf - ↑ Cite error: Invalid
<ref>tag; no text was provided for refs namedWinchester p. 26
Bibliography
- Barnes, C.H. Bristol Aircraft since 1910. London: Putnam, 1964. ASIN B0000CMCQ8
- "Bristol Type 138." Flight, 1 October 1936. pp. 338–340, 348.
- Thetford, Owen. Aircraft of the Royal Air Force 1918–57, 1st edition. London: Putnam, 1957.
- Winchester, Jim. "Bristol Type 138". X-Planes and Prototypes. London: Amber Books Ltd., 2005. ISBN 1-904687-40-7.
External links
