The Bristol High Altitude Monoplane (factory name Type 138) was a research aircraft manufactured by Bristol Aeroplane Company.
It was developed and built in particular to surpass the existing absolute altitude world record.
The Bristol 138 was a low-wing cantilever monoplane designed to fly at extremely high altitudes for the era.
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.
The 138 was powered by a single Bristol Pegasus engine fitted with a high pressure two-stage supercharger, which was critical in enabling the engine to deliver the required performance at altitude.
The first-stage compressor was permanently engaged, while a clutch was used to manually engage the second stage on attaining the correct altitude, which was needed to avoid an excessive charge when flown at low altitudes.
It employed an intercooler between the first and second stages.
Weight saving was a priority and the airframe, other than the steel tube engine mount and cowling, used a wood shell.
It with a plywood skin glued to the mahogany longerons and struts that formed the internal structure, which was faired throughout to reduce drag.
A conventional fixed undercarriage was used as it was more important to reduce the weight than the drag, and a retractable undercarriage would have been counterproductive.
The wings were constructed in three sections with a centre section integral with the fuselage.
Three spars with plywood webs and mahogany flanges were used, covered with plywood sheeting.
In order to cope with the extreme altitudes, the pilot used a specially developed two-piece suit.
This was principally made up of rubberised fabric joined at the waist using a type of pipe-clip.
It was provided with a helmet, which featured a large forward window to provide a view.
It was completed with closed-circuit breathing apparatus with oxygen being delivered via a small injector jet to provide air circulation.
Exhaled air travelled via an external tube to a canister containing carbon dioxide-absorbing chemicals to restore it prior to it returning to the pilot again.
The 138 had an internal fuel capacity of 82 imp gal (370 L; 98 US gal), split between a 70-imp gal (320 L; 84 US gal) lower tank and a 12-imp gal (55 L; 14 US gal) 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.
