Bristol Type 138

https://en.wikipedia.org/wiki/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 (16,440 m).

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 (RAE) 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 (15,440 m), which
was homologated as 49,967 ft (15,230 m). 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.

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.


Role
High-altitude research aircraft

Manufacturer
Bristol Aeroplane Company

Designer
Frank Barnwell

First flight
11 May 1936

Primary user
Royal Aircraft Establishment

Number built
1 (+1 Type 138B airframe not completed)

Squadron Leader F.R.D. Swain, 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 (15,440 m), during which he
engaged the auxiliary supercharger at 35,000 ft ( m). 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 (15,230
m).

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 (15,240 m). During this period, Italy had
been able to recapture the record, achieving a recorded maximum altitude of
51,364 ft (15,656 m). 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 (16,440 m). 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 (370 kW). 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.

Specifications (138A)

General characteristics
Crew: one
Length: 44 ft 0 in (13.41 m)
Wingspan: 66 ft 0 in (20.12 m)
Height: 10 ft 3 in (3.13 m)
Wing area: 568 ft² (52.8 m²)
Empty weight: 4,391 lb (1,996 kg)
Loaded weight: 5,310 lb (2,414 kg)
Powerplant: 1 × Bristol Pegasus P.E.6S radial, 500 hp (373 kW)

Performance
Maximum speed: 107 kn (123 mph, 198 km/h)
Service ceiling: 54,000 ft (16,500 m)
Rate of climb: 1,430 ft/min (7.27 m/s) at 40,000 ft
Wing loading: 9.35 lb/ft² (45.7 kg/m²)
Power/mass: 0.0942 hp/lb (0.155 kW/kg)
Endurance: 2¼ hours




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