The Airspeed AS.57 Ambassador is a British twin piston-engined airliner that was designed and produced by the British aircraft manufacturer Airspeed Ltd. It was one of the first postwar airliners to be produced.
The Ambassador was developed in response to a requirement identified by the Brabazon Committee for a twin-engined short-to-medium-haul airliner as a replacement for the ubiquitous Douglas DC-3. Airspeed assembled a dispersed design team at Fairmile Manor, Cobham, Surrey, in 1943, which initially worked on a smaller proposal powered by Bristol Hercules radial engines; this was quickly superseded by a larger-capacity design aimed at better fulfilling the expansion in postwar civil air travel, although several entities doubted Airspeed's projected growth in air travel. Fitted with the new but conventional Bristol Centaurus radial engines, the Ambassador was designed to accommodate four Napier Nomad turbo-compound engines, a then-recent innovation. Its fuselage was compatible with pressurisation, this being an optional feature offered to customers.
On 10 July 1947, the first prototype Ambassador conducted its maiden flight at Christchurch with chief test pilot George Errington at the controls; a total of three prototypes were built. Early on, British European Airways (BEA) emerged as a key customer for the type, having placed a £3 million order for 20 aircraft in September 1948. Introducing the Ambassador to service in 1951, BEA often referred to the aircraft as the "Elizabethans", as the aircraft were used for the airline's "Elizabethan Class" passenger service. However, the type was quickly outperformed by the turboprop-powered Vickers Viscount, with BEA opting to withdraw its Ambassadors in 1958.
Secondhand aircraft were operated by several other airlines, including Dan-Air and Northeast Airlines. Despite being produced in relatively small numbers, the Ambassador was operational for longer than had been anticipated by planners.
Development Background The origins of the Ambassador can be traced to 1943 and the work of the Brabazon Committee, which sought to define what types of aircraft would be desired by the postwar civil aviation sector. One of the committee's recommendations was a twin-engined short-to-medium-haul replacement for the Douglas DC-3, known as the Type 2.Phipp 2007, pp. 75–77. The British manufacturer Airspeed Ltd. was interested in this requirement and established a small design team to work on the project from a dispersed office in Fairmile Manor at Cobham in Surrey in 1943. This effort was headed by ex-de Havilland aeronautical engineer Arthur Hagg, who had joined the company earlier that year.
Early concepts included an unpressurised aircraft in the 14.5-ton gross weight class to be powered by a pair of Bristol Hercules radial engines.Taylor 1970, pp. 110–111. Work on the endeavour was taken as far as possible without impacting Airspeed's wartime production activity, as the Second World War was still raging.Taylor 1970, p. 111. In response to changes in planner's preferences, dictating that the interim aircraft be procured so that airliners such as the in-development Ambassador had more development time, thus the design was revised substantially to expand its capacity. By 1944, it was a considerably larger design than the DC-3, with improved comfort and space.Taylor 1970, pp. 111–112.
While Airspeed's management believed that postwar demands would demand greater passenger capacities, several figures, including those within the British Overseas Airways Corporation (BOAC) and the Air Ministry, were less enthusiastic with their projections.Taylor 1970, pp. 112–113. The company also promoted the economics of higher utilisation rates and elevated cruise speeds, whereas conventional wisdom amongst airlines was to avoid increasing speeds to reduce fuel consumption. For the Ambassador, Airspeed opted to pursue a cruising altitude of 20,000 ft, relatively high for the era. A comprehensive mockup was completed by November 1944, largely for promotional purposes, while the projected operating cost figures were released during early 1945.Taylor 1970, p. 113.
Immediately following the end of the conflict, the British Ministry of Aircraft Production placed an order with the company to produce a pair of prototypes. While the first prototype was to be unpressurised and powered by a pair of Bristol Centaurus radial engines, it was envisioned that the second prototype would feature a reinforced fuselage compatible with pressurisation and a wing that could accommodate four Napier Nomad turbo-compound engines. Several officials, including company chairman Alan Samuel Butler, favored the adoption of the new turbopropeller engines.Taylor 1970, p. 114. By the end of 1945, work had commenced on building structural test sections and jigs.
Into flight On 10 July 1947, the first prototype, registered G-AGUA, conducted the type's maiden flight, flown by chief test pilot George B.S. Errington. After the generally successful first flight, the prototype underwent a month of alterations, including deleting the spring tab and replacing it with geared tabs fitted to the outer rudders and locking the inner rudder; reversible propellers were also installed. During September 1947, the prototype made its first public display alongside elements of the second prototype, including its cockpit and control system, at the Society of British Aircraft Constructors (SBAC) show at Radlett.Taylor 1970, p. 115.
On 22 November 1947, the programme received a setback when the prototype was forced to perform a belly landing after the port undercarriage leg failed to deploy due to a loss of hydraulic pressure. This did not seemingly deter customer confidence, as the newly created operator British European Airways (BEA) continued to openly express their preference for the type, and in September they placed a £3 million order for 20 Ambassadors.Taylor 1970, p. 119. This support was so impactful that, according to aviation author H. A. Taylor, rival aircraft manufacturer Vickers was close to cancelling development of the Viscount over fears that airlines held a preference for advanced piston-powered airliners over the unfamiliar turboprop.Taylor 1970, pp. 115–116.
During flight testing, it was found that the aircraft possessed particularly favourable low speed characteristics. Having been explored in depth beforehand, this was dramatically displayed at the 1948 Farnborough Airshow, becoming the only aircraft in history to perform such an aerial display with one engine out throughout. This was a deliberate choice, with the inactive engine's propeller being feathered prior to taking off and remaining so until after landing.Taylor 1970, p. 117. The first prototype performed the majority of flight testing through late 1948.Taylor 1970, pp. 118–119.
A total of three prototypes were constructed for the test programme.Singfield 2000, p. 12. The second Ambassador, G-AKRD,Jackson 1973, p. 30. was the first to feature pressurisation and was equipped with a full passenger cabin configuration for demonstration flights. This second prototype was subsequently used by the Bristol Aeroplane Company from 1953 for flight-testing the Bristol Proteus 705 turbine engine. From March 1958, it was used by Rolls-Royce for testing the Dart and Tyne turboprops. The third prototype and first Ambassador 2 G-ALFR was initially used for BEA proving trials, and in 1955 it also supported the development trials of the Napier Eland turbine engine. During 1958, intensive simulated airline flying, to pave the way for BEA Vanguards, was performed using two Tyne-powered Ambassadors.Gunston 1989, p. 128.Taylor 1970, pp. 127–128.
During the latter stage of development, some issues were uncovered. The wing, which had been designed to induce laminar flow characteristics, was relatively unconventional in its design; testing revealed that it lacked sufficient strength, and redesigning the wing was not straightforward. Technical assistance was provided by de Havilland and the issue was proven to be resolved in December 1949. Three minor accidents during demonstration flights, while causing no serious damage, often delayed testing and did not inspire confidence amongst potential customers.Taylor 1970, pp. 119–121. Quantity production of the Ambassador commenced during 1950, leading to the first production standard aircraft, destined for BEA, performing its first flight on 12 January 1951.Taylor 1970, pp. 121–122.
Design The Airspeed Ambassador was an all-metal twin-engine airliner, designed to serve short-to-medium haul routes.Taylor 1970, p. 112. It was relatively advanced for the era, being relatively aerodynamically clean, having a high aspect–ratio wing with hoped-for near-Laminar flow characteristics, a tricycle undercarriage, a steerable nosewheel, integral fuel tanks, and other recent innovations.Taylor 1970, pp. 116–117. Distinctive external features of the Ambassador included its three low tailfins and a long pointed nose, giving it a resemblance to the larger transcontinental Lockheed Constellation. Its tricycle gear gave it a more modern appearance than contemporaries such as the DC-3, Curtiss Commando, Avro Lancastrian and Vickers Vikings that were common on Europe's shorter airline routes.
In a standard configuration, the Ambassador could accommodate up to 47 passengers; somewhat unusually, its passenger cabin could be either pressurised or not as the customer preferred. To reduce cabin noise levels, considerable effort went into soundproofing and general furnishings; Taylor described the Ambassador as being particularly quiet for a piston engine airliner, although also noting it could not equal the lower noise levels of turboprop-powered airliners such as the Vickers Viscount.Taylor 1970, pp. 117–118. It was typically powered by conventional Bristol Centaurus radial engines, although individual aircraft were reengined with various powerplants, including turboprops.Taylor 1970, p. 118.
To increase cruise performance, designers made great efforts to minimise drag, using a NACA laminar-flow wing section. The predicted low-drag performance was not completely realized, however, due to structural features for wing construction, propeller slipstream over the inner wing, and increased skin roughness in actual service conditions. The engine nacelles were initially designed with inwardly-opening louvres for exhaust gases and cooling air rather than the usual outwardly opening "gills". However, these proved inadequate for cooling the engine, thus the gills were reinstated. Engine-out climb performance was enhanced by being able to feather the propeller using a zero-torque signal from the engine.Boot 1990, pp. 25–26.
Operational history During 1952, British European Airways (BEA) introduced the first of its 20 strong fleet of Ambassadors to revenue service, the airline referred to them as the "Elizabethan Class" in honour of the newly crowned Queen Elizabeth II. The planned 1951 introduction date had been pushed to 1952 to address development issues. By May 1951, intense proving trials of the Ambassador were being conducted by a joint team of Airspeed and BEA employees, while official BEA training commenced in August of that year. The flagship of BEA's fleet was G-ALZN, appropriately named "RMA Elizabethan".Taylor 1970, p. 122. The first "Elizabethan" scheduled flight was from Heathrow Airport to Paris Le Bourget on 13 March 1952, soon the type was used across BEA's main UK routes.Taylor 1970, p. 125.