The Armstrong Whitworth A.W.27 Ensign was a British four-engine monoplane airliner and the largest airliner built in Britain during the Interwar period.Tapper 1988, p. 237.
The British airline Imperial Airways requested tenders for a large monoplane airliner with four Armstrong Siddeley Tiger engines in 1934. Armstrong Whitworth designed the Ensign to seat up to 40 passengers for the airline's European and Asian routes while also carrying airmail. It connected Britain with seaplane flights that continued on to Australia and South Africa. Early operations were hindered by mechanical problems and modifications only ever marginally improved performance and reliability.
During the Second World War, the Ensigns were operated by the British Overseas Airways Corporation (BOAC), which had been formed out of the merger of Imperial Airways and British Airways. The type would be flown between Britain and various locations in the Middle East, Africa and India, often in support of military operations. During 1940, two Ensigns were destroyed by enemy action, while one would be captured in 1942 and subsequently operated by the French. The Ensigns were retired following a final passenger flight in June 1946 and the remaining aircraft were scrapped the following year.
Development Background
The Ensign's origins can be traced to 1934 when Imperial Airways expressed a need for a large monoplane airliner powered by four Armstrong Siddeley Tiger radial engines. The airline was expanding and modernizing its fleet, driven in part by the obsolescence of its biplane airliners, such as the Handley Page H.P.42, and a new British Government policy stipulating that first class mail with the Empire travel by air. Imperial Airways also issued a specification for a large flying boat, which became the Short Empire. Imperial Airways approached Armstrong Whitworth directly and various configurations were examined including several low-wing monoplanes with two, three and four engines, but Imperial Airways' managing director insisted on a four-engine shoulder wing monoplane, as this was expected to be more popular with passengers.Tapper 1988, p. 235.
Imperial Airways paid £27,000 for the design work, and a further £43,300 for the construction of the first example on 22 September 1934, with delivery anticipated in 1936. In May 1935, an order for eleven more aircraft, at £37,000 each, was issued, and in December 1936, a final order for an additional two aircraft, priced at £39,766 13 shillings 4 pence each, brought the number of aircraft ordered to 14.
In October 1934, a final specification was drawn up with particular attention being paid to the positioning of the tailplanes in relation to the wing as well as avoiding wing flutter and tail buffeting. Mass-balanced flight controls were adopted to avoid flutter, while wing fillets smoothed airflow along the fuselage. The structure was built to meet military strength requirements.Tapper 1988, pp. 235-236. Performance guarantees required a cruise speed of at , and a take-off distance of under .Tapper 1988, p. 236.
During the late 1930s, production of Whitley bombers for the Royal Air Force had priority, which slowed work on the Ensign, and due to a lack of production capacity, assembly was not undertaken at Armstrong Whitworth's main factory in Coventry,Tapper 1988, pp. 236-237. but at another member of the Hawker-Siddeley group, Air Service Training in Hamble.
Throughout its development, changes were being requested by Imperial Airways, which delayed progress by up to two years, and the Ensign didn't make its first flight until 24 January 1938.Jackson, 1973,
The two Mark.IIs saw their service entry being further delayed by Imperial Airways and Armstrong Whitworth studying their use as piggyback motherships in the same manner as the Short Mayo Composite's Maia, carrying a smaller long-range parasite aircraft. However this idea was abandoned before any changes were completed.Ricco 2017, p. 18. Due to this delay, Everest only flew on 20 June 1940 and Enterprise did not make its first flight until 28 October.Ricco 2017, p. 18.
Flight testing and production The Ensign's first flight revealed heavy ailerons, and an excessively heavy rudder which was resolved with an adjustment to its servo. The undercarriage was retracted for the first time on the second flight and was subject to exhaustive tests before being passed to the Aeroplane and Armament Experimental Establishment (A&AEE) for Air Ministry testing.Tapper 1988, pp. 239-241. During testing, the elevators intermittently jammed while at altitude, which was resolved by modifying the connecting wire runs, to account for the fuselage contracting due to the cold. A disaster was narrowly averted on 8 March 1938, when all four engines cut out due to an incorrect fuel cock settings, and the aircraft glided to RAF Bicester, making a dead-stick landing.Tapper 1988, p. 241.
Flight testing of the prototype found its handling characteristics to be generally acceptable, although Royal Air Force pilots criticised its slow climb rate, which was attributable to the Tiger XI engines that left it underpowered. Upon completion of the flight testing, the type was issued a certificate of airworthiness, clearing it for operational use, following the completion of a redesign of the trailing edge covering, the installation of an engine-driven compressor to charge the brakes, throttle control locks, and additional passenger escape hatches.Tapper 1988, p. 242. Once completed, the aircraft made its first airline service flight between Croydon Airport and Paris, France on 24 October 1938.Tapper 1988, p. 243.
Design
The Ensign was a high-wing cantilever monoplane of light alloy construction and an oval fuselage with a conventional tailplane. The cantilever structure of the wings was built around a single massive internally braced box spar attached directly to both the front and rear ribs. The leading edge of the wing was metal, but aft of the box spar, the wing was covered by fabric, as were the tailplane and fin. Otherwise, it featured semi-monocoque construction, using a combination of stressed skin, transverse frames, and longitudinal stringers. The wing was recessed into the top of a very deep fuselage, while tubular girders provided the floor with the necessary strength.
The Ensign had a hydraulically retractable undercarriage which folded into the nacelles of the inner engines, and a castoring tail wheel. Considerable difficulties with the retraction mechanism were encountered and the wheels on the undercarriage were the largest to have been produced in the UK at that time,Tapper 1988, p. 278. with Dunlop wheels and tyres tall.NACA 1937, p. 7. While most components were designed and manufactured by Armstrong Whitworth, the 16 hydraulic jacks used for the flaps, undercarriage legs, locks and doors were Lockheed units sourced locally. Retraction was often uneven and unpredictable and took significantly longer than specified.
The Ensign was initially powered by four moderately supercharged Armstrong Siddeley Tiger XI 14 cylinder two-row radial engines of displacement mounted ahead of the wing leading edge on a tubular steel framework with flexible rubber mountings and enclosed in Siddeley long chord cowlings. These engines drove de Havilland three-blade two-position adjustable pitch propellers. When it was found to leave the aircraft underpowered, these were replaced when the aircraft were returned to Armstrong Whitworth to resolve numerous mechanical problems, with Armstrong Siddeley Tiger XIC engines,The main differences between the two engines was that supercharger boost on takeoff was raised from , and rocker arm bracket bolts were replaced with studs, a common point of failure on the initial version. and constant speed propellers although these didn't fully resolve the problem, and the final two aircraft ordered by Imperial Airways in 1936 were equipped with 9 cylinder single bank Wright GR-1820-G102A Cyclone geared radial engines and Hamilton constant speed propellers, as A.W.27A Ensign Mk.IIs and all eight surviving Mk.I aircraft were upgraded between 1941 and 1943 to Mk.II standard with the American engines.Flight, 1957, p.247 This improved performance finally allowed the Ensign to be used in hot climates, however production of the engine ended early in the war, and not only did finding spares become difficult, but the rate of climb still generated complaints by the pilots.Tapper 1988, p. 251. A boiler to produce steam was fitted to the engine exhaust, which was channelled through the aircraft to provide heat. An auxiliary petrol-driven generator supplied electricity for lighting and to recharge the aircraft's batteries while on the ground.
The cockpit seated two pilots, side-by-side who were provided with dual controls, along with a radio operator seated sideways behind the copilot, on the right side of the aircraft. The radio operator had a Marconi wireless transmitter/receiver, radio direction finding (RDF) equipment, and on empire routes, also operated a short-wave radio.NACA 1937, p. 6. It was intended for the Ensign to be operated by a crew of five with two pilots, a radio operator, a steward, and a flight clerk. On the Eastern route a second steward replaced the flight clerk.Tapper 1988, p. 279.
Eight aircraft were fitted for Empire routes and four for European routes. The former carried 27 passengers in three cabins or 20 sleeping in bunks, while the latter spread 40 passengers across four main cabins. Armstrong Whitworth claimed that the interior could be reconfigured between each type, including the installation of partition walls and curtains, in 15 minutes. Chair cushions became bunk bed mattresses, and could serve as flotation devices in a ditching. The centre section had a large freight compartment aligned with the engines to reduce noise penetration, a pantry, a lavatory on the starboard side, and a narrow corridor sometimes referred to as the 'promenade' deck between the forward and rear compartments. On Empire routes a flight clerk replaced a steward.Moss, January 1957, p.204 Aside from the interior setup chosen and the installation of a short-wave radio, there was little difference between the two variants.
Operational history
Imperial Airways named the prototype "Ensign", which also became the Class name for the type, as was their standard practice. Three more Ensigns – G-ADSS Egeria, G-ADST Elsinore and G-ADSU Euterpe were completed by Christmas 1938, and were dispatched to Australia with the holiday mail, but all three suffered mechanical problems that prevented them from reaching their destinations. Elsinore was returned home, over a distance of , with its undercarriage lowered as they could not be retracted. Consequently, Imperial Airways returned all five to Armstrong Whitworth, where control runs were modified, rudder area reduced, new constant-speed propellers and more powerful Armstrong Siddeley Tiger IXC engines were fitted to improve performance.Tapper 1988, p. 244. The modified aircraft were evaluated at Martlesham Heath, during which the type's rate of climb was improved while the use of automatic mixture controls, modifications to the priming system, the installation of constant speed propellers and the replacement of bolts with studs in the engine rocker arm brackets greatly improved reliability.