Avro Tudor

British airliner with 4 piston engines, 1945

The Avro Type 688 Tudor was a British piston engined airliner based on Avro's four engine Lincoln bomber, itself a descendant of the famous Lancaster heavy bomber, and was Britain's first pressurised airliner. Customers saw the aircraft as little more than a pressurised DC 4, and few orders were forthcoming, important customers preferring to buy US aircraft. The tailwheel undercarriage layout was also dated and a disadvantage.

Avro began work on the Type 688 Tudor in 1943, following Specification 29/43 for a commercial adaptation of the Lancaster IV bomber, which was later renamed Lincoln. The specification was based on recommendations of the Brabazon Committee, which issued specifications for nine types of commercial aircraft for postwar use. Avro first proposed to build the Avro 687 (Avro XX), which was a Lincoln bomber with a new circular section pressurized fuselage and a large single fin and rudder in place of the predecessor's double ones. During the design stage, the idea of a simple conversion was abandoned and the Avro 688 was designed, which retained the four Rolls Royce Merlin engines. It was designed by Roy Chadwick who, due to wartime restrictions, could not design a completely new aircraft, but had to use existing parts, tools and jigs. Using the Lincoln's wing, Chadwick, who had worked on the Lancaster, designed the Tudor to incorporate a new pressurized fuselage of circular cross section, with a useful load of 3,765 lb (1,705 kg) and a range of 3,975 mi (6,400 km). Two prototypes were ordered in September 1944 and the first, G AGPF, was assembled by Avro's experimental flight department at Manchester's Ringway Airport and first flew on 14 June 1945. It was the first British pressurised civilian aircraft, although the prototype initially flew unpressurised. The prototype Tudor I had 1,750 hp (1,305 kW) Rolls Royce Merlin 102 engines, but the standard engines were 1,770 hp (1,320 kW) Merlin 621s.

The Tudor was a low wing cantilever monoplane with four engines, a single fin and rudder and a retractable tailwheel undercarriage (in its original configurations). The wing was of NACA 23018 section at the root, and was a five piece, all metal, twin spar structure. The untapered centre section carried the inboard engines and main undercarriage, while the inner and outer sections were tapered on their leading and trailing edges, with the inner sections carrying the outboard engines. The ailerons were fitted with trim and balance tabs, and there were hydraulically operated split flaps in three sections on each side of the trailing edges of the centre section and inner wings. A fuel capacity was given by eight bag tanks, one on either side of the fuselage in the centre section and three in both inner wings. The all metal tail unit had a dorsal fin integrated with the fuselage, and a 43 feet (13 m) twin spar tailplane with inset divided elevators. The control surfaces were mass balanced, and each had controllable trim and servo tabs. The circular cross section fuselage was an all metal semi monocoque structure, of 10 feet (3 m) diameter, fitted with kapok noise insulation above floor level. The hydraulically operated main wheel units were similar to those of the Lancaster, had single Dunlop wheels and retracted rearward into the inboard engine nacelles. The twin tailwheels retracted rearward into the fuselage and were enclosed by twin longitudinal doors.

The Tudor I was intended for use on the North Atlantic route. At the time, the United States had the Douglas DC 4 and Lockheed Constellation, which could both carry more passengers than the Tudor which only carried 12, and also weighed less than the Tudor weight of The Tudor tailwheel layout was also obsolete. Despite this, the Ministry of Supply ordered 14 Tudor I aircraft for BOAC, and increased the production order to 20 in April 1945. The Tudor I suffered from a number of stability problems, which included longitudinal and directional instability. The problem was handed over to the Royal Aircraft Establishment at RAE Farnborough, where an extensive programme of testing was carried out, the test pilot being Eric Brown. Following the RAE's recommendations, a larger tailplane was fitted, and the original fin and rudder were replaced by larger vertical surfaces. BOAC added to the delays by requesting more than 340 modifications, and finally rejected the Tudor I on 11 April 1947, considering it unacceptable for North Atlantic operations. It had been intended that 12 Tudors would be built in Australia for military transport, but this plan was abandoned. Twelve Tudor I aircraft were built, of which three were scrapped, while others were variously converted to the Tudor IVB and Tudor Freighter I configurations. As a result of all the Tudor I delays, BOAC – with the support of the Ministry of Civil Aviation – sought permission to purchase aircraft with known performance and reliability, such as the Lockheed Constellation and the Boeing Stratocruiser, for its Atlantic routes instead of the Tudor. Despite BOAC's reluctance to purchase Tudors, the Ministry of Supply continued to subsidize the aircraft.