Development
Origins-
In the 1930s, the Royal Air Force (RAF) was interested primarily in twin-engine bombers. These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. Power limitations were so serious that the British invested in the development of huge engines in the 2,000 horsepower (1,500 kW) class to improve performance. During the late 1930s, none of these were ready for production. The United States and the Soviet Union were pursuing the development of bombers powered by arrangements of four smaller engines, the results of these projects proved to possess favorable characteristics such as excellent range and fair lifting capacity and in 1936, the RAF also decided to investigate the feasibility of the four-engined bomber.
The Air Ministry published Specification B.12/36, for a high-speed, long-range four-engined strategic bomber aircraft, that would be capable of being designed and constructed at speed. The bomb load was to be a maximum of 14,000 lb (6,350 kg) carried to a range of 2,000 miles (3218 km) or a lesser payload of 8,000 lb (3,629 kg) to 3,000 miles (4,800 km) (very demanding for the era). It was to have a crew of six and was to have a normal all-up weight of 48,000 lb, while a maximum overload weight of 65,000 lb was also envisioned. The aircraft would have to be capable of cruising at speeds of 230 mph or greater while flying at 15,000 ft (4,600 m), carrying three gun turrets (located in the nose, amidships and rear positions) for defense.
The aircraft should also be able to be used as a troop transport for 24 soldiers and be able to use catapult assistance for take off when heavily laden. The concept was that the aircraft would fly troops to far corners of the British Empire and then support them with bombing. To help with this task as well as ease production, it needed to be able to be broken down into parts, for transport by train. Since it could be operating from limited "back country" airfields, it needed to lift off from a 500 ft (150 m) runway and be able to clear 50 ft (15 m) trees at the end, a specification most small aircraft would have a problem with today. Aviation author Geoffrey Norris observed that the stringent requirements given in the specification for the prospective aircraft to be able to make use of existing infrastructure, specifically the specified maximum wingspan of 100 feet, adversely affected the Stirling's performance, such as its relatively low ceiling and its inability to carry anything larger than 500 lb bombs.
In mid 1936 Specification B.12/36 was sent out to Supermarine, Boulton Paul, Handley Page and Armstrong Whitworth. In August the Specification was issued to the rest of British industry. Left out of those asked to tender designs, Shorts were later included because the company already had similar designs in hand while possessing ample design staff and production facilities to fulfill production commitments. Shorts were producing several four-engined flying boat designs of the required size and created their S.29 proposal by removing the lower deck and boat hull of the S.25 Sunderland. The new S.29 design was similar to the Sunderland: the wings and controls were the same, construction was identical and it even retained the slight upward bend at the rear of the fuselage, which had originally been intended to keep the Sunderland's tail clear of sea spray. As originally designed, the S.29 was considered to be capable of favorable high-altitude performance.
Following a Tender Design Conference in October 1936, the S.29 was low down on the short list of designs considered. Vickers Type 293 submission was first followed by the Boulton Paul P.90, Armstrong Whitworth's AW.42, the Supermarine Type 316, and then the Short S.29. The Supermarine was ordered in prototype (two aircraft) form as the revised Supermarine Type 317 in January 1937. An alternative design to the Supermarine was needed for insurance and Shorts should build it as they had experience with four-engined aircraft. The original design had been criticized when considered and in February 1937 the Air Ministry suggested modifications to the design, including considering the use of the Bristol Hercules radial engine as an alternative to the Napier Dagger inline, increasing service ceiling to 28,000 ft (carrying a 2000 lb of bombs) and reducing the wingspan. Shorts accepted this large amount of redesign work. The project had added importance due to the death of Supermarine's designer, Reginald Mitchell, which had generated doubt within the Air Ministry about the ability of Supermarine to deliver. Two prototypes were ordered from Shorts.
The S.29 used the Sunderland's 114 ft (35 m) wing and it had to be reduced to less than 100 ft (30 m). To get the needed lift from a shorter span and excess weight, the redesigned wing was thickened and reshaped. It is often said that the wingspan was limited to 100 ft so the aircraft would fit into existing hangars but the maximum hangar opening was 112 ft (34 m) and the specification required outdoor servicing. "The wing span was limited by the Air Ministry to 100 ft". The wingspan limit was a method of stopping aircraft from being too large. In June 1937, the S.29 was accepted as the second string for the Supermarine Type 317 and formally ordered in October; Shorts and Supermarine were issued with instructions to proceed.
Prototypes-
The Air Ministry issued Shorts with contract number 672299/37, under which a pair of prototype S.29s were ordered. However, prior to this, Shorts had decided to undertake a successful practice which had been performed with the earlier Empire flying boat in producing a half scale version of the aircraft, known as the S.31 (also known internally as the M4 – as per the title on the tailfin), to prove the aerodynamic characteristics of the design. The S.31, which was largely composed of wood, was powered by an arrangement of four Pobjoy Niagara engines and featured a retractable undercarriage, operable bomb bay doors, and other measures to realistically represent the larger production aircraft. It was constructed at Short's Rochester facility.
The Short S.31 half-scale testbed used for aerodynamic tests of the Stirling design
On 19 September 1938, the S.31 conducted its maiden flight, piloted by Shorts' Chief Test Pilot J. Lankester Parker. Impressed with its performance, on 21 October 1938, Parker flew the S.31 to the RAF Martlesham Heath, Suffolk, where it was evaluated by the Aeroplane and Armament Experimental Establishment and received mostly favourable reviews. There was one notable criticism amongst the feedback from pilots, being that the length of the take off run was considered to be excessive and that improvements would be desirable. Fixing this required that the angle of the wing to be increased for take off; however, if the wing itself was modified, the aircraft would fly with a nose-down attitude while cruising (as in the Armstrong Whitworth Whitley); making this change was also complicated by the fact that work on the production line had already reached an advanced stage. Thus, Shorts lengthened the undercarriage struts to tilt the nose up on take-off, leading to its spindly gear which in turn contributed to many take off and landing accidents.
The S.31 also received the lengthened undercarriage in order to test this; subsequent trials found that there was no need for further modification in this respect. Other modifications made included the adoption of a larger tailplane with conventional elevators to improve aft controllability. The sole S.31 was scrapped after a take off accident at RAF Stradishall, Suffolk in February 1944. Meanwhile, prior to the maiden flight of either of the prototypes had flown, the Air Ministry had decided to order the S.29 into production "off the drawing board" in response to reports of further increases in strength on the part of the German Luftwaffe.
On 14 May 1939, the first S.29, which had by this point received the service name "Stirling" after the Scottish city, performed its first flight. The first prototype was outfitted with four Bristol Hercules II radial engines, and was reported as having satisfactory handling in its two months of flying. However, the entire program suffered a setback when the first prototype suffered severe damage and was written off as a result of a landing accident, in which one of the brakes locked, causing the aircraft to slew off the runway and the landing gear to collapse. A resulting redesign of the undercarriage led to substantially stronger and heavier struts being installed upon the second prototype. On 3 December 1939, the second prototype made its maiden flight. During its first sortie, one of the engines failed on takeoff but the second prototype managed to land with relative ease.
Production-
Prior to the Munich Agreement of 1938, Shorts had received a pair of orders for the Stirling, each for the production of 100 aircraft; however, as a result of Munich, the Ministry of Aircraft Production (MAP) enacted 'Plan L', under which Stirling orders were rapidly increased to 1,500 aircraft. In addition to contracts extending the projected work at Rochester and Belfast; some of the additional contracts were placed with Austin Motors to be produced at their Longbridge facility and with Rootes, who were to manufacture the type at their new shadow factory in Stoke-on-Trent. At its height, manufacturing activity on the Stirling was being performed at a total of 20 factories. According to Norris, while the aircraft's design had incorporated an inherent ability for production of the Stirling to be broken down: in practice, strict supervision of the work remained necessary. In order to coordinate the dispersed production approach adopted for the Stirling, Shorts and MAP operated a traveling team of 600 production engineers and draughtsmen that routinely travelled throughout the United Kingdom to the manufacturing facilities involved.
On 7 May 1940, the first production Stirling, conducted its first flight. According to Norris, initial rates of production were disappointing, and were in part due to delays in the delivery of machine tools and forgings. It has also been alleged that production of the Stirling was negatively impacted by a decision by Lord Beaverbrook, the Minister of Aircraft Production, which had ordered a change in priority from four-engined bombers towards fighters and twin-engined aircraft to replace those lost during the Battle of Britain. In August 1940, series production of the Stirling commenced at the Rochester factory.
Production of the Stirling was delayed by the ongoing bombing campaign of the Luftwaffe. The area, which included a number of major aviation firms, was heavily bombed in the opening days of the Battle of Britain, including one famous low-level raid by a group of Dornier Do 17s. A number of completed Stirlings were destroyed on the ground and the factories were heavily damaged, setting back production by almost a year. Some production was moved to Austin's Longbridge factory at Cofton Hackett just south of Birmingham, the Longbridge production line eventually produced nearly 150 Stirlings.
From this point on, the Belfast factory became increasingly important as it was thought to be well beyond the range of German bombers. However, Belfast and the aircraft factory were subjected to bombing by German aircraft during the Easter week of 1941. To meet the increased requirement for its aircraft during the war, satellite factories near Belfast were operated at Aldergrove and Maghaberry, producing 232 Stirlings between them. In 1940, bombing damaged Supermarine's factory at Woolston and the incomplete Type 316 prototypes. In November 1940, development of the 316 was formally cancelled, leaving the Stirling as the only B.12/36 design.
The first few Stirling Mk.Is were furnished with Bristol Hercules II engines, but the majority were built with more powerful 1,500 hp (1,100 kW) Hercules XI engines instead.
Proposed developments-
Even before the Stirling went into production, Short had improved on the initial design with the S.34 in an effort to meet specification B.1/39. It would have been powered by four Bristol Hercules 17 SM engines, optimised for high-altitude flight. The new design featured longer span wings and a revised fuselage able to carry dorsal and ventral power-operated turrets each fitted with four 20 mm Hispano cannons; despite the obvious gains in performance and capability, the Air Ministry was not interested.
In 1941, it was decided that the Stirling would be manufactured in Canada and an initial contract for 140 aircraft was placed. Designated as the Stirling Mk.II, the Hercules engines were to be replaced by 1,600 hp (1,193 kW) Wright GR-2600-A5B Twin Cyclone engines; a pair of prototypes were converted from Mk.I aircraft. However, it was decided to cancel the contract in favor of manufacturing other aircraft; thus, no production Mk.IIs were ever completed.
Shorts also pursued the development of the Stirling for potential use on the civil market. Designated S.37, it was a full-furnished transport aircraft that was capable of seating 30 passengers and was constructed to conform with civilian standards. A single prototype, known as the Silver Stirling, was converted from a Mk.V aircraft; however, partially due to greater levels of interest being expressed for a more promising civil version of the Handley-Page Halifax, the proposal met with little official interest.
In 1941, Short proposed the development of a new variant of the Stirling, the S.36, which was nicknamed "The Super Stirling" in a company publication. This aircraft would have featured a wing span of 135 ft 9 in (41.38 m), four Bristol Centaurus radials and a maximum takeoff weight of 104,000 lb (47,174 kg). The projected performance estimates included a speed of 300 mph (483 km/h) and a 4,000 mile (6,437 km) range, along with a weapons load of 10,000 pounds (4,500 kg) over 2,300 miles (3,700 km), or 23,500 pounds (10,700 kg) over 1,000 miles (1,600 km). The defensive armament of the S.36 was to be an assortment of ten .50 BMG machine guns that were set into three turrets.
The S.36 was initially accepted for testing under Specification B.8/41, which had been specifically written to cover the type, and an order for a pair of prototypes was placed. However, Arthur Harris, as commander of Bomber Command, felt that achieving bulk production of the type would take too much time and that the effort would be better expended on outfitting the existing design with improved Hercules engines with the aim of providing a higher operational altitude ceiling. However, despite the Air Staff having initially found the proposal to have some attraction, it was eventually decided to favor increased production rates of the rival Avro Lancaster instead. In May 1942, Shorts were informed that the Air Ministry would not be continuing the project; in August 1942, Shorts decided to terminate all work.
