Short Stirling
The Short Stirling was a British four-engined heavy bomber of the Second World War. It has the distinction of being the first four-engined bomber to be introduced into service with the Royal Air Force during the war.
The Stirling was designed during the late 1930s by Short Brothers to conform with the requirements laid out in Air Ministry Specification B.12/36. Prior to this, the RAF had been primarily interested in developing increasingly capable twin-engined bombers, but had been persuaded to investigate a prospective four-engined bomber as a result of promising foreign developments in the field. Out of the submissions made to the specification, Supermarine proposed the Type 317, which was viewed as the favourite, whereas Short's submission, named the S.29, was selected as an alternative. When the preferred Type 317 had to be abandoned, the S.29, which later received the name Stirling, proceeded to production.
In early 1941, the Stirling entered squadron service. During its use as a bomber, pilots praised the type for its ability to out-turn enemy night fighters and its favourable handling characteristics, but its low ceiling was often criticised. The Stirling had a relatively brief operational career as a bomber before being relegated to second-line duties from late 1943, due to the increasing availability of the more capable Handley Page Halifax and Avro Lancaster, which took over the strategic bombing of Germany. Decisions by the Air Ministry on certain performance requirements had played a role in limiting the Stirling's performance; the 100 ft limit also affected earlier models of the Halifax though the Lancaster never adhered to it.
During its later service, the Stirling was used for mining German ports; new and converted aircraft also flew as glider tugs and supply aircraft during the Allied invasion of Europe in 1944–1945. In the aftermath of the Second World War, the type was rapidly withdrawn from RAF service, having been replaced in the transport role by the Avro York, a derivative of the Lancaster that had previously displaced it from the bomber role. Several ex-military Stirlings were rebuilt for the civilian market.
Development
Origins
In the 1930s, the Royal Air Force was interested primarily in twin-engined 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 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 favourable 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 carried to a range of or a lesser payload of to . 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, carrying three gun turrets for defence.
The aircraft should also be able to be used as a troop transport for 24 soldiers and be able to use catapult assistance for takeoff 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 runway and be able to clear trees at the end, a specification with which most small aircraft would have a problem 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 fulfil 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 favourable 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 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 criticised 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 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 wing and it had to be reduced to less than To get the needed lift from a shorter span and excess weight, the redesigned wing was thickened and reshaped. The wingspan was often said to be limited to 100 ft so the aircraft would fit into existing hangars, but the maximum hangar opening was and the specification required outdoor servicing. 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 was 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, 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.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 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 takeoff 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 takeoff; however, if the wing itself were modified, the aircraft would fly with a nose-down attitude while cruising ; 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 takeoff and landing accidents.
The S.31 also received the lengthened undercarriage to test this; subsequent trials found 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 takeoff accident at RAF Stradishall, Suffolk, in February 1944. Meanwhile, before 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, but on landing, one of the brakes locked, causing the aircraft to slew off the runway and the landing gear to collapse. The resulting damage caused the first prototype to be written off. 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.