Short Sunderland
The Short S.25 Sunderland is a British flying boat patrol bomber, developed and constructed by Short Brothers for the Royal Air Force. The aircraft took its service name from the town and port of Sunderland in North East England.
Developed in parallel with the civilian S.23 Empire flying boat, the flagship of Imperial Airways, the Sunderland was developed specifically to conform to the requirements of British Air Ministry Specification R.2/33 for a long-range patrol/reconnaissance flying boat to serve with the Royal Air Force. Sharing several similarities with the S.23, it had a more advanced aerodynamic hull and was fitted with various offensive and defensive armaments, including machine gun turrets, bombs, aerial mines, and depth charges. The Sunderland was powered by four Bristol Pegasus XVIII radial engines and was fitted with various detection equipment to aid combat operations, including the Leigh searchlight, the ASV Mark II and ASV Mark III radar units, and an astrodome.
The Sunderland was one of the most powerful and widely used flying boats throughout the Second World War. In addition to the RAF, the type was operated by other Allied military air wings, including the Royal Australian Air Force, Royal Canadian Air Force, South African Air Force, Royal New Zealand Air Force, French Navy, Norwegian Air Force, and the Portuguese Navy. During the conflict, the type was heavily involved in Allied efforts to counter the threat posed by German U-boats in the Battle of the Atlantic. On 17 July 1940, an RAAF Sunderland performed the type's first unassisted U-boat kill. Sunderlands also played a major role in the Mediterranean theatre, performing maritime reconnaissance flights and logistical support missions. During the evacuation of Crete, shortly after the German invasion of the island, several aircraft were used to transport troops. Numerous unarmed Sunderlands were also flown by civil operator British Overseas Airways Corporation, traversing routes as far afield as the Pacific Ocean.
During the post-war era, use of the Sunderland throughout Europe rapidly declined, while greater numbers remained in service in the Far East, where large developed runways were less prevalent. Between mid-1950 and September 1954, several squadrons of RAF Sunderlands saw combat action during the Korean War. Around a dozen aircraft also participated in the Berlin airlift, delivering supplies to the blockaded German city. The RAF continued to use the Sunderland in a military capacity up to 1959. In December 1960, the French Navy retired its aircraft, which were the last remaining examples in military use in the Northern Hemisphere. The type also remained in service with the RNZAF up to 1967, when they were replaced by the land-based Lockheed P-3 Orion. A number of Sunderlands were converted for use within the civil sector, where they were known as the Hythe and the Sandringham; in this configuration, the type continued in airline operation until 1974 – despite being originally made for military use, the Sunderland had a far longer commercial lifespan than its civilian Empire sibling and was one of the last large WWII-era flying boats in airline service. Several examples have been preserved, including a single airworthy Sunderland which has been placed on display in Florida at Fantasy of Flight.
Development
Origins
During the early 1930s there was intense international competition to develop suitable aircraft to operate new long-range intercontinental passenger service between the United Kingdom, the United States, France and Germany. It was recognised that the United Kingdom had no existing equivalent to the new American Sikorsky S-42 flying boats or the German Dornier Do X. Accordingly, in 1934, the British Postmaster General declared that all first-class Royal Mail sent overseas was to travel by air, establishing a subsidy for the development of intercontinental air transport in a fashion similar to the U.S. domestic programme a decade earlier. In response, Imperial Airways announced a competition to design and manufacture a fleet of 28 large flying boats, each weighing and having a range of with a capacity for 24 passengers. A corresponding contract was issued to Short Brothers of Rochester for their design, which became the S.23 Empire.While the Empire flying boat has often been credited as a predecessor of the Sunderland this impression is not strictly true. During November 1933 the British Air Ministry released Specification R.2/33, which called for the development of a next-generation long-range general purpose flying boat, intended to perform ocean reconnaissance missions. The specification called for an aircraft, either a monoplane or biplane, which would have a performance equal to the recently constructed Short Sarafand flying boat, but powered by a maximum of four engines and much more compact than the Sarafand. Specification R.2/33 was released before the publication of the commercial Imperial Airways requirement; by the time that Short received Imperial Airways' request the company had already started planning the design of the prospective military flying boat. After reviewing both sets of requirements, Short decided to prioritise the development of the civil S.23 design but also to work on a response to specification R.2/33.
Chief designer Arthur Gouge had originally intended for a COW 37 mm gun to be mounted in the bow of the craft to accompany the single Lewis gun installed in its tail. As with the S.23, he aimed to produce a fuselage which generated the lowest amount of drag possible, while a much longer nose than had been used for the S.23 was ultimately adopted.
Selection
During October 1934, Shorts settled upon the general configuration and geometry of the design, opting for a four-engine shoulder-wing monoplane configuration, similar to the Short Empire which had been ordered at the same time. The military flying boat design received the internal designation of S.25. While the S.25 design bore a strong resemblance to the civil S.23, it had an improved aerodynamic form, and sheet metal with curvature in more than one direction. This compound curve was more complex to manufacture but gave a superior shape.During late 1934 the S.25 proposal was submitted to the Air Ministry. Rival firm Saunders-Roe had also designed and submitted its own proposal, the Saro A.33, in response to Specification R.2/33. Following evaluation of the submissions, the Ministry decided to place orders for the production of prototypes of both the S.25 and A.33.
During April 1936 the Air Ministry was sufficiently confident in Shorts' submission to place a contract for an initial batch of 11 further aircraft. On 4 July 1936, the first of the Empire flying boats to be built, G-ADHL, named Canopus, made its first flight which confirmed the basic principles of the S.25's design. while the crucial final design conference was held around the same time as the flight. The sole A.33 was damaged beyond repair due to a structural failure, resulting in the S.25 being the only candidate.
As construction of the prototype S.25 proceeded, several design changes were made. In response to feedback from Air Ministry and the Royal Air Force, it was decided to change its defensive armament to a single 0.303 Vickers K machine gun in the nose turret and four 0.303 Browning machine guns in the tail turret. The tail turret was also changed to a powered version; Gouge therefore had to devise a solution to account for the resulting movement aft of the aircraft's centre of gravity, which was initially achieved via the presence of ballast positioned in the forward area. By the end of September 1937, the prototype had been completed.
Flight testing
On 16 October 1937, the prototype made its maiden flight, fitted with Bristol Pegasus X radial engines each capable of producing of power. The more powerful Pegasus XXII was unavailable at the time. Flown by Shorts' chief test pilot John Lankester Parker and Harold Piper, the initial flight lasted for around 45 minutes; later that day, a second flight of a similar duration was made. Parker later declared his satisfaction with the basic design. Prior to the first flight, the type had received the name Sunderland.Following early flight trials the aircraft was returned to the workshop where it underwent further modifications; the adoption of a wing sweepback of 4° 15' was achieved by the addition of a spacer into the front spar attachments. This was a response to the changes in defensive armament, and moved the centre of lift enough to compensate for the altered centre of gravity; further alterations were necessary to maintain the hydrodynamics properties. On 7 March 1938, K4774 made its first post-modification flight, having been fitted with the intended Bristol Pegasus XXII radial engines, each one capable of producing.
On 21 April 1938, the first aircraft of the development batch made its first flight. By this point, manufacturer testing of the prototype had already been completed and the prototype had been transferred to the Seaplane Experimental Station at Felixstowe, Suffolk for evaluation by the Marine Aircraft Experimental Establishment ; on 8 March 1938, it was joined by the second production aircraft. On 28 May 1938, this second aircraft, which had been cleared for operations under tropical conditions, flew a record-breaking flight to Seletar, Singapore, stopping off at Gibraltar, Malta, Alexandria, Habbaniyah, Bahrain, Karachi, Gwalior, Calcutta, Rangoon, and Mergui.
Testing showed that the aircraft could be fully refuelled in 20 minutes, and that its most economical cruising speed was about at. At this speed and altitude, a consumption rate of gave the aircraft an endurance of 18 hours, during which it could cover. The take-off distance was found to be.
Design
Overview
The Short S.25 Sunderland was a large four-engined flying boat developed for military use. The design of the S.25 shared much in common with the civil-orientated S.23, principally differing in its use of a deeper hull profile. As with the S.23, the interior of the Sunderland's fuselage contained two individual decks; the lower deck contained a total of six bunks, along with a galley outfitted with a twin kerosene pressure stove, a yacht-style porcelain flush toilet, an anchoring winch, and a small machine shop for performing inflight repairs. The crew was originally intended to total seven members; this was subsequently increased for later versions of the Sunderland to around 11 crew members and sometimes greater, dependent upon the specific mission being undertaken.The Sunderland featured all-metal, mainly flush-riveted construction, except for the flight control surfaces, which used a fabric-covered metal frame construction. Of these, the flaps employed unusual Gouge-patented devices that slid backwards along curved tracks, moving rearwards and downwards to increase the wing area and generating 30% greater lift for landing. The thick wings, upon which the aircraft's four nacelle-mounted Bristol Pegasus XXII radial engines were carried, also accommodated a total of six drum-style fuel tanks, which had a total capacity of 9,200 litres. In addition to the main fuel tanks, an arrangement of four smaller fuel tanks was installed behind the rear wing spar later on; with the extra tanks fitted, the Sunderland had a combined total fuel capacity of 11,602 litres, which was enough to enable the type to conduct eight- to 14-hour patrols.
The specification to which the Sunderland was developed to conform with had called for an offensive armament of a 37 mm gun and up to of bombs, mines or depth charges. The ordnance was stored inside the fuselage in a purpose-built bomb room and was winched up to racks, under the wing centre section, that could be traversed out through doors on each side of the fuselage above the waterline to the release position. Defensive armament included a Nash & Thompson FN-13 powered turret with four.303 British Browning machine guns in the extreme tail and a pair of manually operated.303 set on either side of the fuselage, firing from ports just below and behind the wings. These machine guns were later upgraded to 0.5-inch calibre Brownings. There were two different nose turret weapons, the most common, later, being two Browning machine guns. The nose weapons were later augmented by four fixed guns, two each side, in the forward fuselage that were fired by the pilot. Much later, a twin-gun turret was to be dorsal-mounted on the upper fuselage, about level with the wing trailing edge, bringing the total defensive armament up to 16 machine guns.