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During the 1920's and the 1930's the development of aircraft capable of spanning the world's oceans became important for long distance overseas flight. With more aviation routes flying into less developed areas, often without airports, aircraft such as the flying boat and amphibian became more valuable. The success of the China Clipper flights in the late 1930's demonstrated the potential of flying boats. These planes also had a military potential, which paralleled their civil use. During the course of this blog, we'll look at the development of one such flying boat, which entered service at a critical time-the Short Sunderland.

During the early 1930's, 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. Britain was behind the curve in flying boat development at the time, since they had no existing equivalent aircraft to the new United States Sikorsky S-42 flying boats or the German Dornier Do X. Acting in support of the United Kingdom aircraft industry, 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 manner similar to the U.S. domestic program a decade earlier. In 1934, Imperial Airways announced a competition to design and manufacture a fleet of 28 large flying boats, each weighing eighteen long tons with a capacity of 24 passengers and a range of 700 miles. A compatible contract was sent to Short Brothers of Rochester for their design, which became the S 23 Empire. Among a number of historians, the Empire flying boat has been credited as a predecessor of the Sunderland, aviation author, Geoffrey Norris, stated this was not completely true. In 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 2/33 specification proposed an aircraft, either a monoplane or biplane, to have performance similar to the recently developed Short Sarafand flying boat, along with other requirements, including the new aircraft be powered by a maximum of four engines and that it be a more compact plane than the Sarafand.

In October of 1934, Short decided upon the concept of the design, opting for a four-engine shoulder-wing monoplane configuration, similar to the Short Empire (S 23) , which was ordered concurrently with the Sunderland (S 25 ). Later in 1934, another proposal was submitted to the Air Ministry from rival firm Saunders-Roe. The Saunders-Roe entry, the Saro A 33, was a response to the requirements of Specification R 2/33. After reviewing both the Short and Saunders-Roe proposals the following year, the Air Ministry authorized flight tests of both aircraft in support of a detailed evaluation, after which a production order would be contracted with one of the competitors. In April 1936, the Air Ministry decided on the S 25 (Sunderland) flying boat after a number of tests, with the initial contract specifying the delivery of eleven aircraft. The S 25 aircraft was awarded the production contract, in part due to a structural failure in an A-33 aircraft during the latter part of the flight tests. The Air Ministry awarded the production contract to Short for the S 23 (Empire), the civil variant of the Sunderland in July 1936. In October 1937, initial flight tests of the initial prototype aircraft began. Flown by Short's chief test pilot John Lankester Parker and Harold Piper, the initial flight was about 45 minutes duration, followed by a second flight the same day. Parker was well pleased with the design of the S 25, which was now called the Sunderland. Though the first aircraft flew with Bristol Pegasus X engines capable of 950 hp. each, the first production aircraft, which flew in May 1938 was powered by four Pegasus XII radial engines, each generating 1,010 hp. This plane flew a record-breaking flight from the Seaplane Experimental Station at Felixstowe to Seletar Airfield in Singapore, with stops at Gibraltar, Malta, Alexandria, Habbiniyah, Bahrain, Karachi, Gwalior, Calcutta, Rangoon and Mergui. Results during the flight proved the Sunderland could be fully refueled in 20 minutes, with an optimum cruising speed of 150 mph. at an altitude of 2,000 ft., consuming approximately 110 imperial gallon per hour, giving the plane a range of 2,750 miles with an endurance of eighteen hours. The take-off distance was about 1,800 ft.

The Sunderland was a well armed plane for its mission. It housed a nose turret, initially mounting a single .303 machine gun (two on later models) with four .303 machine guns in the tail turret with four fixed .303 waist guns, later replaced by .50 caliber machine guns. A dorsal turret was later mounted on the upper fuselage, bringing the total to 16 machine guns. Up to 2,000 lbs. of depth bombs or mines could be stored on racks in the bomb room, just underneath the wings. The Sunderland's crew ranged from seven to twelve, depending upon the mission. The Sunderland began World War II by rescuing the entire 34-man crew of the merchant ship Kensington Court in September 1939. While the .303 machine guns may have lacked the power of larger bore guns, such as the .50 caliber and 20mm cannon, they still gave a good account. In April 1940, a Sunderland flying off Norway was attacked by six Junkers JU-88 fighters. The Sunderland shot one fighter down, damaging a second, sending it to a forced landing, while forcing the remaining four to flee. Sunderlands also proved themselves in the Mediterranean Theatre, carrying out a number of evacuations from the German invasion of Crete while under fire. One flying boat flew the reconnaissance mission to observe the Italian fleet at anchor in Taranto before the Royal Navy Fleet Air Arm attack on November 11, 1940, which wiped out most of the ships at the anchorage. In the Atlantic, a Sunderland performed the first unassisted U-boat sinking on July 17, 1940.

As the war progressed, new weapons made the flying boats more lethal. While Sunderlands carried the relatively light 100 lb. anti-submarine bomb at the beginning of the war, these were replaced in early 1943 by more larger torpex-filled depth bombs that would sink to a predetermined depth and then explode. While Sunderlands rarely used the high-powered Leigh searchlights, the flying boat's ASV Mark II radar enabled the Sunderlands to attack U-boats on the surface. The Germans responded by equipping their submarines with a radar warning system known as Metox, which was tuned to the ASV frequency, giving U-boats an early warning that an aircraft was in the area. Though U-boat kills fell off for a time, with the introduction of the ASV III radar in early 1943, German losses began to increase again, due to the Metox system being unable to pick up centimetric radar pulses of the ASV III radar. The ASV III was so effective that Admiral Karl Donitz, commander of the U-boat arm, believed British spies were furnishing details of U-boat movements to the Royal Navy. The ASV III system also forced the U-boats to fight it out on the surface, increasing their anti-aircraft armament to one or two 37mm and twin quad 20mm flak guns to shoot it out with the attackers.

After the war, Sunderlands were removed relatively quickly from Europe and sent to the Far East, where well developed runways were less common and new maritime patrol aircraft, such as the Avro Shackleton weren't fully compatible with the smaller runways. However, the Sunderland returned to European service as part of the Berlin Airlift of 1948-49. Some ten Sunderlands and two transport variants, known as Hythes, were utilized to transport goods from Finkerwerder, on the Elbe near Hamburg to Berlin, landing on the Havel river beside RAF Gatow, until iced over. The Sunderlands were of particular value in transporting salt, since their airframes were already were already protected against corrosion from seawater. Carrying salt in a land-based aircraft could be risky, in the event of a spill. Sunderlands were active during the Korean War, performing daring air-sea rescue missions from Japan during the entire war. Sunderlands operated by the Royal Navy were phased out in 1959, with those flown by the French Navy retired in December 1960. Sunderlands were also flown by the Royal New Zealand Navy, and these saw service until 1967. Whatever its short comings may have been as an aircraft, the Short Sunderland was long on service.

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