Short Sunderland - Design

Design

The S.25 shared much in common with the S.23 but it had a deeper hull profile. As construction proceeded the armament was changed to a single 0.303 Vickers K machine gun in the nose turret and four 0.303 Browning machine guns in the tail. Then there was a change in the tail turret to a powered version and Gouge had to devise a solution for the resulting movement aft of the aircraft's centre of gravity. The unarmed prototype (K4774) first flew, on 16 October 1937. Following some flight trials it was modified with a wing sweepback of 4° 15' by adding a spacer into the front spar attachments. This moved the centre of lift enough to compensate for the changed centre of gravity. The modified K4774 flew on 7 March 1938 with Bristol Pegasus XXII engines of 1,010 hp (750 kW).

As with the S.23, the Sunderland's fuselage contained two decks with six bunks on the lower one, a galley with a twin kerosene pressure stove, a yacht-style porcelain flush toilet, an anchoring winch, and a small machine shop for inflight repairs. The crew was originally intended to be seven but increased in later versions to 11 crew members or more.

It was of all-metal, mainly flush-riveted construction except for the control surfaces, which were of fabric-covered metal frame construction. The flaps were Gouge-patented devices that slid backwards along curved tracks, moving rearwards and down, increasing the wing area and adding 30% more lift for landing.

The thick wings carried the four nacelle-mounted Pegasus engines and accommodated six drum fuel tanks with a total capacity of 9,200 litres (2,025 Imperial gallons, 2,430 U.S. gallons). Four smaller fuel tanks were added later behind the rear wing spar to give a total fuel capacity of 11,602 litres (2,550 Imperial gallons, 3,037 U.S. gallons), enough for eight- to 14-hour patrols.

The specification had called for an offensive armament of a 37 mm gun and up to 2,000 lb (910 kg) of bombs, mines or (eventually) depth charges. The ordnance was stored inside the fuselage in a 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 & Thomson FN-13 powered turret with four .303 British Browning machine guns in the extreme tail and a manually operated .303 on either side of the fuselage, firing from ports just below and behind the wings. These 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.

Portable beaching gear could be attached by ground crew so that the aircraft could be pulled up on land. The gear consisted of a pair of two-wheeled struts that could be attached to either side of the fuselage, below the wing, with a two- or four-wheel trolley and towbar attached under the rear of the hull.