Short S.29 Stirling
In 1936 the RAF also decided to investigate the feasibility of the four-engined bomber. The Air Ministry Specification B.12/36 had several requirements. 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) (incredibly demanding for the era). It had to cruise at 230 or more mph at 15,000 ft (4,600 m) and have three gun turrets (in nose, amidships and rear) 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. The idea was that it 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 able to clear 50 ft (15 m) trees at the end, a specification most small aircraft would have a problem with today.
Initially left out of those asked to tender designs, Shorts were included because they already had similar designs in hand and they had ample design staff and production facilities. Shorts were producing several four-engined flying boat designs of the required size and created their S.29 by removing the lower deck and boat hull of the S.25 Sunderland. The new S.29 design was largely identical otherwise: the wings and controls were the same, construction was identical and it even retained the slight upward bend at the rear of the fuselage, originally intended to keep the Sunderland's tail clear of sea spray.
In October 1936, the S.29 was low down on the shortlist of designs considered and the Supermarine Type 317 was ordered in prototype form in January 1937. However it was decided that an alternative design to Supermarine was needed for insurance and that 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 original Short design, including considering the use of the Bristol Hercules engine as an alternative to the Napier Dagger, increasing service ceiling (28,000 ft) and reducing the wingspan. Shorts accepted this large amount of redesign. The project had added importance due to the death of Supermarine's designer causing doubt in the Air Ministry. 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), the same limit as that imposed on the P.13/36 designs (Handley Page Halifax and Avro Manchester). In order 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. "The wing span was limited by the Air Ministry to 100 ft" but the maximum hangar opening was 112 ft (34 m) and the specification required outdoor servicing. The limitation was to force the designer to keep overall weight down.
The original layout of the bomber was tried out by the construction of a half-scale model S.31 fitted with four 97kW Pobjoy engines. Flying trials with this proved the feasibility of the design. Short had originally decided on an incidence of 3° giving the best possible cruise performance, but the RAF asked that the incidence be increased to 6.5°, being more concerned with improving take-off performance than the cruising speed. In order to accommodate the RAF request for increased wing incidence a major re-design of the central fuselage would have normally be undertaken, but because of time restraints, Short decided on a "quick fix" by lengthening the main landing gear legs to give a higher ground angle.
At the end of 1938, this change was incorporated on the Short S.31 prototype.
While testing with the S31/M4, construction began on two full size prototypes now officially known as the Stirling MkI/P1. Shortly after construction of the prototypes began, the Air Ministry decided to order the Stirling into production with a contract of 100 Stirling MkI's as the second string for the Supermarine 316 and formally ordered in October. 1938. The prototype S29 was rolled out of the company's Rochester factory on 13 May 1939.
Given the RAF serial number L7600, the prototype made its maiden flight on 14 May 1939 (with four Bristol Hercules II engines). After a graceful takeoff and short test flight it suffered an undercarriage failure on landing and was damaged beyond repair. The failure was traced to the light alloy undercarriage back arch braces which were replaced on succeeding aircraft by stronger tubular steel units.
The second prototype (L7605) was fitted with the strengthened undercarriage and made its maiden flight on 3 December 1939. For this flight the gear was left down, but happily for both Short and the RAF, the revised undercarriage held up when put to the tests of retraction, lowering and landing. During the spring of 1940, the prototype spent four months undergoing service tests at Boscombe Down.
Deliveries of production aircraft to the RAF began in August 1940. It was built initial-ly by the parent firm at Rochester and by Short and Harland at Belfast, where the Air Transport Auxiliary (ATA) opened No.8 Ferry Pool (FP) to clear them.
The first production version for the RAF was the Stirling I powered by four 1185kW Bristol Hercules XI radial engines and without a dorsal turret fitted. First in action in February 1941, the Stirling carried 7 tons of bombs for 590 miles, and was armed with eight machine guns. It went into service with 7 Squadron at Leeming in August 1940, and remained in pro-duction throughout the war. Prior to its first operational sortie, ATA is recorded as hav-ing ferried 12.
Short S.29 Stirling