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 initially 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 production throughout the war. Prior to its first operational sortie, ATA is recorded as having ferried 12.
Both the factories were bombed in the summer of 1940, after which production was further distributed to the Austin Motors shadow factory at Longbridge, Birmingham, and to a new Shorts factory at South Marston, Swindon.
The Stirling II, only a few of which were completed, was a conversion of the Mk I with Wright R-2600-A5B Cyclone engines. The Mk III had four 1,230kW Bristol Hercules XVI engines and featured a mid-upper turret.
In order to train pilots on the new aircraft, each Stirling squadron formed its own conversion flight and in December 1941, a training unit was created at Thruxton with the specific purpose of training ATA pilots who would have to ferry them. This unit moved to Hullavington in May 1942, to Marharn in August and to Stradishall in October.
When by 1943 the output of heavy bombers had risen to over 400 a month, more ATA four-engined rated pilots were urgently needed to move them. When in February 1943 an ATA Halifax training unit, which had been opened at Pocklington stood down, to fill the gap in four-engined training, ATA reverted to a previous arrangement for its pilots to be given conversion courses with 1647 Stirling Conversion Unit at Stradishall.
From 1943, when the Stirling was no longer a suitable bomber, unlike the Mk III, the Stirling IV was produced from new as a long-range troop transport and glider tug (Horsa glider), the nose and upper turrets being removed and replaced by fairings, although the four-gun tail turret was retained. Up to 24 paratroops or 34 airborne troops could be carried. The final version of the Stirling was the Mk V, an unarmed military transport and freighter with a redesigned nose.
Total production of the Stirling – the Mk III of which was the major variant – was about 2,380.
On the night of 7-8 April 1945, RAF Stirlings of 38th Group dropped two battalions of French parachutists, including both regular soldiers and members of the French Resistance, into Holland south of Groningen. The aim was to support the advance of the Canadian Second Division.
Stirling I
Max speed: 260 mph
Range: 1930 miles
Crew: 7/8
Armament: 8 x .303 Browing mg
Bombload: 14,000 lb
Stirling Mk III
Engines: 4 x Bristol Hercules XVI, 1230kW / 1627 hp
Max take-off weight: 31751 kg / 69999 lb
Empty weight: 19595 kg / 43200 lb
Wingspan: 30.2 m / 99 ft 1 in
Length: 26.59 m / 87 ft 3 in
Height: 6.93 m / 23 ft 9 in
Wing area: 135.63 sq.m / 1459.91 sq ft
Max. speed: 235 kt / 435 km/h / 270 mph
Cruising Speed: 200mph (323kmh)
Service ceiling: 5180 m / 17000 ft
Max range: 1747 nm / 3235 km
Range w/max.payload: 950 km / 590 miles
Range: 2,010 miles (3,242km) with 3,500lb (1,589kg) bombload
Armament: 8 x .303in / 7.7mm machine-guns
Max. bomb load: 14,000 lb / 6,350 kg
Crew: 7-8
All Aero 