The Stirling was a World War II heavy bomber design built by Short Brothers. It was the first British design meant to use four engines from the outset, an otherwise successful experiment that was hampered by a number of odd design requirements. In the end it would be relegated to second-line duties, while four-engine conversions of earlier twin-engine designs took over its role. Through the 1930’s the Royal Air Force was interested primarily in twin engine bombers. These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. However. the limitations in terms of power were so serious that they invested heavily in development of huge engines in the 2,000 horsepower class in order to improve performance. Meanwhile the US and USSR were developing bombers with four smaller engines instead, which proved to have excellent range and fair lifting capacity. So in 1936 the RAF decided to try their hand at the four engine bomber as well. It wasn’t entirely clear what they wanted in an aircraft, and the resulting Specification B.12/36 was an odd mix of features. In addition to a 14,000 lb (6,350 kg) bombload carried to a range of 3,000 miles (4,800 km)(incredibly demanding for the era), the aircraft should also be able to be used as a troop transport for 100 soldiers. 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 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. In addition, so it would fit into existing hangars, the wingspan should not exceed 100 ft (30 m). This last requirement was particularly odd, as the most common Type C hangar opened to over 125 ft (38 m). Shorts was one of eleven designs returned in response to the requirements, but were likely the only company that could have realistically started production in a short time. They had several four-engine flying boat designs of the needed size, and created their S.29 design by basically removing the lower deck and boat hull of their S.25 Sunderland. The S.29 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. Shorts managed to persuade the RAF to dismiss most of its unrealistic design goals when they saw that the S.29 would be an excellent bomber. But on one point the RAF stood firm: the S.29 used the Sunderland’s 114 ft (35 m) wing, and they demanded it be reduced to less than 100 ft (30 m). In order to get the needed lift, the resulting redesigned wing was thickened and reshaped. Shorts first constructed a half scale version as the S.31, which flew in September 1938. Everyone was happy with the design in general, with the exception of the takeoff run was thought too long. Fixing this required the angle of the wing to be increased for takeoff, meaning the aircraft would be flying nose down while cruising. Instead, Shorts lengthened the landing gear to tilt the nose up more on the runway, leading to its spindly gear. The first S.29, now known as the Stirling, took to the air in May 1939 with four Bristol Hercules engines. Upon landing one of the brakes locked, causing it to skew off the runway and the landing gear collapsed. A redesign added a much stronger and heavier set of gear on the second prototype. On its first mission two months later one of the engines failed on take-off, but the plane landed easily. From then on the record improved and service production started in August 1940 at Shorts’ factory. Several were destroyed during Battle of Britain raids, and operational status wasn’t reached until January 1941. Although it wasn’t as large as the US and Soviet experimental designs, the Stirling had considerably more power and far better payload/range than anything then flying. The massive 17,000 lb (7,700 kg) bombload put it in a class of its own, double that of any other bomber. In fact it was larger than the Avro Lancaster and Handley-Page Halifax which would eventually replace it, but both of these were originally designed to have twin engines. The design mounted nose and tail turrets (the later was notable for the wide angles it could point its guns), and included a retractable ventral turret just behind the bomb bay that was fired by remote control. This proved to be almost useless due to the limited visibility through the sighting system, with the added distraction that the turret tended to drop when taxing over bumps and hit the ground. It was removed almost from the start, to be replaced by a new dorsal turret instead. This turret had its own problems: it had a metal back with an escape hatch in it, but it turned out to be almost impossible to use. The later Stirling Mk.III instead used a fully glazed turret that removed the hatch, and had more room and an improved view. The first few Mk.I’s received the Hercules II, but the majority received the 1,500 hp (1.100 kW) Hercules XI. The Mk.III was similar with the exception of the new dorsal turret and the improved 1,635 hp (1.2 MW) Hercules VI or XVI engines, which improved maximum speed from 255 to 270 mph (410 to 435 km/h). The Stirling proved to be an excellent design and pilots were delighted to discover that it could outturn the Junkers Ju 88 and Messerschmitt Bf 110 nightfighters they faced: a side effect of that thick wing. Another side effect of the thick wing was poor altitude performance, and many missions were flown as low as 12,000 ft (4,000 m). This made the plane particularly difficult to use in many scenarios, notably if they were attacking Italy and had to fly over the Alps. When operated along with other RAF bombers flying at higher altitudes, the Luftwaffe ignored the latter and concentrated on the low-flying Stirlings. Within five months of being introduced, 67 out of the 84 aircraft delivered had been lost to enemy action or written off after crashes. In addition the Stirling’s huge bomb load was only able to be delivered at short ranges. On typical missions into Germany or Italy a smaller 3,500 lb (1,600 kg) load was carried, consisting of seven 500 lb (230 kg) bombs. This was the sort of load being carried by the RAF’s medium bombers such as the Vickers Wellington, and by 1944 by the deHavilland Mosquito. Perhaps the biggest problem with the design was that the bomb bay had dividers running down the middle, limiting it to carrying 500 lb (230 kg) bombs at the largest. As the RAF started using the 4000 lb (1,800 kg) ‘cookies’, and even larger ‘specials’, the Stirling became less useful to them. By March of 1943 the Stirlings were already being phased out of bomber duty, seeing more use for dropping mines outside German ports, and dropping spies deep behind enemy lines at night (through the now unused ventral turret ring). In 1944 a new Mk.IV series was built without the nose and dorsal turrets for towing gliders and dropping paratroops, and the otherwise similar Mk.V removed the tail turret as well. Total production : 2383.

Serials

Mk I

Mk III

Mk IV

Mk V

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