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Short-stirling-bomber-02

Short Stirlings in formation[1]

The Short S.29 Stirling was a four engine heavy bomber used by the RAF during World War 2.

HistoryEdit

By the mid-1930s new, more efficient technologies had encouraged the development of faster bombers carrying ever-greater loads over long distances, essential for the RAF, of which bombers would form the backbone. Operational requirements called for a mainstream twin-engined bomber force supplemented by a few long-range bombers to attack capital ships and specialised distant targets.

The former was prescribed in Specification P.13/36, the B12/36 (from which the Stirling emerged) being 25 per cent larger and 50 per cent heavier to carry double the P.13/36's load. The 2,000|b Armour Piercing (AP) bomb would be its heaviest and largest weapon. A need for 100 large B.l2/365, inevitably four-engined, was agreed in April 1936. The aircraft was to carry an 8,000lb load for 3,000 miles, 14,000lb for 2,000 miles, and fly at 230 m.p.h. at 15,000ft. Protection would be provided by three power-operated turrets; four guns at the rear, two at the front, and two in a retractable ventral “dustbin”. Wingspan was limited to 100ft, not because of hangar entry dimensions as generally thought, but to keep the aircraft's overall size and weight in check in order to curtail take-off and landing runs and hard runway requirements and allow easier storage in 150ft-wide hangars. The maximum weight for a 700yd take-off run was set at 36,000lb; or 46,000lb if assisted take-off could be devised.

Studies in May and June 1936 showed that a B.12/36 crew would need meal facilities, rest bunks and a toilet if they were to undertake journeys of up to 3,000 miles. The need for a rapid initial climb to operational height to reduce fuel consumption during cruising would influence total tankage. A wide-track undercarriage, with tyre pressure of 35lb/in2, would allow operation from grass fields.

On July 9, 1936, manufacturers were invited to tender to B.12/36. Short Brothers, privately designing a four-engined landplane based upon its superb flying-boats, received a late invitation. Most favoured by the Ministry was a Vickers four-engined, elliptical-wing Wellington variant with four load-spreading undercarriage units. A design by Supermarine, the only completely original layout, which also had an elliptical wing, was placed last in the line of "favourites" and soon just ahead of Shorts late-entry "Night Bomber".

Being involved with the Handley Page Hereford[N 1] through its Short & Harland tie-up, Short proposed four Napier Dagger sleeve-valve engines for its 86ft 6in-long, 1121t-span, B.12/36, and gave the design the company type number S.29. Four tiers each of five bombs would be carried in four cells placed well forward, the fuselage cross-section allowing easy crew and troop movement, as the B.12/36 specification was to be a bomber/transport. Short estimated a loaded weight of 38,100lb and a maximum permissible weight of 53,100lb.

The new design's size, weight, untried engines and bomb carriage drew criticism, so Short was asked to redesign the aircraft and reduce its wingspan. in April 1937 the company submitted a proposal for a 102f span wing, the shortest desirable. Told that 100ft was the maximum, the company reluctantly reduced it to 99ft 1in. The all-up weight now seemed likely to be 41,600lb, and maximum permissible weight 56,900lb. Conventional bomb stowage was in three 42ft-long, 19in-wide cells, supplemented by six inner-mainplane cells. There was provision for 28 x 500lb high-explosive (HE) bombs or 7 x 2,000lb AP bombs. Power would come from four Bristol Hercules radials or Napier Daggers.

The Air Staff now favoured the Supermarine Type 337 bomber, relegating the 8.29 to a fall-back position in case their first choice failed. His Majesty’s Treasury needed much persuading to support two projects, and not until October 6, 1937, was funding sanctioned. Reginald J. Mitchell's untimely death placed a question mark over the Supermarine bomber, and engendered a belief that its elliptical wing may bring production problems, so Short’s contender took prime place.

The final requirement of December 1937 called for a cruising speed of 230 m.p.h. at 15,000ft during a normal loaded 1,500-mile sortie, and take-off to clear 50ft optimistically not exceeding 500yd. For a 2,000-mile operation, now carrying a 4,000lb load, take-off was listed as 700yd, and the landing run was not to exceed 800yd. Assisted take-off might allow an 8,000lb load for a 3,000-mile sortie. An altitude of 10,000ft had to be maintained on three engines, 20,000ft was to be reached in a 25min climb, and service ceiling was set at 28,000ft.

Short suggested higher speeds of 325 m.p.h. maximum and 280 m.p.h cruising, while the Air Staff limited take-off weight to 45,700lb. Short’s chief designer, C.T.P. Lipscomb, soon stated that a much higher take-off weight of 60,000lb would be needed in order to attain the specified range and load requirements, and suggested double Gouge flaps to shorten the take-off run. The high structure weight stemmed not only from the designs flying-boat origins, but from stressing for catapult launch using either rocket assistance or a track system. This requirement was abandoned in August 1938, leaving the new bombers overstressed and overweight. Assisted-take-off ideas lingered until 1941, when runways for bomber airfields were accepted on a scale of one 1,800yd runway and three of 1 ,100yd. The 1,500yd maximum chosen later would have been suitable for Short’s original design.

Increasingly concerned about take-off run, the company built a half-scale wooden replica, the 8.31, powered by four 114 h.p. Pobjoy Niagara engines, first flown from Rochester Airport on September 19, 1938, by John Lankester Parker and Hugh Gordon. Testing confirmed a long take-oft run and a pronounced swing to the right. Once airborne the 8.31 handled like a fighter, with the short, broad-chord wings conferring a good rate of roll.

After pilots of the Aeroplane and Armament Experimental Establishment (A&AEE] at Martlesham Heath had flown the 8.31, the Air Ministry asked for a reduction in the S.29’s take-off run, suggesting a 3° increase in wing incidence. The low angle of attack was intended to reduce the drag imposed by the deep wing section, which accommodated fuel and bombs, but design and construction was too advanced for radical change. Instead, the tall undercarriage was further lengthened, and tested on the S.31 from November 1938.

The P/1 production specification for the bomber (now named Stirling) was issued in January 1939, and accepted reduced loads. A 1,500-mile cruise carrying 4,000lb load and 2,000lb for a 2,000-mile sortie were stipulated. On January 3, 1939, agreement was reached for a normal loaded weight of 50,844lb and an even higher maximum of 67,000lb.

Excessive demands on engine production led to a plan for Canadian-built Stirling lls powered by Wright Cyclone engines. Several Mk ls re-engined with Cyclones proved inferior even to early Mk ls, so the Mk ll was abandoned. Short had its sights on a different Mk ll, the 8.34 “Stirling ll” to Specification B.1/39, alias “The Ideal Bomber”. With increased wingspan, projected Bristol Hercules high-altitude engines and seemingly excellent load/range characteristics, it reached only the mock-up stage. A slimmed rear fuselage carried twin fins, and set amidships were two Boulton Paul twin 20mm-cannon turrets, forward of which was a capacious bomb bay. Bomber Command had already chosen its new types, however, and the B.1/39 faded.

The Stirling’s weight increased repeatedly with various equipment additions, raising the normal loaded weight to 57,000lb. Air Ministry calculations suggested that even a normal 63,000lb required a 1,000yd take-oh‘ run and a tyre pressure of 43-5lb/in? tor grass-field operations. The prototype, L7600, had Bristol Hercules I engines, the intended Is being unavailable. On Sunday, May 14, 1939, with the workforce at home, the bomber's flying career started well in John Lankester Parker's skilled hands but soon ended dramatically. On touchdown the light alloy back arch of the undercarriage collapsed and L7600 was wrecked. Steel tubing was substituted in the second aircraft, L7605, which made a successful first flight, four months late, on December 3, 1939.

The A&AEE, now at Boscombe Down, received L7605 on April 22, 1940, for four months‘ assessment. Stirling production was already under way at Rochester, and began at Short & Harland, Belfast, in June 1940. The first production Stirling, N3635, flew on May 7, 1940, and like the next nine aircraft had Hercules IIs giving 1,100 h.p. at 5,000ft, which was unsuitable for operations. Changing them was difficult, as they were installed in monocoque nacelles.

Tested at a take-off weight of 57,400lb, L7605 unstuck after 640yd and cleared 50ft after 1,200yd. At 64,000lb that became 1,500yd. It took 121/2 min to reach 10,000ft, and the highest speed attained at that altitude was 249~5 m.p.h. TAS (true air speed) before performance fell away. With the aircraft loaded to 64,000lb, a top speed of 246 m.p.h. was attained at a mere 4,000ft. At the higher loading the best cruising speed was around 184 m.p.h. at 10,000ft. Service ceiling was 15,000ft. A 10,000lb bomb load could be carried with 1,096gal of fuel aboard; a 14,000|b load reduced that to a mere 584gal. The following figures illustrate how far production aircraft fell short in terms of expected flying performance:

Specification:

  • 8,000lb for 3,000 miles
  • 14,000lb for 2,000 miles
  • Cruising speed 230 m.p.h. (15,000ft)

Actual

  • 5,000lb for 1,930 miles
  • 14,000|b for 740 miles
  • Cruising speed 184 m.p.h. (10,000ft)

Improved performance from superior engines was forecast.

The Stirling was too slow for daylight operations, and its low ceiling prevented the use of armour-piercing bombs to maximum effect. However, it could accommodate six slender sea mines, whereas the Handley Page Halifax could carry only two. September 1940 brought a demand for a superior dorsal turret to replace the beam guns and ventral turret.

Production was hard hit late on August 14, 1940, when 15 Heinkel He 111s of KG100 bombed the works at Belfast, wrecking four completed Stirlings and causing splinter damage to others. The next afternoon Do 17Zs of KG3 attacked Rochester Airport, destroying six Stirlings and crippling several others. Widely dispersed production was quickly devised, using Gloster’s Hucclecote works for final assembly until new factories were established around Swindon. Assembly and flight testing took place from South Marston.

Short had envisaged a small production run. Instead, increased orders demanded a fourth major production batch, and Austin Motors at Longbridge near Birmingham was chosen. The first Longbridge Stirling was completed in March 1941, by which time production Mk ls had more powerful Hercules Xls adding 3,000lb to the all-up weight. The FN7A dorsal gun turret was a standard item of the Mk l Series lll, whose engines were fitted in a steel frame, N3662 being the first production example.

in 1941 Short designed an enlarged Stirling, the Short S.34, to Specification B.8/41, related to the Shetland flying-boat. A six-engined version also came to nothing.

More important were the Stirlings with Hercules VI “power eggs", whose superior cooling was expected to enhance higher-altitude performance. The decision to fit them was made in May 1941, and tests started in June 1942 using R9309. They proved disappointing, however, and improved cooling tests were halted on September 6 when an engine fire caused R9309 to crash at Poiton Down, near Salisbury. Stirling l series ll R9188, a modified replacement, flew within a few weeks. ln BK648 and BK649, true prototype llls, cable throttles replaced the troublesome Exactor hydraulic system, and were fitted in subsequent marks. In the Mk lll the ceiling rose to over 17,000ft, but speed showed little improvement.

Output remained unacceptably slow, so in March 1943, under Defence of the Realm Regulation No 78, the Ministry of Aircraft Production (MAP) took control of Short Bros, imposing mass-production methods and a Mk III modification programme. With loss and accident rates increasing, a conference on July 30, 1943, decided that Stlrlings should be withdrawn from front-line bomber squadrons by April 1944. Spare Mk Ills would be convened into GT Mk IV glider tugs/troop transports, a tow coupling being installed under the rear fuselage and a floor hatch being cut for paratroop drops. Shorn of the defensive turrets, the Mk lV could tow a loaded Airspeed Horsa or carry supply containers.

Output switched to the Mk IV tug at the end 01 1943, and others were produced by modifying Mk IIIs. The C Mk V entered production in mid-1944 as an unarmed, dedicated transport which could carry up to 40 troops, mixed freight or 12 stretchers. Its top-hinged nosecone allowed loading using a fitted beam and tackle. A 9ft-wide starboard side aft door and ramp permitted stowage of two jeeps, or one and a six-pounder gun. Converted into the 8.37 “Silver Stirling", PJ958 was a civilian-style transport, but a similarly-converted Halifax was rated superior. A dozen C.Vs were converted by Airtech in 1947 as 00-XAK to O0-XAV for the Far East runs by Trans-Air of Belgium. Several were sold to the Egyptian government, which converted them back lnto bomber transports, thereby completing the cycle of development.

SpecificationEdit

  • Powerplant: Four 1,600 h.p. Bristol Hercules engines
  • Accommodation: Two pilots, navigator/bomb-aimer, wireless operator/front gunner, flight engineer/gunner, two gunners.
  • Dimensions: Wing span 99ft 1in; Length 87ft 3in; Height (tail down) 22ft 9in Wing area 1,322ft2
  • Weights: Empty 44.000lb; Maximum gross 59,400lb
  • Performance: Maximum speed 260 m.p.h. Service ceiling 15,500ft Range 2,330 miles
  • Armament: Three power-operated turrets with Browning 0.303in machine-guns. Maximum bomb load 18,000lb

ReferencesEdit

NotesEdit

  1. Basically a Handley Page Hampden with Napier Dagger sleeve-valve engines.

SourcesEdit

  1. WW2 vehicles

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