Short Stirling

Air Ministry Specification B.12/36 called for the design and development of a strategic four-engined heavy bomber that could quickly is placed into production, giving the RAF Bomber Command a high-speed aircraft capable of delivering a large bombload at long ranges. It was to be crewed by seven or eight men with defensive armament consisting of multi-gunned nose, ventral and tail turrets. The initial maximum take-off weight had to be between 48,000 lbs (21769 kg) and 53,000 lbs (24036 kg), but with the capability of that figure being increased to around 65,000 lbs (29478 kg). The weapons bays also had to be compatible with all standard RAF bomb ordnance in use at that time. The specification also demanded that the aircraft be capable of lifting off a 500 ft (152.4 m) runway and is able to clear 50 ft (15.2 m) trees at the end, with the wingspan not exceeding 100 ft (30.48 m).

Several companies submitted their designs to the Air Ministry, these being Armstrong Whitworth, Short Brothers and Supermarine. The Armstrong Whitworth design was rejected, and both Short Brothers and Supermarine were asked to construct prototypes. Supermarine's Type 317 prototype was still under construction when the factory was bombed by the Luftwaffe early in the war. The factory and prototype were virtually destroyed, causing Supermarine to withdraw from the competition leaving only the Short Brothers design.

Short initially proposed a design that would give good high-altitude performance provided by a wing spanning 112 ft 0 in (34.14 m) and was to be powered by four Rolls Royce Goshawk engines. Provision was also made for a remote control turret in the lower portion of the rear fuselage. Short would incorporate the same structural and aerodynamic concepts they had used on the Short S.25 Sunderland (maritime reconnaissance flying boat). The RAF rejected this proposal based on the wingspan, demanding it to be made shorter so that the aircraft would fit in RAF aircraft hangers that had standard door openings of 100 ft (30.48 m). This requirement would severely restrict the Stirlings operational altitude. The Short design team had therefore to revise its concept with a wing of reduced span and greater chord, the resulting decrease in aspect ratio inevitably reducing high-altitude capability. Even though this meant a reduction of capabilities of the Stirling, the need for an aircraft of this type was so urgent, the Air Ministry was forced to continue with the project and ordered two prototypes designated Short S.29 Stirling. Production orders for the aircraft followed even before the prototypes flew.

To test the aerodynamics and controllability of the new type, the S.31 was designed as the half-scale prototype with a powerplant of four 90 hp (67 kW) Pobjoy Niagara III radial engines. The Short S.31 made its first flight on 19 September 1938 and revealed good overall handling characteristics. 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. Other changes included the installation of four 115 hp (86 kW) Pobjoy Niagara IV radial engines. In order to address longitudinal control problems horn-balanced elevators were installed but these were soon replaced by a larger tailplane with conventional elevators.

A Short S.29 Stirling Mk III (W7455 OJ-B) based in Mildenhall in early 1942 with No. 149 Squadron. Later in the war this particular aircraft was lost to a night intruder over Great Thurlow.

The construction of two Short S.29 prototypes started in 1939 and the first prototype (L7600) was flown for the first time on 14 May 1939 powered by four 1,375 hp (1025 kW) Bristol Hercules II engines. It would also be its last flight, as on landing one of the wheel brakes seized causing one of the landing gear legs to shear off slamming the aircraft into the ground. Damage was so extensive, the aircraft was written off. The failure was traced to the light alloy undercarriage back arch braces which were replaced on succeeding aircraft by stronger tubular steel units. The main landing gear units on the second prototype (K7605) were redesigned, with this aircraft first flying on 3 December 1939. During the spring of 1940, the prototype spent four months undergoing service tests at Boscombe Down. Main production had already started by this time, with the first Short S.29 Stirling Mk I flying on 7 May 1940 powered by four 1,595 hp (1189 kW) Hercules XI radial engines. The revised landing gear would later give the aircraft a tendency to swing violently unless handled carefully during take-off and landing.

Initial deliveries began in August 1940 to No. 7 Squadron based at Leeming, replacing their Handley Page Hampdens. The Stirling was used operationally for the first time on the night of 10/11 February 1941, when three aircraft from No.7 Squadron attacked oil storage tanks at Rotterdam. The Stirling was thus the RAF's first four-engined monoplane bomber into service, the first to be used operationally in World War II, and also the first to be withdrawn from the bomber role after a final operational sortie on 8 September 1944. This occurred when there were adequate supplies of the Avro Lancaster and Handley Page Halifax bombers for Bomber Command requirements, for the Stirling had an inadequate operational ceiling and could not carry the larger high-explosive bombs that had been introduced by that time. Total production of bomber versions then amounted to 1,759 aircraft, comprising of the Stirling Mk I (712) and Stirling Mk III (1,047) aircraft. The designation Stirling Mk II was allocated to a planned production version to be built in Canada with 1,600 hp (1193 kW) Wright R-2600-A5B Cyclone radial engines, but this was cancelled after only two prototypes were produced by converting two Mk I aircraft.

As 1942 progressed, No. 7 squadron (with its Stirling's) and other Bomber Command squadrons were transferred to form the nucleus of the newly formed No. 8 (Pathfinder) Group. By the end of the year, the new Mk.III Stirling's, equipped with 1,675 hp Bristol Hercules XVI engines and a new dorsal turret design, were entering service and slowly replacing the Mk I's.

By late 1943, German flak defences were inflicting serious losses to the Stirlings, mostly due to its low ceiling caused by its restricted wing span. It was soon evident that such losses could not continue and Air Marshal Harris, was forced to withdraw the aircraft from operations. By early 1944, as supplies of the Avro Lancaster became available, most of the Stirling squadron's began to re-equipped with this type. Although, it would not be until September 8, 1944 that No. 149 squadron, flew last operation Stirling sorties against Le Havre.

The Stirling never lived up to it potential as a great bomber (in no small part to the RAF Bomber Commands requirement changes) but it a proved very popular aircraft with its crews, who dubbed it the "fighter bomber" due to its excellent manoeuvrability and rugged construction. On one occasion four German night fighters attacked a Stirling from No. 218 Squadron on a night raid in 1942. Manoeuvring for its life, the Stirling managed to shoot down three of the attackers before returning to base safely, although a little battered. As a result of its high wing loading, the Stirling had a high roll rate and was manoeuvrable enough to out-turn the Junkers Ju 88 and Bf 110 night fighters.

From early 1944 the Stirling's primary role changed to that of glider tug and transport. For the former role two Stirling Mk IIIs were converted as prototypes, losing their nose and dorsal gun turrets, retaining the tail turret and gaining glider towing equipment to become designated Stirling Mk IV. They proved efficient in this new role, towing one General Aircraft Hamilcar or two Airspeed Horsas for assault and up to five General Aircraft Hotspurs on a ferry flight or for training. The Stirling Mk IV also saw service With No. 100 (Bomber Support) Group, carrying out Electronic Counter Measure (ECM) sorties. They also took part in the D-Day operations in Normandy, in the airborne operations at Arnhem and the March 1945 crossing of the Rhine. Production of the Stirling IV totalled 549.

The Stirling Mk V transport was the last version of the aircraft built for RAF Transport Command. This was configured to carry 40 troops, or 20 fully equipped paratroops, or 12 stretchers and 14 seated casualties. It could be used also for loads such as two jeeps with trailers, or a jeep with a field gun, trailer and ammunition. The Mk Vs were the last Stirlings in service, being gradually replaced by the Avro York, with the last of them withdrawn from use in 1946. Production on Belfast built Mk V totalled 160 aircraft. During 1947 Airtech Limited of Thame, Oxon, converted 12 Stirling Mk Vs for use by a Belgian civil operator under the name Silver Stirling.

Official service figures credit the Stirling with 18,440 sorties flown in which 27,821 tons (28268 tonnes) of bombs were dropped and 20,000 mines were laid, for the loss of 769 aircraft. 

Specifications (Short S.29 Stirling Mk III)

Type: (Mk I & III) Seven or Eight Seat Heavy Bomber (Mk IV) Glider Tug & Troop Transport (Mk V) Transport, Heavy Freighter & Air Ambulance

Accommodation/Crew: A crew of eight was carried on early Stirlings comprising of the Pilot and Co-Pilot, Navigator/Bomb-aimer, Wireless/Radio Operator, three air-gunners and a Flight Engineer. On later aircraft the position of the second pilot was removed. Crew positions also changed slightly depending on defensive armament carried. The Bomb-aimer was in the nose below the pilot's floor and under the nose gun turret. Pilots coupé gives not only good forward view but is designed to permit fighting controller to operate with minimum of interference during enemy fighter attack. The navigator is also seated within the coupé boundry. Retractable astral dome superimposed with escape hatch just aft of back end of coupé. Armoured bulkhead with hinged door separates flight compartment from engineer and wireless-operator. First pilot has additional armour to his back and head and the fighting controller has armour protection to his chest when attending to the air-gunner's position. Centre-section above bomb floor is braced to allow egress aft and also provides stowage space and rest quarters for any member of the crew. A bunk is fitted on the starboard side of this compartment. Aft of centre-section is the mid-upper turret and the servo-feed ammunition boxes to the tail turret. Aft of the bomb-bay are the multi flare chutes and a walkway to the tailplane spar frames and through them to the tail turret. Main entrance door to fuselage is fitted aft of the flare station. There were escape hatches in the nose at the Bomb-aimers position, above the Pilot's seat, two on top of the fuselage and one near the tailgunner's position.

Design: Designer Arthur Gouge of Short Brothers Limited

Manufacturer: Short Brothers (Rochester & Bedford) Limited based in Rochester, Kent, England. The company was founded in 1898 by the brothers Eustace and Oswald Short, originally building spherical balloons but they later concentrated on the building of Flying Boats. In 1936, Short Brothers Limited and Harland & Wolff Limited (Shipbuilders) of Queen's Island, Belfast, Northern Ireland, formed a partnership under the name Short & Harland Limited. Also built by Austin Motors Limited in Longbridge. Production would eventually be dispersed to over 20 different factories.

Powerplant: Four 1,675 hp (1250 kW) Bristol Hercules XVI 14-cylinder sleeve-valve double-row air-cooled radial engines rated at 2,900 rpm at 4,500 ft (1370 m); 1,615 hp (1205 kW) at 2,900 rpm for take-off; 1,050 hp (783 kW) at 2,400 rpm at 10,250 ft (3130 m). Engine weight (dry) 1,930 lbs (875 kg). The fuel used was 100 or 130 octane. The propeller was a metal three-bladed de Havilland Hydromatic Type 55/10 variable pitch, constant speed full-feathering airscrew with a diameter of 13 ft 6 in (4.11 m).

Performance: Maximum speed 270 mph (435 km/h) at 14,500 ft (4420 m); economical cruising speed of 233 mph (375 km/h) at 11,000 ft (3355 m); service ceiling 17,000 ft (5180 m); initial rate of climb 800 ft (245 m) per minute.

Fuel Capacity: Seven cylindrical self-sealing (except the leading-edge tank) fuel tanks in each wing outboard of the wing bomb cells, giving a total of 2,254 Imperial gallons (2,707 US gallons or 10246 litres), plus provision for 220 Imperial gallons (264 US gallons or 1000 litres) of auxiliary fuel in tanks installed in each of the wing bomb cells for an additional total of 440 Imperial gallons (528 US gallons or 2000 litres). The systems could be interconnected if necessary by operating an inter-system balance cock in the centre section.

Oil Capacity: Each engine had its own oil tank with a capacity of 33 Imperial gallons (40 US gallons or 150 litres).

Range: 590 miles (950 km) on internal fuel with a bombload of 14,000 lbs (6350 kg). 2,010 miles (3237 km) on normal internal fuel with a bombload of 3,500 lbs (1587 kg). Ferry range (clean) of over 3,000 miles (4831 km) was possible using auxiliary fuel tanks. Range on the Short S.29 Stirling was one of its major weaknesses. While it was capable of carrying a tremendous amount of ordnance, it could only do this a very short distance. As a result, on most missions to get the desired range, bombload was sacrificed.

Weights & Loadings: Empty (clean) 43,200 lbs (19595 kg), empty (equipped) 59,400 lbs (26939 kg) with a maximum take-off weight of 70,000 lbs (31751 kg). Wing loading 48 lbs/sq ft (234 kg/sq m); power loading 10.6 lbs/hp (4.85 kg/hp).

Dimensions: Span 99 ft 1 in (30.20 m); length 87 ft 3 in (26.59 m); height 22 ft 9 in (6.93 m); wing area 1,460.0 sq ft (135.63 sq m).

Armament: A total of eight 7.7 mm (0.303 in) Browning machine-guns plus up to 14,000 lbs (6350 kg) of bombs. Disposable stores were carried in a lower fuselage weapons bay rated at 11,000 lbs (4989 kg) and in 6 wing cells each rated at 500 lbs (227 kg). Normal loadout usually consisted of 2,000 lbs (907 kg) armour-piercing bombs and/or 500 lbs (227 kg) general-purpose high explosive bombs. The main bomb-bay in the fuselage is formed of two main longitudinal girders with arched members to the main floor. The bay is 42 ft 7 in (13.0 m) long and fitted with six hinged doors. Internal stowage for bombs is also provided in the centre-section inboard of the inner engine nacelles. A bomb overload of up to 25,500 lbs (11567 kg) was possible but it reduced the range considerably.

  • 2 × 7.7 mm (0.303 in) Browning trainable forward-firing machine-guns in the power-operated Frazer-Nash F.N.5 nose turret.

  • 2 × 7.7 mm (0.303 in) Browning trainable machine-guns in the power-operated Frazer-Nash F.N.50 (Boulton-Paul) dorsal turret.

  • 4 × 7.7 mm (0.303 in) Browning trainable rearward-firing machine-guns in the power-operated Frazer-Nash F.N.20A tail turret.

  • 1 x 7.7 mm (0.303 in) Browning trainable rearward firing machine-gun in a manually operated ventral position (Some aircraft only).

Variants: Stirling Mk I, Stirling Mk II (two prototypes only), Stirling Mk III, Stirling Mk IV, Stirling Mk V (transport).

Equipment/Avionics: Standard communications and navigation equipment. Aircraft in the role of pathfinder carried H2S bombing radar. It was also the first RAF Bomber to carry "Oboe" navigation radar. Standard equipment would have also included de-icing equipment fitted to the leading-edges of the wings, tailplane and fin. Several fire extinguishers and crash axes were positioned inside the aircraft fuselage. There was also equipment on board to destroy radio and bombsight equipment or set the aircraft on fire in case of an emergency landing. Individual dinghy kits were stacked in racks inside the Stirling, usually in combination with parachute stowage racks. Dinghy radio equipment was stowed in the fuselage. The Type J Dinghy for eight men was stowed in the Port wing. Complete with topping up bellows, leak stoppers, rescue line and knife. The Dinghy could be released from inside the fuselage or from the outside, or automatic by flooding of the immersion switch located in the fuselage nose. Emergency equipment was carried in a Lindholme Dinghy Container, including a first aid pack, corned beef cans, services and RAF flying rations, rum and cigarettes. Engine maintenance platforms and ladders were carried in the fuselage. Oxygen equipment was provided for all crew members. First aid kits were located on the fuselage sides behind the Pilot seat (two on the starboard and one on the port side). A Mk.XIV bomb sight control panel used in conjunction with a Mk.XIV bomb sight computer was used by the Bomb-aimer.

Wings/Fuselage/Tail Unit: The wings are of a mid-wing cantilever monoplane type with a two-spar all-metal structure similar to that of the Short "Empire" flying boat. Gouge type trailing-edge flaps with chord equal to 48 percent of the total chord. The leading-edges of the wings are armoured and are provided with barrage-balloon cable cutters. The fuselage is a rectangular section with rounded corners with an all-metal structure built up of transverse frames covered with aluminium-alloy sheet with intercoastal stiffeners and all joints joggled flush with flush riveting. The tail unit is a cantilever monoplane type with a single fin and rudder similar in form and construction to those of the "Empire" flying boats.

Landing Gear: The landing gear was a two-stage retractable type with the main wheels retracting vertically and then backwards into the inner engine nacelles taking part of their fairings with them. Retraction was powered by electric motors with alternative hand operation. Twin castoring retractable tailwheels type Dunlop WS30. In order to accommodate the RAF Bomber Command requirement to shorten the take-off and landing, a very long landing gear was utilized making the aircraft prone to swing violently on take-off and landing. The undercarriage retraction motors were originally located inside the nacelle, but were later relocated inside the fuselage to allow for manual retraction in the event of motor failure. The electric retraction motors often failed, being wholly inadequate for the task. The Stirling had one of the largest tires, manufactured by Dunlop, on a British aircraft at the time.

History: First flight (S.31 research aircraft) 19 September 1938; first flight (S.29 prototype) 14 May 1939; first flight (production Mk I) 7 May 1940; final operational sortie 8 September 1944; final production (Mk V) November 1945; withdrawn from service (RAF) 1946.

Operators: United Kingdom (RAF).

Units: They initially entered service with No.7 Squadron and at the peak of their service they equipped 13 RAF Bomber Command Squadrons (Nos.7, 15, 75, 90, 101, 149 166, 199, 214, 218, 513, 622 and 623). Starting in 1944 the main role of the Stirling was that of glider-tug and transport with RAF Transport Command. For D-Day on 6 June 1944 RAF Transport Command Squadrons Nos 190 and 622 from Fairford and Nos 196 and 299 from Keevil towed Airspeed Horsa gliders into Normandy. Late in the war, Squadron Nos 171, 295, 570, 620 and 624 also used the Stirling and participated in the airborne landings in Arnhem and the March 1945 attack across the Rhine. Squadron Nos 138 and 161 were (Special Duties) Squadrons, flying for the Special Operations Executive (SOE) from Tempsford, near Cambridge. They performed Covert Operations supplying arms to the Resistance in occupied countries. Based in Blida, North Africa, No. 624 Squadron performed the same task in the Mediterranean area.

The Super Stirling
By Raul Colon

In the early part of 1941, inside the corridors of power of the Royal Air Force (RAF) Bomber Command, there was a growing concern regarding the Halifax and Lancaster bomber force in contrast to the Sterling platform. The perceived disadvantage of the Sterling had many in the Command clamouring for an upgrade to the existing aircraft. This lead to the RAF to send a formal request for an enhanced Sterling in the form of Specification B.8/41. The result of this was the project called “Super Stirling” which was based on the new Centaurus CE3Sm radial engines.
The new blue print for the Short Super Stirling, tagged project S36, began with the introduction of the Centaurus CE3Sm radial engines early in the summer of 1941. With the power plant in hand, it was time for the engineering team at the Short Brothers (from 1943 forward, the company was named Short Brothers and Harland) to start developing the S36 concept. The first thing they did was redesign the whole wing structure. The base Stirling fuselage was extended for the installation of a larger central bomb bay indented to carry the huge and still in the developmental stages 8,000lb free fall bomb. This alone represented a major upgrade over the original Stirling bomber. In addition, six more wing-based cells were installed. Each cell could carry up to 1,000lb of ordinance.

In the autumn of 1941, the RAF’s Controller of Research and Development (CDR) Department issued a paper covering, among other things, the expected operational characteristics of the S36 design. The CRD viewed the new bomber as a “typical night bomber having high useful load at a comparatively slow, economical cruising speed (214mph at 15,000’) just six miles per hour faster than the Stirling”. Despite the uninspired report, the CDR still recommended that the project go to full production mode.

The first, true outline of the project now known as the Stirling III, which was revealed to the RAF’s top brass in July 15th 1941, offered an insight into the Short engineers’ vision. The “III” design had a powerful defensive armament system. Two .5 inch machine guns were placed in the nose of the aircraft, four additional ones in both mid upper and tail turrets alignment. Another machine gun, a .303 inch calibre, was installed under the fuselage in an under turret mechanism. Maximum takeoff weight for the new bomber was estimated at 103,100lb. Top operational speed was 311mph at a 20,000 service ceiling. Maximum operational ceiling was determined at 29,300’. In August a further revision of the S36’s profile was made. But the outlines of it were the same, a similar airframe to the original bomber with an increased bomb load and extended longer. The S36 was conceived to be able to carry a powerful 23,500lb total bomb load, compare to the original Stirling’s 14,000lb capacity, for 2,300 miles.

On the 19th of November 1941, the Ministry of Defence issued Specification Order B8/41 to cover the program costs and allocation of resources. Nevertheless, questions were raised regarding the new aircraft’s feasibility. The Controller General Office was sceptical of the Stirling III production success. In a report made public in the fall of 1941-42, the CRD issued some reservations about the development of the S36 as it compared with the Halifax and Lancaster platforms. Or even the much anticipated Avro Super Bomber design, still years away from presentations. Still, the CDR endorsed the project with an order of two sample aircraft on 9th of January 1942. The CRD assigned Serials JR540 and JR543 to the two units. One, without a certain operational requirement, the other ready to fly once completed.

It was estimated that the first unit would take to the air in the autumn of 1943. Short was also encouraged to prepare a production blue print for an Initial Production Order of twenty aircraft with a possible extension of 130 units. On May 11th, the Commander in Chief of the RAF’s Bomber Command, Air Chief Marshall Sir Arthur Harris wrote that “the B8/41 was expected to eradicate the weakness of the present Stirling force and with much bigger span wings should be a better aircraft. But the new potential given does not justify the change over the switch will cost at least 126 Stirlings at Rochester plus a ratio of two B8/41s for three Stirlings. The best course is to concentrate on the Hercules VI Stirling which will go a long way to improve the really weak feature, its operational ceiling at weak mixture. The Hercules VI should push this up to 19,000’ from 16,000’ which is superior to the B8/41”. Coming from Harris, these words were gospel to the MoD.

A fortnight later, the Short brothers were told to cease all work on the S36 project. The Ministry estimated that by the time the S36 achieved operational status, and taking into the equation the expected increases in additional weight that usually goes into a new aircraft, the new profile of the bomb load would be insufficient to justify the losses of standard production Stirling. The decision shocked Shorts who, for awhile, entertained the idea of privately continuing with the program. But on August 5th they decided to abandon the whole program. The valuable data gained during the program’s life was used on another air platform concept; the Vickers Long Range High Altitude Super Bomber.

Air Power: The men, machines, and ideas that revolutionized war; from Kitty Hawk to Gulf War II, Stephen Budiansky, Penguin Books 2004
International Air Power Review, AIR Time Publishing,Volume 22, 2007