The Stirling Prototypes
Until 1936, the Royal Air Force had operated either single or twin engine bombers; however, the activities of the Americans and Russians in building four engine bomber prototypes led the Air Staff to begin discussions aimed at producing a similar aircraft for the RAF. These discussions led to issuing a specification for the RAF's first four engine bomber under the title Specification B12/36. Short was one of the companies asked to submit proposals based on Specification B12/36. The Short's proposal emerged as a four engine, mid-wing bomber originally to be powered by four liquid cooled Rolls Royce Goshawk engines. The aircraft would have a crew of six; two pilots, an observer/navigator, wireless operator, and two gunners manning the nose and tail turret. Provision was also made for a remote control turret in the lower portion of the rear fuselage. Armor would be fitted along with sound proofing and even a toilet. By late 1936, the Air Staff ordered a fuselage mock-up of the design. In order to keep the takeoff and landing run within limits, Short's Chief Designer, Mr.Lipcombe, felt that the wing length should be enlarged from under 100 feet to around 112 feet. This request was rejected, with the Air Ministry stating that existing RAF hangars would not accomodate wing spans of more than 100 feet. This decision created a severe altitude and range limitation because of a decreased wing aspect ratio.
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The S.31 half scale stirling on test in the windtunnel at RAF Farnborough in 1940. |
Short constructed a half-scale flying prototype during 1938, which was intended to reveal any aerodynamic problems. Powered by four 90 hp Pobjoy engines and constructed of plywood, the aircraft was officially designated the S31, but was better known as M4. The S31/M4 first flew on 19 September 1938 with Short's Chief Test Pilot John Lankester Parker at the controls. Within a few weeks the flight tests had been completed, while generally satisfied, RAF officials expressed concern over the prototype's takeoff and landing runs, which were felt to be excessive. The proposed solution was to almost double the wing angle. Short decided to drastically lenghten the undercarriage legs to achieve the required additional 3 degrees of wing incidence.
| The single stage landing gear leg was discarded due to the increased length of the undercarriage rods which proved too long to be retracted into the engine nacelle wheel wells. A two stage undercarriage was built which retracted vertically and then back- wards into the nacelle. 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... |
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The powerplants were also changed from the 90 hp Pobjoy engines to 115 hp Niagara IV engines. While testing resumed 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. The prototype S29 was rolled out of the company's Rochester factory on 13 May 1939.
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Given the RAF serial number L7600, the prototype made its maiden flight on 14 May. 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. |
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For Questions or Suggestions E-mail me at Stirling@box.nl
