The Short Stirling represents a departure point in British design. It also indicates a British return to a military design philosophy abandoned for many years. When the United States began experimenting with the Flying Fortress family a decade ago, a group of British experts, led by the noted Americanophobe C G Grey, roundly condemned the idea as stupid, impractical, and completely beyond the pale of military usefulness. Their campaign resulted in the neglect of four-engined military aircraft designs for nearly twenty years.
This, in a way, was unfortunate. British design experience in the four-engined class was well founded. It went back to the days of World War I when the desire to bomb Berlin brought forth the great twin-tandem Handley Page V-1500. DeHavilland, Armstrong-Whitworth, Vickers, Short, as well as Handley Page made major contributions to the quad-power-theory, but certain reactionary forces kept it out of the bomber class.
But despite civilian critics, an order to develop a four-engined bomber went out in 1936. The Short brothers, whose experience in large craft stemmed chiefly from flying boats but whose history in metal plane construction reached back to the Silver Streak biplane of 1919, took the first whack at the revival.
The first step was to build a half-scale prototype, powered by four Pobjoy Niagara IV engines. Once the model was test flown and the design's flight characteristics determined, a prototype was started. Still the small plane faction argued against the four-engined bombers and, just about the time the Munich pact was signed, the first full-sized Stirling prototype was test flown by Lankester Parker.
The twenty-month gap between the appearance of the prototype and the military debut of the Stirling in 1940 was filled with arrangements for one of the most extensive subcontracting programs in history. Twenty different plants built main sections of the airframe in cooperation with a network of several hundred smaller subcontractors.
In general design, the Stirling is a mid-wing cantilever monoplane with four air-cooled engines mounted in the leading edge. The standard powerplants are 1,600-hp Bristol Hercules 14-cylinder two-row sleeve-valve engines. Alternate engines are Wright Cyclones of similar output. The Stirling was built still subject to a prewar restriction on wingspan when airplanes were expected to be hangared regularly instead of being picketed outdoors except for major airframe repairs. British hangars have a door span of 100 feet, therefore it was necessary to limit the size of the airplane wingspan to less than that figure. The Stirling emerged with the odd span of 99' 1". The size of the Stirling's fuselage was limited by the dimensions of packing cases in which the subassemblies could be shipped from their builder to the assembly plant. Subject to those odd restrictions, the Stirling took final shape.
Short did not, in the construction of a large landplane, abandon their boat experience. There is more similarity between the structure of the Stirling and the Sunderland flying boat than meets the eye. The outward appearance of the fuselage, aside from the lack of marine provisions such as flotation and a step, is remarkably like the flying boat, the top is straight, the sides flat as is the bottom to the end of the bomb bay, where it curves and tapers to the fuselage end. The back terminates in a gun turret. Corners, of course, are rounded off.
The fuselage structure is made up of Z-channel bulkheads connected by angular U-channel stringers, running continuously the length of the structure. The bulkheads are notched to accommodate the longitudinal members. Transept points are reinforced with gussets. Over this, a smooth skin is flush-riveted with all joints butt-fitted to make an aerodynamically flat surface.
The wing structure was lifted practically whole from the flying boat design. Basically, it is a two-spar structure, with the spar flanges made of T-section extruded aluminum alloy connected by a tubular N-girder arrangement. Interspar bracings and compression members are made of the same tubing. This is the main difference from the flying boat wing, which uses the ordinary square tie rods. The ribs are conventional Warren truss type.
Despite its size, the wing is made up in two complete panels, assembled into a single unit and running the entire span. The skin on the top of the wing it allowed to remain on, even in the section that runs through the fuselage. This is done for strength purposes, but the space offers an excellent storage unit for small articles.
The Stirling's landing gear is installed in independent, electrically motivated units in each of the inboard nacelles. The gear consists of two broken-upright struts which carry hydraulic shock units on each side of a six-foot medium-pressure tire. In the down position, a set of supporting struts lie at an angle between the bottom of the shock strut and the main supporting member to which the gear is attached. At a point about a third of the way down, the main struts are broken. At the break, two horizontal bars are attached, which in turn are attached to a mechanical gear box. When the power is applied to the horizontal member, it breaks the main struts, folding the gear, knee-like, into the engine nacelle.
The tail wheel uses two shoes, linked by chains and sprockets, distributing the tail weight. Thus, small wheels can be used, giving the engineers more space for the installation of the tail turret. The engines, either Bristol Hercules 14-cylinder 1600-hp units or Wright Cyclones of the same size, are installed in monocoque nacelles, bolted to the front spar. The rear portions of the nacelle fair into the wing contour. All fuel is carried in eight drum-shaped self-sealing tanks, four located in each outboard panel. The engines drive three-bladed, thirteen-and-one-half-foot DeHavilland hydromatic full-feathering propellers, all of which are equipped with de-icing slinger rings.
The Stirling carries a fighting crew of seven men; Pilot and co-pilot, navigator-bombardier, front gunner-radio operator, flight engineer-gunner and two spare gunners. In combat position, the radio operator mans the nose gun position, which contains two .303-cal Brownings in a power turret. The bombardier-navigator lies on his stomach, directly below him. The pilots sit aft of this station, in a large glass-enclosed compartment which affords sufficient all-directional view for one of them to act as fire-control officer. This is done while standing up, with hinged sections of armor plate protecting the officer directing fire. While seated, the pilots are protected with armor plate behind their backs and heads, as well as armor incorporated into the sliding door at the end of their compartment.
The navigator's table is also situated in the flight compartment. His station is provided with a retractable astrodome which is incorporated into the top escape hatch in the radio operator's compartment. The flight engineer and his array of engine instruments and controls occupy the next compartment, while his seat is in the compartment behind, with the radio operator.
Like all of the English four-engined bombers, the Stirling's bomb "bay" is no bay at all, but a flat cell, running the greater part of the fuselage length, from in front of the center section forty-two-and-one-half feet back. It spans the width of the fuselage and is divided by two sets of girders into three compartments. Its capacity is eight tons.
The Boulton-Paul dorsal turret on the fuselage is fitted with two .303-cal Brownings. The tail turret carries four similar guns. All three gunners are provided with adequate armor. The dorsal turret is equipped with a set of guide tracks so arranged that the gun is incapable of being pointed at any part of the airplane itself. This prevents the gunner from shooting off his own tail in the excitement of combat. The device bears the interesting title of "taboo tracks."
The Stirling will probably be the last British airplane held arbitrarily to a wingspan of just under one hundred feet. It has a wing area of 1460 square feet and an aspect ratio of 6.5. The Stirling, fully loaded, weighs in the neighborhood of 68,000 pounds, which brings the total wing loading to forty-seven pounds per square foot. Despite this load, it is a fairly maneuverable airplane with a top speed of 300 mph at its best altitude. Its ferrying range is about 2000 miles. The fuselage is 87' 3" long, while the maximum interior (aside from the pilot's greenhouse) is 8' 8" high and 6' 8" wide.
This article was originally published in the April, 1943, issue of Air News magazine, vol 4, no 4, pp 23-25.
The original article includes 8 photos and the cutaway diagram above.
Photos credited to British Information Service, Black Star, British Combine.