The SAAB 21 is a Swedish single-seat low-wing monoplane fighter and attack aircraft designed and manufactured by SAAB. Its twin boom fuselage with a pusher engine gives the aircraft an unusual appearance. Work began at SAAB following a Swedish Air Force decision to embark on a major expansion programme in preparation for the possibility of being drawn into the Second World War. The company designed a monoplane twin-boom aircraft, powered by a single Daimler-Benz DB 605B engine that was positioned at the rear of the fuselage nacelle, driving a pusher propeller. This arrangement allowed guns to be carried in the aircraft's nose while providing the pilot with good visibility. An ejection seat was adopted to enable the pilot to bail out without hitting the propeller. On 30 July 1943, the 21 performed its maiden flight and on 1 December 1945, the first examples of the J 21A-1 were introduced to service. It was quickly followed by the improved J 21A-2, which featured heavier armament, and the A 21A-3 fighter-bomber. With jet-powered aircraft rapidly overtaking piston-powered aircraft, in 1947 SAAB produced a conversion of the 21 using the British de Havilland Goblin jet engine, the resulting airframe being designated the SAAB 21R. Along with the Soviet Yakovlev Yak-15, the SAAB 21 was one of only two jet fighters to be successfully converted from piston power to jet power. The 21 was replaced in the mid-1950s after less than 10 years of service by the similarly configured de Havilland Vampire and the Saab 29 Tunnan.
Design and development
Background SAAB was carrying out design studies during the late 1930s into possible options for a new fighter aircraft. Many of these had been based around the use of a British Bristol Taurus radial engine and some were unconventional for the time. One of the configurations studied was of a monoplane pusher configuration twin-boom aircraft, with the engine behind the pilot at the rear of a central nacelle. This unorthodox design possessed several advantages, such as the ability to concentrate most of the guns in the aircraft's nose, good pilot visibility, and ease of service. While this promising design study was completed, it remained dormant until 1941, when defence considerations heightened the imminent need for it. At the start of Second World War, the Swedes became concerned about maintaining their neutrality and independence as they would soon be threatened by one or more European nations. As an emergency measure, the Swedish Air Force embarked upon a major rearmament and expansion of their military during the 1939–1941 period, which included the procurement of foreign-sourced aircraft as well as the local development of new, modern designs. However, as a consequence of the war, few nations at war were willing to supply fighters to a neutral country, while Sweden's own production capability would be insufficient until at least 1943. As a consequence, Sweden was forced to purchase already obsolete Fiat CR.42 biplanes from Italy as an interim measure, and which were of little value against modern monoplane fighters. Accordingly, SAAB began looking at solutions to various anticipated production problems for their proposed fighter.
Design
The SAAB J 21 needed a top speed of at least 480 km/h (300 mph), which required a powerful engine. It was decided to substitute the Taurus engine for the American Pratt & Whitney R-1830 Twin Wasp radial engine. However Svenska Flygmotor was also asked to provide an alternative to the Twin Wasp. Options were limited by the urgency involved, leaving a license-produced engine as the only option. Accordingly, a locally built version of Germany's new 1,100 kW (1,500 hp) Daimler-Benz DB 605B inline engine was selected, however, due to the DB 605B's lack of maturity, a great deal of refinement and modification by Swedish engineers was required to ready it for operational use.
The SAAB 21 was an unorthodox twin-boom low wing pusher configuration fighter aircraft with a tricycle landing gear, and a heavy forward-firing armament. Several recent innovations were incorporated into its design, including an ejection seat for the pilot while the pusher layout later allowed the type to be readily re-engined with a turbojet. The advantages of a pusher design include an unobstructed forward view for the pilot, and the armament can be concentrated in the nose, however, a major drawback is difficulty in making an emergency exit as the pilot could get drawn into the propeller blades. Many solutions were examined, such as jettisoning either the propeller or the engine via explosive charges prior to bailing out, before it was decided to adopt an ejection seat developed by Swedish defense firm Bofors, in parallel with the fighter. The J 21 was one of the first operational aircraft in the world with an ejection seat. The wing of the 21 was built around a SAAB-designed laminar airfoil. As the wings could not readily accommodate the main landing gear when retracted, wells were provided in the tail booms, aft of the rear wing spar. To reduce drag, coolers and ducts for the engine were located in the wing section between the fuselage and tail booms, and integral fuel tanks were fitted. The armament initially consisted of one nose mounted 20 mm (0.79 in) Hispano-Suiza Akan m/41A and four 13.2 mm (0.52 in) Akan m/39AA autocannons, two in the wings, and two in the nose. On the A-2 the 20 mm (0.79 in) guns were replaced with belt fed 20 mm (0.79 in) Bofors Akan m/45. The J 21A-3 (later designated A 21A) could carry rockets and bombs. Later in the Saab 21's service life the 13.2 mm (0.52 in) autocannons were rebarreled to fire American 12.7 mm (0.50 in) ammunition due to economics. These improved firing performance but the Saab 21 was now only viable as a ground attack aircraft due to the rate of change in fighter design. Three prototypes were completed of which only two were to be flyable, while the third was a static airframe for stress testing purposes. On 30 July 1943, the first J 21 prototype conducted its maiden flight, flown by SAAB test pilot Claes Smith. During takeoff, he used too much flap, impairing acceleration and climb. This resulted in it hitting a fence and damaging the undercarriage, although he was able to land successfully afterwards.
Further development
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