page_title: SAAB 21
section_title: Design
hierarchical_section_title: SAAB 21 / Design
caption_attribution_description: Русский: SAAB J-21
context_page_description: The SAAB 21 was a Swedish fighter and attack aircraft designed and manufactured by Swedish aviation company SAAB. It used a relatively unorthodox and visually distinctive combination of a twin boom fuselage and a pusher configuration, giving the aircraft a unique appearance.
  Work on the development of the 21 commenced at SAAB following a decision by the Swedish Air Force to embark on a large-scale expansion programme in preparation to fears that the nation might get dragged 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 within the rear of the fuselage, directly behind the pilot, that drove a rear-facing propeller, commonly known as a pusher configuration. This arrangement allowed armaments to be contained within the aircraft's nose section as well as providing the pilot with a high degree of external visibility. To safely enable the pilot to bail out without striking the propeller behind him, it was decided to adopt an ejection seat.
  On 30 July 1943, the 21 performed its maiden flight; on 1 December 1945, the first model of the fighter, the J 21A-1 was introduced to service.
context_section_description: The Saab 21 was an unorthodox twin-boom pusher configuration fighter aircraft. It featured a low-mounted wing, a tricycle landing gear arrangement, and was furnished with heavy forward-firing armament. Several recent innovations were incorporated into its design, such as an ejector seat for the pilot; the pusher layout later allowed the type to be readily modified with a turbojet engine as well. The advantages of a pusher design are of an unobstructed forward view for the pilot, while the armament can also be concentrated in the nose; however, a major drawback is difficulty in escaping from the aircraft in an emergency, as the pilot could get drawn into the propeller blades. Saab resolved this issue via the installation of an early ejector seat, developed by Swedish defense firm Bofors, which was developed in conjunction with the fighter itself.
  The wing of the 21 was based on a SAAB-designed low drag airfoil; as the wings could not reasonably accommodate recesses for the main landing gear when retracted, alcoves were instead provided within the tail booms, directly aft of the rear wing spar. Other systems, such as the center-section fuel tanks, were incorporated into the wing, which complicated their design considerably. To minimise drag, the coolers and ducts for the engine were housed within the contours of the airfoil between the fuselage and tail booms.
  The armament initially consisted of two 13.2 mm automatkanon m/39A (akan m/39A) autocannons in the wings and two more 13.2 mm akan m/39A autocannons and a 20 mm akan m/41A in the nose. On the A-2 model the 20 mm akan m/41 was replaced with a belt fed Bofors designed 20 mm akan m/45. On the J 21A-3 (later designated A 21A) it was possible to load rockets and bombs of various sizes and types. Later in the Saab 21's service life the 13.2 mm akan m/39 autocannons were rebarreled to fire American 12.7x99 mm ammunition due to economics. This was not a bad change though as the 12.7 mm AP-rounds had better performance than the 13.2 mm AP rounds and the Saab 21 was basically only viable as an attacker after WWII due to the jet era.
caption: Artist's depiction of a J 21 inflight
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