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ⓘ Saab 21R




Saab 21R
                                     

ⓘ Saab 21R

The Saab 21R was a Swedish twin-boom fighter/attack aircraft developed and produced by Saab. It was a jet-powered development of the earlier piston-engined Saab 21. It was the first jet aircraft to be produced by Saab. Along with the Soviet Yakovlev Yak-15, the 21R was one of only two jet fighters to have been successfully converted from a piston-powered aircraft.

During the Second World War, the nation of Sweden, feeling under threat during the expanding conflict, ordered Saab to develop an advanced front-line fighter aircraft. The corresponding aircraft was an unorthodox twin-boom pusher configuration fighter aircraft, featuring a low-mounted wing, a tricycle landing gear arrangement, and was furnished with heavy forward-firing armament. Throughout 1945, several different options were explored with the intention of improving the 21s performance, leading to SAAB working on producing a version of the 21 that would harness jet propulsion. On 10 March 1947, the maiden flight of the 21R occurred.

During 1947, at the direction of the Swedish Air Force, SAAB began converting the services piston-engined J 21s to use jet propulsion instead, which required each aircraft to receive extensive modifications. A production run of 124 aircraft was originally envisaged, including four prototypes, however, the type was instead mainly used as a fighter-aircraft and orders were almost halved to 64 aircraft. As a fighter, its service designation in the Swedish Air Force was J 21R, and saw service in the late 1940s and early 1950s before it was replaced by a new generation of fighters that had been designed from the onset to harness jet propulsion, such as the de Havilland Vampire and the Saab 29 Tunnan.

                                     

1.1. Design and development Background

During the early stages of the Second World War, the nation of Sweden was concerned that its neutrality and its independence could come under threat by one or more of the belligerent powers, enacted a series of emergency measures to increase its militarys combat preparedness and deterrence value against would-be aggressors. During the period of 1939–1941, the Swedish Air Force committed itself to a major expansion programme, which included the procurement of large number of foreign-sourced and indigenously developed fighters. As a consequence of the raging conflict across Europe, there were not many nations who possessed the available production capacity or a willingness to prioritise the supply of modern fighter aircraft to Sweden, being a relatively small neutral country; while Swedens own domestic production capability had been deemed to be insufficient until at least 1943, it was recognised that it would be necessary for the nation to build up its own first-rate designs to meet its needs.

During 1941, in response to demand from the Swedish Air Force, the Swedish aviation company SAAB commenced work upon bringing a radical new fighter design. The company envisioned an unorthodox twin-boom pusher configuration fighter aircraft, featuring a low-mounted wing, a tricycle landing gear arrangement, and was furnished with heavy forward-firing armament. It was powered by a piston engine, specifically a localised version of Germanys new Daimler-Benz DB 605B inline engine, which was refined and built by Swedish firm Svenska Flygmotor AB for the programme. Designated as the J 21 by the Swedish Air Force, the proposal was accepted and authorised to proceed to the prototype stage; on 30 July 1943, the first J 21 prototype, powered by the 605B engine, conducted its maiden flight, flown by SAAB test pilot Claes Smith.

Throughout 1945, several different options were explored with the intention of improving the 21s performance. Many of these speculative upgrades involved the substitution of the 605B engine with a more powerful alternative powerplant. During the first half of the year, the company had planned out a variant of the fighter that would be powered by a Rolls-Royce Griffon engine, capable of producing a maximum of 2.000hp; according to estimate, the Griffon would have provided the 21 with a projected top speed of 416mph. Other projects, such as the studied SAAB 27, were intended to use the Griffon as well; however, by the end of the year, all work on piston-engine design had been abandoned by the company in favour of another means of improvement.

In parallel with the piston-engine studies, SAAB and several other Swedish companies had been evaluating the options for the adoption of an innovative, brand new type of engine - the jet engine. Two early studies, designated as RX 1 and RX 2, had been examined, both of which being twin-boom aircraft similar to the 21. Recognising enthusiasm on the part of the Swedish Air Force for a jet-powered aircraft to be made available, during late 1945, SAAB resolved itself to producing a version of the 21 that would harness jet propulsion. As such, Saab launched a design study with the aim of best determining how to power the J 21A with a jet engine. It was viewed as being highly desirable to catch up with recent innovations in the field of fighter development, which was visibly advancing rapidly in nations such as the United Kingdom where, among others, British aircraft manufacturer de Havilland had already placed its own jet fighter, the de Havilland Vampire, into production.

For some months after starting work during late 1945, the design study proceeded without focusing on a specific engine, the selection of which was an open question for some time. As SAAB continued its work on the initiative, it became possible for Sweden to purchase the "Goblin 2", a turbojet engine, directly from de Havilland Engine Company, along with the license to manufacture it in Sweden. Acquired as per this manner, the engine became the first jet engine to be used by the Swedish Air Force, who designated it as the RM1. On 10 March 1947, the maiden flight of Saabs first jet aircraft, being a converted J 21, outfitted with a single Goblin 2 engine, took place. The results of the subsequent flight test programme was largely viewed as satisfactory.

                                     

1.2. Design and development Conversion programme

During 1947, at the direction of the Swedish Air Force, SAAB began to receive the services piston-engined J 21s for the purpose of converting them to jet propulsion. This conversion work involved an extensive rebuilding programme, requiring each aircraft to be extensively modified. In all, in excess of 50 per cent of the airframe, tailplane and wing was changed and, most significantly, each of the fighters was furnished with a single British-sourced de Havilland Goblin turbojet engine, which took the place of the DB 605B and propeller unit. The resulting jet-propelled fighter was adopted by the Swedish Air Force and became the services first jet aircraft. In light of its extensive changes and to differentiate them from its predecessors, rebuilt 21s were assigned the designation of J 21R.

Many changes had to be made in order to accommodate the Goblin engine, which had considerably different properties and requirements than the earlier DB 605B unit. In order to sufficiently address the position of the new engines exhaust, the stabilizer of the tailplane had to be raised from its former position, effectively requiring the tail section to be redesigned. Additionally, to sufficiently feed the hungry turbojet engine, the fighters fuel volume was increased significantly via the installation of additional fuel tanks within the middle section of the wing and a pair of large wingtip tanks. At the same time, various measures to improve the fighters aerodynamic properties were implemented during the rebuild process, leading to the adoption of curved glass in the forward section of the cockpit canopy, while the leading edge of the wing also received modifications. It was decided to implement air brakes in the form of an additional flap located on the outer wings trailing edge. The ejection seat, which was a relatively new innovation, received various alterations and improvements that enabled it to correctly eject during higher speed flight.

The attack version, designated as the A 21R, saw the addition of 14.5 cm and 18 cm ground attack rockets, which could be installed underneath the middle wing. Furthermore, an alternative external gun pod called "Paddan", meaning the toad, could also be installed underneath the middle wing in place of the rockets. Paddan contained eight 8 mm ksp m/22 machine guns with 800 rounds of ammunition per gun. The fire control system of the plane allowed it to fire all guns and all its rockets or gunpod at the same time. This was an interesting "sensation" for the pilot as it made the airplane lose airspeed and almost stop mid air.

A production of 124 planes was planned originally, including four prototypes. When experience with the type as a fighter was gained with the F 10 wing it was concluded that all aircraft needed to be changed to attack aircraft. The production series was then reduced to 64 aircraft. Of these, 34 fighters were designated as the J 21RA later re designated to A 21RA, powered by the Goblin 2 engine, known as the RM1 in Sweden, which was capable of generating a maximum of 1360kp of thrust. The other 30 were of the A 21RB standard, which was powered by a Swedish-made Goblin 3 engine, designated in Swedish Air Force service as the RM1A, capable of producing 1500kp of thrust.

                                     

2. Operational history

The first prototype Saab 21R first flew on 10 March 1947, almost 2 years after the Second World War. The aircraft first entered service with F 10 in August 1950. Although the type was originally intended as a fighter aircraft, a newly developed fighter, the Saab J 29 first flew in October 1948, the number to be produced was halved from 120 to 60, and eventually all 21Rs were converted to attack aircraft as A 21RA or the A 21RB depending on the engine type.

                                     

3. Variants

J 21RA / A 21RA First production series, powered by British-built engines, 34 built in 1950 including four prototypes, retired in 1953. A 21RB Second production series, powered by Swedish-built engines, 30 built between 1950 and 1952, retired in 1956.
                                     

4. Surviving aircraft

No original planes were preserved after they were taken out of service. In the 1990s a group of volunteers took one of the remaining unairworthy Saab 21 airframes and rebuilt it to a 21R for display at the Swedish Air Force Museum.

                                     

5. Specifications Saab 21RA

Data from Beskrivning over fpl typ 21R, hafte 1 description of airplane type 21R, booklet 1, Beskrivning over fpl typ 21R, hafte 6 kap L. Bevapning, SAAB J21/J21R

General characteristics

  • Wingspan: 11.37 m 37 ft 4 in
  • Crew: 1
  • Length: 10.55 m 34 ft 7 in including 20mm cannon
  • Wing area: 22.1 m 2 238 sq ft
  • Airfoil: Saab laminar airfoil
  • Empty weight: 3.090 kg 6.812 lb J 21RA / A 21RA
  • Height: 2.9 m 9 ft 6 in
3.200 kg 7.055 lb A 21RB
  • Fuel capacity: 1.690 l 450 US gal; 370 imp gal total maximum ; 590 l 160 US gal; 130 imp gal in fuselage tank, 300 l 79 US gal; 66 imp gal in wing tanks and 800 l 210 US gal; 180 imp gal in optional tip tanks
  • Gross weight: 4.340 kg 9.568 lb
  • Max takeoff weight: 5.615 kg 12.379 lb
  • Powerplant: 1 × de Havilland Goblin II Svenska Flygmotor RM1 centrifugal-flow turbojet engine, 13.24 kN 2.980 lbf thrust J 21RA / A 21RA
Svenska Flygmotor RM1A DH Goblin 3 14.71 kN 3.310 lbf A 21RB

Performance

  • Maximum speed: 800 km/h 500 mph, 430 kn J 21RA / A 21RA
930 km/h 580 mph; 500 kn A 21RB
  • Service ceiling: 12.000 m 39.000 ft
  • Take-off run: 650 m 2.133 ft
  • Range: 450 km 280 mi, 240 nmi internals only
  • Ferry range: 900 km 560 mi, 490 nmi with wing-tip drop tanks
  • Cruise speed: 610 km/h 380 mph, 330 kn
  • Rate of climb: 17 m/s 3.300 ft/min
  • Landing speed: 155 km/h 96 mph; 84 kn

Armament

  • Guns
  • 2x 12.7 mm 0.50 in akan m/39A with 350 rpg in the nose
  • 2x 12.7 mm 0.50 in akan m/39A with 325 rpg in the wings
  • 1× 20 mm akan m/45 with 140 rounds in the nose
The J 21RA had 13.2 mm barrels installed on its akan m/39As but these were changed to 12.7 mm once they were converted to A 21RA
  • Belly 700 kg 1.543 lb maximum
  • 10x 15 cm sprangraket m/51A & B HE-rocket Bofors 10.3 cm rocket with 15 cm HE head
  • 5x 18 cm halvpansarraket m/49A & B SAP-rocket Bofors 18 cm rocket with 18 cm APHE head. At the start used primarily against ships but in the end against basically everything else.
  • Vapenkapsel paddan "the toad" gunpod containing 8x 8 mm ksp m/22 machine guns with 800 rounds per gun
  • 10x 14.5 cm pansarsprangraket m/49A & B HEAT-rocket Bofors 10.3 cm rocket with 14.5 cm HEAT head
  • 10x 8 cm pansarraket m/46 AP-rocket
  • 10x 15 cm sprangraket m/46 HE-rocket
  • Wingtips
  • 2x vingspetstank drop tanks Could be armed in flight as incendiary bombs. They were also tested with napalm.


                                     
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