|Designer||Kosta Sivčev, Ljubomir Ilić, Slobodan Zrnić|
|First flight||late May 1938|
|Introduction||late March 1940|
|Primary user||Royal Yugoslav Air Force|
The Rogožarski IK-3 was a 1930s Yugoslav low-wing monoplane single-seat fighter with retractable landing gear, and was designed by Ljubomir Ilić, Kosta Sivčev and Slobodan Zrnić as a successor to the Ikarus IK-2 fighter. It was considered comparable to foreign aircraft such as the Messerschmitt Bf 109E, and came into service in 1940. Six IK-3s were serviceable when the German-led Axis invasion of Yugoslavia began on 6 April 1941. Pilots flying the IK-3 claimed 11 aircraft during the eleven-day conflict, but all surviving aircraft and incomplete airframes were destroyed by their crews and factory staff to ensure they did not fall into German hands. The IK-3 design was used as the basis for the post-war Yugoslav-built Ikarus S-49 fighter.
In the late 1920s, a scheme promoted by the Royal Yugoslav Air Force (Serbo-Croatian: Vazduhoplovstvo vojske Kraljevine Jugoslavije, VVKJ) and the Royal Aero Club of Yugoslavia sent aspiring aeronautical engineers to France to develop their knowledge. It was intended that after this advanced training, they would return to Yugoslavia and be offered specialist roles in the VVKJ or in the aeronautical industry. Ljubomir Ilić and Kosta Sivčev went through this program, but when they returned to Yugoslavia, both were employed in administrative work. Frustrated by this, in 1931 they decided to design a replacement for the Czechoslovakian-built Avia BH-33E biplane fighter then in service with the VVKJ. Working in a basement in Belgrade then in Ilić's apartment in Novi Sad they devoted their spare time to secretly working on their design. Their original concept was for a low-wing monoplane with retractable landing gear, but contemporary thinking led them to evolve their initial ideas into a strut-braced high-wing monoplane armed with a hub-firing autocannon and fuselage-mounted synchronised machine guns. The gull-wing design emphasised power, speed, maneuverability, climb and firepower. The design concept for this aircraft, which became the Ikarus IK-2, was submitted to the VVKJ on 22 September 1933, which freed up time for Ilić and Sivčev to start preliminary development of a low-wing monoplane that could meet and defeat the high performance bomber prototypes then in development.
Design and development
The new streamlined low-wing monoplane design was to have a retractible undercarriage and, like the IK-2, Ilić and Sivčev developed it privately at first. A scale model was tested in the Eiffel-built wind tunnel in Paris, but the pair soon realised that they needed a third engineer to take some of the strain of evaluating the design and determining the structural details. The man that joined the team was Slobodan Zrnić, who had worked as a specialist aircraft engineer in France and was head of construction at the Yugoslav State Aircraft Factory at Kraljevo. The project designation used for the IK-2 was changed from IK, standing for (Ljubomir) Ilić and Kosta (Sivčev), to IKZ, thus incorporating Zrnić. This designation did not stick, however, possibly due to the similarities between the Cyrillic "З" (Z) and the Arabic numeral "3", and the aircraft became known as the IK-3. The aircraft was to mount a Hispano-Suiza 12Y29 engine, generating 980 hp (730 kW) at 5,000 m (16,000 ft). The designers chose to favour maneuverability over speed, trying to find a compromise between the German and British concepts of a modern monoplane fighter. The design concept had a smaller wing area than the Hawker Hurricane and the Supermarine Spitfire, to achieve a relatively higher speed for the engine power. In comparison to the Messerschmitt Bf109, the Yugoslav design had a shorter fuselage and smaller turning radius. It differed from both British and German design concepts in that its proposed armament was concentrated in the fuselage.
The designs were delivered to the VVKJ in good time for approval by mid-1936, but the general reluctance to adopt new design concepts faced with the IK-2 also delayed the IK-3. The contract for the production of the prototype was not concluded until March 1937. The company selected for construction was Rogožarski A.D. in Belgrade. The first flight for the prototype was carried out by the VVKJ Test Group towards the end of May 1938, and the aircraft was then flown by a group of VVKJ officers who were charged with the responsibility of determining the best employment for it within the VVKJ, along with tactics to be used in Yugoslav conditions. These pilots observed that the controls were highly sensitive, but the only real criticisms related to the distortion caused by the convex/concave panels of the canopy. Some pilots believed that the fuselage-mounted armament should be supplemented by two wing-mounted machine guns. One of the key tasks of the pilot group was to compare the performance of the IK-3 with the Hawker Fury, Heinkel He 112, Morane-Saulnier M.S. 405 and Hawker Hurricane. The pilot group concluded that the IK-3 most closely matched the Morane-Saulnier, although the Yugoslav aircraft was 40 km/h (25 mph) faster. In November 1938, the VVKJ placed an order with Rogožarski for 12 aircraft.
On 19 January 1939, test pilot Kapetan Milan Pokorni put the prototype into a steep dive and when he reached 400 m (1,300 ft) the windscreen detached from the aircraft. Pokorni pulled up hard and the strain broke off half of the starboard wing. The aircraft crashed and Pokorni was killed. The crash inquiry determined that modifications to the windscreen had contributed to the accident, but the main factor had been the pilot's handling of the sensitive controls. Following the crash, engineers recalculated the stress factors on the airframe and they were found to be safe. Ultimately, the crash was attributed to the highly sensitive controls and pilot error.
The loss of the prototype and some changes in the construction of the production model delayed the delivery of the contract. Additional tests were conducted on the wing structure, and it was found to withstand a g-force of 14. A number of modifications were made for the production model. These included the use of flat Plexiglass panels in the windscreen and the canopy to provide better visibility for the pilot. The instrumentation layout was improved and the upper rear fuselage behind the pilot's seat was re-shaped. The folding undercarriage leg covers were also replaced by single plates. The major changes were; the replacement of the engine with a modified version of the Hispano-Suiza 12Y engine made under licence by the Czechoslovak company Avia, and the replacement of the cannon by a Swiss-made Oerlikon FF 20 mm cannon. German-made Telefunken radios were to be installed, but delivery delays meant that only the first aircraft was actually delivered with a radio fitted. The production aircraft were numbered 2–13, as the prototype bore the number 1. They were constructed at the Rogožarski factory in Belgrade and then the parts were delivered to the company hangar at Zemun for assembly. The first six aircraft were delivered by late March 1940, but delivery of the rest of the order was not completed until July due to delays by foreign suppliers. The first production aircraft was delivered to the VVKJ Test Group where it was determined that the production aircraft were free of the issues evident in the prototype. The Test Group determined that the maximum speed, previously estimated at 540 km/h (340 mph), was actually 527 km/h (327 mph).
When they entered service, the IK-3s suffered from minor equipment and instrument faults, largely caused by deficiencies in the Yugoslav aeronautical industry. This resulted in instrumentation that was a mix of foreign and Yugoslav-made equipment. The Yugoslav Minister of War approved the acquisition of a further 48 IK-3s in 1941–42. The aircraft were allocated to the 51st Independent Fighter Group at Zemun, with six aircraft allotted to both the 161st and 162nd Fighter Squadrons. Once in service, the IK-3 was evaluated against Yugoslav Messerschmitt Bf 109Es in a series of mock dogfights. The evaluation concluded that the IK-3 had a number of advantages over the German-made aircraft, in particular, the Yugoslav-made aircraft was more maneuverable in level flight, enabling it to quickly get behind a following Bf 109E with a series of tight horizontal turns.
During its first year of service, one IK-3 was lost when one of the IK-3 squadron commanders, Kapetan Anton Ercigoj was making a mock attack on a Potez 25 over the confluence of the Sava and Danube rivers. After passing below the Potez, he went into a climb with the intention of performing a loop. His rate of climb was too steep, and the aircraft went into a spin at low altitude and he was unable to recover before the aircraft hit the water. In the lead-up to the German-led Axis invasion of Yugoslavia in early April 1941, the 51st Fighter Group was placed under the 6th Fighter Regiment, which was responsible for the defence of Belgrade. The 51st Fighter Group was further reinforced on the day before the invasion began, with the addition of the 102nd Fighter Squadron, equipped with Bf 109Es. On the day the invasion began, 6 April, the two IK-3 squadrons had only three serviceable aircraft each, the five remaining production IK-3s being in the workshops. The 161st Fighter Squadron was commanded by Kapetan Savo Poljanec, and the 162nd Fighter Squadron was led by Kapetan Todor Gogić.
The invasion commenced with an initial wave of 234 German dive bombers and medium bombers attacking Belgrade, escorted by 120 fighters. This force reached Belgrade at 07:00, where it was met by the whole 51st Fighter Group, although one IK-3 from the 161st Fighter Squadron developed engine trouble after take off and was unable to join the fray. The five remaining IK-3s were the first to meet the first bomber wave, but they were almost immediately engaged by Bf 109Es of Jagdgeschwader 77. In this first engagement, five enemy aircraft were claimed by the pilots of the IK-3s, but one aircraft from each squadron were lost. Poljanec claimed a twin-engined bomber and a Bf 109E, but when he returned to Zemun in his badly damaged aircraft, he was strafed by a Messerschmitt 110, which further damaged his aircraft and wounded him. After this initial encounter only three IK-3s were serviceable, the IK-3 that had developed engine difficulties prior to the first German wave having been repaired.
A second wave of German aircraft arrived over Belgrade at 10:00, and the remaining IK-3s were scrambled with the rest of the 51st Fighter Group, but no victories were claimed by the IK-3 pilots. A joint claim was made during the third German attack at 14:00, a twin-engined bomber by Gogić and another pilot from the 162nd Fighter Squadron. On the following day, the IK-3 pilots did 5–6 sorties against German bomber formations and their fighter escorts, and shot down three bombers between them. At 17:00, Milislav Semiz attacked a tight formation of three bombers, during which his aircraft received a total of 56 hits from return fire, 20 of which were in the engine and propeller. Despite this, he managed to land his badly damaged aircraft. The return of another IK-3 from the workshops meant that the number of serviceable IK-3s remained at three.
It became difficult to continue activity from the 51st Fighter Group airfield at Zemun due to ongoing attacks, so on 8 April the remaining IK-3s and Bf 109Es flew to an auxiliary airfield at Veliki Radinci, some 50 km (31 mi) northwest of Belgrade where the surviving aircraft of the entire 6th Fighter Regiment were concentrated. Poor weather conditions made operations impossible until 11 April, when Semiz shot down a Bf 110 that had strafed the airfield. Later that day, Gogić and another IK-3 pilot both claimed Ju 87s during a patrol. That night, German ground troops approached within 15 km (9.3 mi) of the airfield at Veliki Radinci, and the following day all remaining aircraft of the 6th Fighter Regiment, including the remaining IK-3s, were burned by their crews. According to aviation writers Dragan Savić and Boris Ciglić, a single serviceable IK-3 was captured by the Germans in April 1941, and by the end of June a second IK-3 had been obtained. Both aircraft were located at Zemun, along with 23 other former VVKJ aircraft in serviceable condition. A fence separated this group from a number of aircraft that had been earmarked for scrapping. In late June, while German guards were distracted listening to news of the invasion of the Soviet Union, local communists, including former VVKJ mechanics, moved the fence. All the serviceable aircraft, including the two IK-3s, were subsequently scrapped.
"The IK-3s put up a valiant resistance against the Luftwaffe," author William Green writes, "scoring a number of 'kills' before they were finally destroyed in combat." Aviation writers Šime Oštrić and Čedomir Janić claim 11 victories for the IK-3, with Semiz as most successful, at four victories.
A shortage of engines was a major obstacle to large-scale production and further development of the IK-3, so tests were conducted on more powerful engines that could be utilised as an alternative to the Hispano-Suiza 12Y. These included the Daimler-Benz DB 601, Rolls-Royce Merlin II, and Hispano-Suiza 12Y51. These tests were incomplete at the time of the invasion, and the only prototype to be fitted with a non-production engine was destroyed by factory personnel with the incomplete production aircraft. Development of a dual-control two-seater trainer variant of the IK-3 had been commenced, but pressure on the design team had delayed the completion of the project when the invasion intervened.
The successful development of the IK-3 encouraged the three designers to pursue the idea of a twin-engined fighter aircraft capable of a range of uses, including long-range reconnaissance, photographic reconnaissance and in a "destroyer" or heavy fighter role similar to the Bf 110. Development of this new aircraft, designated the IK-5, commenced when the production IK-3s were being completed. A decision to order a prototype was made in early July 1939, and its first flight was scheduled for autumn 1941. The outstanding Yugoslav Ikarus S-49 fighter, produced after World War II, was based on the IK-3.
Specifications (Rogožarski IK-3)
Data from 
- Crew: 1
- Length: 8 m (26 ft 3 in)
- Wingspan: 10.3 m (33 ft 10 in)
- Height: 3.25 m (10 ft 8 in)
- Wing area: 16.5 m2 (178 sq ft)
- Empty weight: 2,048 kg (4,515 lb)
- Gross weight: 2,630 kg (5,798 lb)
- Fuel capacity: 330 l (73 imp gal; 87 US gal)
- Powerplant: 1 × Avia-built Hispano-Suiza 12Ycrs liquid-cooled V-12 piston engine
- Propellers: 3-bladed adjustable pitch
- Maximum speed: 421 km/h (262 mph; 227 kn)
- Cruising speed: 400 km/h (249 mph; 216 kn)
- Range: 785 km (488 mi; 424 nmi)
- Service ceiling: 9,400 m (30,840 ft)
- Time to altitude: 7 minutes to 5,000 m (16,000 ft)
- Wing loading: 159.4 kg/m2 (32.6 lb/sq ft)
- Related development
- Ikarus S-49
- Aircraft of comparable role, configuration and era
- Related lists
- Oštrić & Janić 1973, pp. 170–171.
- Oštrić & Janić 1973, p. 182.
- Oštrić & Janić 1973, p. 183.
- Oštrić & Janić 1973, pp. 187–188.
- Shores, Cull & Malizia 1987, p. 188.
- Oštrić & Janić 1973, p. 188.
- Shores, Cull & Malizia 1987, pp. 196–198.
- Oštrić & Janić 1973, p. 189.
- Shores, Cull & Malizia 1987, p. 199.
- Savić & Ciglić 2002, pp. 59–60.
- Green 1969, p. 207.
- Oštrić & Janić 1973, p. 192.
- Oštrić & Janić 1973, pp. 185–186.
- Oštrić & Janić 1973, pp. 186–187.
- Oštrić & Janić 1973, pp. 189–190.
- Oštrić & Janić 1973, p. 187.
|Wikimedia Commons has media related to Rogozarski IK-3.|
- Green, William (1969). War Planes of the Second World War, Volume Four: Fighters. London, England: MacDonald & Co. ISBN 0-356-01448-7.
- Oštrić, Šime I.; Janić, Čedomir J. (1973). "IK Fighters (Yugoslavia: 1930–40s)". In Cain, Charles W.; Windrow, Martin. Aircraft in Profile. 241–246 13. Windsor, Berkshire: Profile Publications. pp. 169–193. ISBN 0-85383-022-3.
- Savić, Dragan; Ciglić, Boris (2002). Croatian Aces of World War 2. London, England: Osprey Publishing. ISBN 978-1-84176-435-1.
- Shores, Christopher F.; Cull, Brian; Malizia, Nicola (1987). Air War for Yugoslavia, Greece, and Crete, 1940–41. London, England: Grub Street. ISBN 978-0-948817-07-6.