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 help evaluate the design and determine the structural details. Like the IK-2 it was initially developed privately by the two men. Ilić and Sivčev's new streamlined low-wing monoplane design had a retractable undercarriage. With this work completed, Ilić and Sivčev had time to start preliminary development of a new low-wing monoplane that could better meet and defeat the high-performance bomber prototypes then in development by potential adversaries. The design concept for what became the Ikarus IK-2 was submitted to the VVKJ on 22 September 1933. Contemporary thinking within the VVKJ led them to evolve their initial ideas into a strut-braced gull-wing monoplane armed with a hub-firing autocannon and fuselage-mounted synchronised machine guns. They originally planned a low-wing monoplane with retractable landing gear. Working in a basement in Belgrade, and later in Ilić's apartment in Novi Sad, they secretly devoted their spare time to work on their design. Frustrated by this, they decided in 1931 to design a replacement for the Czechoslovakian-built Avia BH-33E biplane fighter in service with the VVKJ. Ljubomir Ilić and Kosta Sivčev went through this program but, when they returned to Yugoslavia, both were employed in administrative work. 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. In the late 1920s, the Royal Yugoslav Air Force ( Serbo-Croatian: Vazduhoplovstvo vojske Kraljevine Jugoslavije, VVKJ) and the Royal Aero Club of Yugoslavia helped send aspiring aeronautical engineers to France to gain knowledge. The IK-3 design was the basis for the post-war Yugoslav-built Ikarus S-49 fighter. Another account suggests that one aircraft survived the invasion and was later destroyed by sabotage. According to one account, to prevent them from falling into German hands, the surviving aircraft and incomplete airframes were destroyed by their crews and factory staff. Pilots flying the IK-3 claimed 11 Axis aircraft had been shot down during the 11-day war. All six were in service with the 51st Independent Fighter Group at Zemun near Belgrade. Six IK-3s were serviceable when the Axis invasion of Yugoslavia began on 6 April 1941. The prototype crashed during testing twelve production aircraft had been delivered by July 1940. It was considered comparable to foreign aircraft such as the Messerschmitt Bf 109E and came into service in 1940. Its armament consisted of a hub-firing 20 mm (0.79 in) autocannon and two fuselage-mounted synchronised machine guns. The Rogožarski IK-3 was a 1930s Yugoslav monoplane single-seat fighter, designed by Ljubomir Ilić, Kosta Sivčev and Slobodan Zrnić as a successor to the Ikarus IK-2 fighter. Kosta Sivčev, Ljubomir Ilić, Slobodan Zrnić
1930s Yugoslav low-wing, monoplane, single-seat fighter