The Rogožarski management observed that the SIM-X characteristics were very close to an acrobatic plane. It was decided to make some modifications to make an acrobatic plane. Sima Milutinovic designed reinforcements, reducing the wing area, and more powerful engines during 1937.

The Rogozarski SIM-XI (Serbian: Рогожарски СИМ-XI) was a single engined, one crew, high wing monoplane, designed and built at the Rogožarski factory in Belgrade.

The test flying was conducted by factory test pilot Captain Milan Bjelanovic by the end of January 1938. Factory tests were carried out until 12 March, then tests the experiment group of the Yugoslav Royal Air Force. The plane got excellent grades and and the YAF bought the prototype SIM-XI. This was the first Yugoslav aerobatic aircraft.

Engine: 1 × Siemens Sh14a, 110 kW (150 hp)
Propeller: 2-blade
Wingspan: 9.40 m (30 ft 10 in)
Wing area: 17.30 sq.m (186.2 sq ft)
Length: 7.16 m (23 ft 6 in)
Height: 2.70 m (8 ft 10 in)
Empty weight: 540 kg (1,190 lb)
Gross weight: 680 kg (1,499 lb)
Maximum speed: 191 km/h (119 mph; 103 kn)
Cruising speed: 168 km/h (104 mph; 91 kn)
Minimum control speed: 88 km/h (55 mph; 48 kn)
Range: 825 km (513 mi; 445 nmi)
Service ceiling: 5,500 m (18,045 ft)
Rate of climb: 4.87 m/s (959 ft/min)
Crew: 1

Aircraft SIM X was designed by Sima Milutinovic at the beginning of the 1936, drawing on the positive experiences of his previous SIM-II and SIM-VIII. The Rogozarski SIM-X (Serbian: Рогожарски СИМ-X) was a 1936s Yugoslav Sports and tourist plane and for basic training of military pilots. It was designed and built at the Rogožarski factory in Belgrade.

The prototype aircraft was built at Rogožarski from August until the end of the 1936. The first test flight took place on 1936. The SIM X had parasol monoplane with one engine, a wooden two blade propeller, with two crew members in tandem. The aircraft was registered YU-PDY.

While the plane was designed for three types of engine mounting: Walter NZR 120 hp, Siemens 110 hp and Gypsy Major 120 hp: the production incorporated a Walter NZR 120 hp air-cooled radial with reduction gear which was produced locally by Vlajković in Belgrade. The plane was mostly wooden construction, the fuselage of elliptical cross-section made entirely of wood covered with plywood, and the wings made of wood covered with cloth, with rounded ends. On each side, the wings are supported by a pair of inclined struts from the fuselage. The fuel tank was located in the central part between the wings. Landing gear was fixed, no axles, completely made of steel pipe.

A trainer single-engine two-seater plane for the initial training (21 units, 1937)

The Rogožarski SEA-JEEP was a modification of SIM X with Gypsy Major engine (1 converted SIM-X, 1940)
A second Rogožarski SEA-JEEP was a modified SIM-X plane for a school night flying (1 converted SIM-X 1941)

Variants:
Rogozarski SIM-X First variant with Walter NZR engines 120 hp, 21 built (1) prototype 1936 (10 airplane) 1937 and (10 airplane) 1938

Rogozarski SI-GIP
Second variant with Gipsy Major engines 132 hp, 1 built (conversion) in 1940.

Rogozarski SIM-Xa
Third variant instrument flight aircraft with Walter NZR engines 120 hp, 1 built (conversion) in 1941.

Engine: 1 × Walter NZR 6-cylinder radial, 89 kW (120 hp)
Propellers: 2-bladed Crew: 2
Wingspan: 10.00 m (32 ft 10 in)
Wing area: 18.50 sq.m (199.1 sq ft)
Length: 6.96 m (22 ft 10 in)
Height: 2.70 m (8 ft 10 in)
Empty weight: 543 kg (1,197 lb)
Gross weight: 785 kg (1,731 lb)
Maximum speed: 202 km/h (126 mph; 109 kn)
Cruising speed: 168 km/h (104 mph; 91 kn)
Minimum control speed: 72 km/h (45 mph; 39 kn)
Range: 560 km (348 mi; 302 nmi)
Service ceiling: 5,500 m (18,045 ft)
Rate of climb: 4.17 m/s (821 ft/min)

The Rogožarski SIM-VI (Serbian Cyrillic:Рогожарски СИМ-VI) was designed by engineer Sima Milutinović in the early 1930s as an inexpensive trainer plane that would enable expansion of sports aviation. It was a single-engined, two-seat, low wing aircraft designed as trainer in Yugoslav before World War II. It was designed and built at the Rogožarski factory in Belgrade.

The project was in suspension until 1936, when a prototype was constructed, and the maiden flight was made in 1937. The aircraft was a low wing monoplane of fabric-covered wooden structure. A four cylinder 50 hp / 35 kW Walter Mikron engine drove a two-bladed wooden propeller. The plane had tandem seating and was intended for civilian use, for the training of sports pilots, demonstration flights and travel. The wings were of thin profile, only 6% of trapezoidal shape with rounded ends. On each side, the wings were stretched with molded wire ties.

Immediately after completing the flight tests of aircraft SIM-VI, engineer Milutinovic undertook the repair of the shortcomings he had observed, so that by the end of the 1937 Rogozarski had a new variant of this airplane which was designated SIM-VI-a. It had a more powerful engine Walter Mikron II 44 kW (60 hp). An increase in engine power required strengthening of the basic structure which Milutinovic used to make general improvements to the aircraft. Wing relative thickness was increased from 6% to 15% which strengthened the wings, and resulting in the removal of wire ties. Now the wings were covered with cardboard to the half swing instead of cloth which resulted in an improved overall stiffness of the aircraft. This also enabled the landing gear and aerodynamic characteristics of new aircraft to be upgraded, while the weight was only slightly increased.

After successful testing and registration (YU-PDX) the SIM-VI was bought by the Central Administration of the Aero Club, which used the it at airshows and for the training of sport pilots. The aircraft did not enter serial production because it could not be used at smaller and under-equipped airports that existed in central Serbia.

Flight tests that were carried out at the Zemun airfield proved that the SIM-VI-a had slightly better performance than its predecessor, and was able to perform basic stunts. After being granted a navigation license the plane was registered under civil designation YU-PEZ and it was bought by Aero Club’s Central Administration. The plane was used until the beginning of the war in 1941 at air shows, sports competitions and for pilot training.

Pilot and engineer Arsenijevic, piloted this plane at the competition Little Entente states in 1938. during the competition the plane flew some 3,274 km (2,034.37 miles) achieving an average speed of 151.01 km/h (93.83 mph). Pilots who flew the plane felt that the plane was agile and easy to operate and fly, but despite good features and relatively low cost, SIM-VI did not go into serial production it could not win over the traditional empirical approach (a robust and powerful aircraft is a good aircraft). Right before the outbreak of war, the aircraft was included into the Yugoslav Royal Air Force, deployed as a signalling plane but was destroyed during the bombing of Lazarevac airfield on 7 April 1941.

Engine: 1 × Walter Mikron, 37 kW (50 hp)
Propeller: 2-bladed
Wingspan: 9.50 m (31 ft 2 in)
Wing area: 15.00 m2 (161.5 sq ft)
Length: 7.05 m (23 ft 2 in)
Gross weight: 500 kg (1,102 lb)
Empty weight: 300 kg (661 lb)
Maximum speed: 120 km/h (75 mph; 65 kn) 150 km/h (93 mph) at sea level
Cruising speed: 60 km/h (37 mph; 32 kn)
Range: 450 km (280 mi; 243 nmi)
Service ceiling: 4,000 m (13,123 ft)
Crew: 2