MMN (1939)
The MMN was an attempt to modernize the SB made between the late 1938 and the beginning of 1939 in Factory n.22 with the aim to increase its speed.
It was characterized by:
- M-105 engines with VISh-22E propellers;
- reduced wing outer panels; the span was 18 m, the wing area was reduced of 8 sqm, the size of the ailerons was reduced;
- the tail unit was modified;
- the nose was replaced with a more streamlined one, armed with one ShKAS machine gun instead of two;
- the dorsal turret was an MV-3, while the ventral mount was deleted.
The profile shows that the fin and rudder were different from standard SB.
The plane looks painted matt light grey, with black (?) engine nacelles.
The reduced size of the wings is well recognizable on this photo.
Note the elegant triangular extension of the black area over the wings.
Left:
the streamlined nose of the MMN, with one ShKAS machine gun instead of two of the production SB.
The state tests were made in August 1939, and showed only a modest increase of maximum speed to 458 km/h, few mre than the standard SB with M-103 already in service, instead of 500 km/h expected, and was not an adequate compensation for the weaker armament. Besides the distance for takeoff and landing increased due to the smaller wings.
The result was unsatisfactory, so the MMN was rejected.
The People's Commissariat for the Aviation Industry ordered to prepare a new prototype of SB satisfying the request for a speed of 500 km/h within 1 October 1939.
SB-RK (april 1940)
SB s/n 2/281 was built in May 1940 in Factory 22. It was characterized by two M-105 engines and a reduced wing span and area, similar to that of the MMN (2.3 m less than the standard width of SB) and enlarged flaps. The coolers are contained inside the outer wing panels, with the outlet on the upper surface of the wings; the engine nacelles were very slim. The fuselage was as the standard for SB of early 1940. It was first called slim-nacelles SB, then SB-RK (Radiatory-Krylo, radiators-wings)
The plane reached a maximum speed of 492 km/h at an altitude of 4700 m, considerably higher than the standard SB.
State tests at the NII VVS (Scientific Institute of the Air Force) were made in May 1940.
The tests were made without dorsal or ventral turrets, and it was pointed that the plane without these installations was not ready for production.
The SB-RK s/n 2/281 required further modifications before being launched into production:
- a new navigator's cabin similar to that of MMN with some duplicated flight controls and one ShKAS mg; it allowed better visibility for the pilot, both over the nose and downwards through the windows;
- the pilot's seat was moved on the right to allow visibility downwards through the windows under the nose, while the flight instuments were grouped on the right;
- a second set of flight controls with a foldable stick was installed for the navigator;
- the front ShKAS was installed on a ball-end similar to that of DB-3F, providing a conical field of fire of 50°; it was fed via a flexible belt from a box on the right;
- the dorsal turret, called TSS-1, could fire on the rear emisphere only; it was a half ring angulated upwards, covered by a trasparent 'turtle' that could slide forwards rotating upwards in the first part ofthe opening to give some field of fire;
- the ventral hath was equipped with a MV-2 retractable mount for a ShKAS aimed with a periscopical gun sight; a cam prevented it from firing on the tail wheel.
- New four NP-1 bomb racks were mounted under the wings next to the fuselage, and allowed to carry bombs up to 500 kg each for dive bombing
- air brakes made by oval steel profiles under the wings and automatic dive recovery system for use as dive bomber; mechanical indicators protruding from the wing uppersurface extended when the air brakes were extended.
- new cupola for the rear gunner;
- retractable strut for the ventral ShKAS, lowerable through the open hatch;
- engine monitoring instruments mounted on the inboard of each nacelle;
- a plywood hump for the loop aerial of the radio direction finder on the back;
- the pilot's canopy lifted at the beginning of its opening, then slided rearwards.
Ar-2
In February 1941, a refinement of production Ar-2 included the moving forward of the engines of 150 mm to improve longitudinal stability, and a VISh-22E propeller of 3,1 m diameter instead of the earlier 3,0 m; the engines received a gear with ratio of 0.59 instead of the previous 0.66, thinner air brakes and other refinements that allowed a max speed of 505-512 km/h insteadof previous 475 km/h.
Neverthless, the plane was phased out of production in favour of the Pe-2 that was faster and had a bigger potential to be modernized.
Factory 22 produced 190 Ar-2 between October 1940 and mid 1941.